Suggested Restoration Alternatives

John A. Downing and Jeff Kopaska

 

Introduction/Summary of Issues

 

            The diagnostic portion of this study shows that Clear Lake has water quality problems, due to historic and present phosphorus and sediment loading, internal resuspension of sediment and nutrients and inputs of fecal-derived bacteria.  These problems derive from the agricultural and urban watersheds and from the lake bottom.  Deep lakes (i.e. >13 ft (4 m) average depth) generally have better water clarity, lower densities of algae, lower concentrations of suspended particles in the water, and are more likely to lack winter fishkills or other oxygen depletion problems.  Shallow lakes like Clear Lake (mean depth=9.6 ft (2.9 m)) have less volume for the dilution of nutrient and sediment inputs.  Accumulated sediments also decompose and resuspend and can exacerbate oxygen and nutrient problems.  Further, even these nutrient-rich sediments derive from watershed impacts since sedimentation rates increase sharply when eutrophication and deposition of eroded material lead to increased plankton production and carbon-rich detritus.  Anthropogenically eutrophied lakes like Clear Lake suffer many undesirable ecological characteristics (Table 9, Chapter 5, p. 96) most of which can be remediated through better watershed management.

 

            Sediment from watershed runoff has had a major impact on this lake over its lifetime.  Sediment flux has reduced the volume of Clear Lake to 38% of its original post-glacial volume, and nearly 25% of that sediment was deposited since 1935.  The rate of sediment deposition in the lake may have been reduced in the last few decades due to improved erosion management.  Sediment deposition still occurs, however, so Clear Lake is becoming shallower and smaller with the passing years. 

 

            Runoff from the watershed contributes bacteria, nutrients and turbidity to the water and leads to algal blooms, reduced transparency and great concentrations of suspended solids.  In the long term, sediments accumulate in the lake basin and cause water quality problems that are common to shallow lakes.  Eventually, lake basins can fill to the point that they are no longer useful for recreation.  Nutrients in the excess quantities found in Clear Lake impair many aspects of water quality.

 

            The following restoration alternative suggestions are designed to reverse the eutrophication and sedimentation processes by improving the nutrient retention of the watershed and by deepening parts of the lake.  Preventative measures in the watershed are necessary to slow the input of new nutrients and sediments into the lake, so that the restored lake can have an enhanced lifetime and improved water quality. 

 

            Principle restoration needs would be aimed to:

·        reduce phosphorus inputs to the lake,

·        reduce bacteria inputs to the lake,

·        improve management of bottom sediments, siltation, erosion and fish populations to reduce turbidity and nutrients due to sediment. 

We project that phosphorus loading to the lake could be reduced by 50-60% by implementing practices designed to address these issues.  This would lead to a substantial increase in water clarity and improved biological function.

 

Issue Identification and Potential Solutions

 

1.  Reduction in phosphorus inputs to the lake

 

a.  Phosphorus sources.  Phosphorus is the most important limiting nutrient in Clear Lake.  The Clear Lake diagnostic study attributed two-year average phosphorus loading to the lake as follows: rainfall, 31%; Ventura Marsh inflows, 24%; north shore agriculture, 14%; internal loading from Ventura Marsh, 9%; groundwater, 7%; City of Clear Lake urban, 6%; south shore agriculture, 5%; City of Ventura urban, 2%; and south shore (Cerro Gordo County) residential, 2%.  Thus, rural areas provided 52% of the phosphorus, rainfall accounted for 31%, urban areas provided 10%, and groundwater provided 7%.  Areas of the lake and watershed west of McIntosh Point provided 54% of the lake’s phosphorus input or about 81% of all non-precipitation surface inputs.  A total mass balance of Clear Lake (not including Ventura Marsh) suggests that internal loading, on average, amounts to about 800 kg annually or <10% of the overall phosphorus budget.  Because of the predominance of agricultural sources to the phosphorus budget of the lake, and the difficulty of managing rainfall and groundwater, mitigation of agricultural phosphorus holds the best hope for substantial phosphorus reduction.

 

Because agricultural P in the western watershed is substantial, AGNPS (Young et al. 1994) modeling of watershed nutrient transport was applied to present land use practices in the watershed.  Much of the phosphorus input in Iowa lakes derives from storm driven inputs (Downing et al. 2000).  We therefore modeled one storm event to simulate the typical rain event-driven loading seen in Iowa watersheds.  This event was a 2-inch, 24-hour storm, which climatic records show to occur at least once annually (National Climatic Data Center 2001).  The results of the modeling of present conditions, which included some AGNPS default values, are shown in Table 1 and Figure 1.  Default values and values appropriate for local conditions were acquired from Young et al. (1994) and the 1978 USDA Agricultural Handbook.  Results of this modeling show sub-basins 2, 3, 4, 5, and 10 (Fig. 1) export the greatest masses of phosphorus and nitrogen to the lake.  The model also predicts that sub-basin 11 loses the most nitrogen and phosphorus on a per unit area basis considering all agricultural basins.  The results from modeling nutrient and sediment loss under current conditions therefore reflect the general trend of field observations.  Field observations also show subbasins 2, 3, 4, 5, and 10 losing the largest masses of nutrients on an annual basis.

 

b.  Phosphorus reductions.  Decreasing the transport of phosphorus and eroded soil into the lake is critical to improving the health of the lake.  Many conservation practices that address this issue are already in use in this watershed.  Contour farming, terracing, and grass waterways are best management practices (BMPs) that are presently in use in the Clear Lake watershed, and many farmers are using field management methods that help to reduce phosphorus losses (see Chapter 11, p. 254).

The diagnostic study indicates a clear need to undertake a reasonable reduction in phosphorus delivery to Clear Lake, through a variety of in-lake, marsh, and watershed modifications.  Little water quality improvement will be achievable without substantial reductions in phosphorus loading.  The amount of reduction necessary to achieve a given degree of water quality improvement and thus determine target levels of phosphorus- and sediment loading to the lake can be assessed using a variety of models available in the published literature.  We used a combinations of models (Table 2, see references below) published in the Wisconsin Lake Modeling Suite 3.1.1 (1999) to perform these analyses.  These models calculate the likely total phosphorus concentration in a water body by making assumptions about the likely phosphorus retention of a lake.  The diagnostic portion of the study determined that the annual average water column total P was 166 ppb while the volume-weighted spring-overturn total P was 186 ppb.  Morphometric and hydrologic data were added to this and entered into the models along with known mass loading rates of P.  Although several of these models fit quite well, the Canfield-Bachmann, Natural Lake model fits the data best, yielding a predicted P concentration at spring circulation of 189 ppb.  Fit of this model was thus within 2% of the observed concentrations.  Using this model, we were able to calculate the likely changes in total phosphorus that would be obtained given hypothetical changes in total phosphorus loading (see Fig. 40 in Chapter 5).  Since substantial water quality improvement is sought, we judged that a useful criterion would be to bring the lake as close as possible to the 100 ppb range in order to decrease algae blooms and increase water clarity.  Such a change would require a decrease of 50-70% in the overall total phosphorus input to Clear Lake.  This should increase water clarity by 100-200%.  The feasibility analysis therefore sought a combination of watershed modifications that could yield this level of phosphorus load reduction.  Because the lake is shallow and mixed by wind, boats and benthic fish, water quality improvements will be contingent upon decreasing internal loading, as well.  Since rainfall and groundwater are now P-enriched in this region, every effort to reduce P input from all sources will be of short-term and long-term benefit to the lake.

 

A principle objective of this feasibility study was to suggest activities that would improve and protect the lake at a reasonable cost.  We therefore applied well-known watershed models and GIS (Geographic Information Systems) to indicate areas in the watershed that currently supply the highest rates of sediment and nutrient delivery. We used the AGNPS (Young et al. 1994) model to simulate sediment and nutrient transport following a rain event in the Clear Lake watershed.  An introduction to the modeling process, and the results for present conditions modeling were presented in Chapter 10 and in the Phosphorus Sources section above.  The methods used in the AGNPS modeling effort are also presented in Chapter 10.  The AGNPS model was applied according to present land use practices, verified through observations of actual field uses in 2000 and was then modified to simulate future land use scenarios designed to reduce inputs to the lake from the watershed.  We modeled one storm event to simulate typical rain event-driven loading seen in Iowa watersheds.  This event was a 2-inch, 24-hour storm.

 

Different land use changes were applied using model simulation to the watershed.  The first land use change was to look at model cells (0.22 acres) lying in row crop fields that exhibited the highest levels of phosphorus export.  These cells tended to be in areas with the highest slopes.  Cells with the highest phosphorus export under optimal nutrient conditions were “changed” from row crops to permanent grass for modeling purposes.  These “changes” from row crops to permanent grass were modeled in stages, “changing” the top 1%, top 5%, and top 10% of phosphorus exporting cells in a step-wise fashion.  The area removed from row crop production by these “changes” is 22.6 hectares (55.8 acres), 111.6 hectares (275.8 acres) and 220.1 hectares (543.7 acres) respectively (Fig. 2).  The results of the model following these changes are shown in Table 1.  These step-wise changes in land use produced an entire watershed average of a 2% reduction, a 10% reduction and an 18% reduction in phosphorus loading, respectively, from agricultural lands.  This would represent a 1% reduction, a 4% reduction and an 8% reduction, respectively, in total phosphorus loading to Clear Lake.  Thus, placing 10% of the agricultural land in permanent vegetation could yield an average reduction in phosphorus load to that lake of around 8%.

 

The next step in the watershed modeling effort was to identify areas in the watershed where water and nutrients could be retained in order to reduce nutrient flux to the lake.  Wetlands and other small impoundments are well known to immobilize significant amounts of sediments and nutrients in agricultural watersheds and the abundance of hydric soils in the watershed (see Fig. 5, Chapter 1, p. 20) suggests that wetlands were quite abundant prior to human habitation.  We used the PONDNET model (Walker 1987) to assess the impact of installing small nutrient retention ponds and restored wetlands in identified potential sites across the watershed.  Four existing wetlands, six areas that were previously wetlands, and six areas for potential wetland construction were located, added hypothetically to the landscape, and nutrient retention was modeled.  The area taken up by these 16 wetlands is estimated to be 135 hectares (335 acres).  The placement of these hypothetical wetlands in the landscape is shown in Figure 3.  The results of the model following these changes, and the ones listed earlier, are shown in Table 1.  The estimated sizes, volumes, phosphorus reduction potential (from PONDNET), and flow paths are listed in Table 3.  The cumulative changes in land use produced a 59% reduction in phosphorus loading, a 57% reduction in nitrogen loading, and a 24% reduction in sediment loading from the agricultural sections of the watershed to the lake, according to the models.  When combined with idling 10% of the agricultural land, these changes represent an 18% reduction in the total phosphorus load to Clear Lake.

 

            Ventura Marsh and the tributary streams that flow into it provide 33% of the total phosphorus budget to Clear Lake.  Marshes are well known to serve as nutrient traps, but the diagnostic study showed that Ventura Marsh exports more phosphorus than it receives from its watershed except after the carp removal experiment apparently decreased internal loading (contrast “Ventura Marsh Internal in Chapter 10, Figs. 3 and 4).  The next stage in identifying potential phosphorus reductions was to suggest restoration activities designed to enhance the nutrient retention capacity of the marsh.  The PONDNET model was used to estimate nutrient reductions in water flowing through Ventura Marsh, if the marsh was functioning properly.  Under that scenario, Ventura Marsh would remove 50% of the phosphorus load from the water flowing through it.  This alone would reduce Clear Lake’s phosphorus load by 17%.  When combined with the watershed modifications and wetland installations discussed above, results in a 26% reduction in the phosphorus load to Clear Lake.  A schematic diagram of how all these wetlands will work in concert to filter water on its trek to the lake is shown in Figure 4. 

 

The steps suggested to restore Ventura Marsh are as follows:

·        Separating the eastern and western sections of the marsh by building a dike across it. This would permit independent level controls of the two marsh basins.

·        Installing islands in the eastern basin of the marsh to limit wind-resuspension of marsh sediments and enhance aquatic vegetation growth.

·        Installing improved fish barriers in both the present grade structure and the new dike.

·        Installing a pumping system to serve as a primary means of water movement between Ventura Marsh and Clear Lake. The outfall could serve as a high-water overflow. Water level could thus be maintained both above and below the lake level.

·        Enhanced fishery management to limit benthic fish populations in Ventura Marsh.

A visual representation of these structural changes is shown in Figures 5 and 6.  There would be many additional benefits from these restoration activities.  By dividing the marsh into an eastern and western basin, the ability to manage water levels would be greatly increased.  This would result in improved nutrient retention in the marsh, and improved aquatic vegetation growth.  An additional result would be improved waterfowl production and increased waterfowl hunting opportunities.  Similarly, the islands planned for the eastern basin of Ventura Marsh would be constructed to limit wind, but also to provide waterfowl nesting habitat and waterfowl hunting opportunities.  This plan also suggests widening the existing grade between Ventura Marsh and Clear Lake when the fish barrier is improved.  The widened grade could be developed into a park, with trees, picnic tables, and educational information, as well as water pumps for marsh level control.  The improved grade would provide safer public access for anglers; a safer access across the grade for bikers, walkers, and runners; give DNR staff safer access to the structures and pumps; and, allow the construction of a goose-proof fence on the west side of the grade without drastically reducing the visual amenities that the lake and marsh provide.  This fence would be necessary because of the greatly increased waterfowl production that should occur in Ventura Marsh.  The widened grade structure would also help to improve fishery management, because it would make it more difficult for anglers to carry or throw fish across the grade into the marsh, and educational signs could be installed that would explain the reasons for keeping fish out of the marsh.  Improving water level management could also enhance fishery management by improving the ability to induce winterkills in the marsh, thus limiting benthic fish populations and allowing levels to fluctuate for optimum enhancement of wetland plants.  Other enhancements to fishery management might include increased monitoring of the fish population in Ventura Marsh, stocking predator fish to reduce benthic fish populations, and occasional chemical applications to reduce fish populations in the marsh.

 

            The urban areas surrounding Clear Lake provide 10% of the annual phosphorus budget to the lake.  This phosphorus enters the lake through storm drains following rain events.  The sources of this phosphorus include pet waste, sediment from construction sites, yard waste, materials washed off of the streets, and numerous other miscellaneous sources.  Phosphorus concentrations in water flowing from storm drains was incredibly high, with an average of 520 mg/L from Ventura, 560 mg/L from Clear Lake and 900 mg/L from the south shore residential area.  Suggestions in this report, if implemented, would reduce phosphorus loading from the agricultural areas of the watershed by 80-90%.  Similar reductions should also be expected from the urban areas of the watershed.  Activities such as improved construction practices, appropriate disposal of pet and yard wastes and regular street cleaning would all aid in reducing phosphorus transport to the lake.  Additionally, structures such as detention basins, constructed wetlands, and other approaches suggested in the Vierbicher Associates (2000) report concerning storm water management should be investigated.  Bonestroo, Rosene, Anderlik, and Associates are already implementing some efficient designs in this area.  Examples of activities that can be undertaken by different groups, such as citizens, cities, regional governments and agricultural communities, are shown in Table 4.

 

All of the above mentioned practices would reduce sediment and phosphorus transport from non-point sources to Clear Lake, but would have no impact upon point sources.  Potential point sources of phosphorus in the watershed may include overflows from the wastewater collection system of the City of Clear Lake, residences in the city of Clear Lake that are not connected to the wastewater collection system and rural residences in the Clear Lake watershed.  We did not specifically observe these problems, but we did find numerous occurrences of caffeine, a known human sewage tracer, in water from field tiles in the Clear Lake watershed.  Additionally, a 1998 study for the City of Clear Lake showed caffeine present in water flowing from 30 storm drains into Clear Lake.  It is assumed that all residences in the cities of Clear Lake and Ventura are connected to the wastewater system.  It is well known that the system is not adequate to prevent overflows during large precipitation events, but now overflows are pumped to areas that do not drain into Clear Lake. Additionally, little is known about the condition of the septic systems in rural areas of the watershed.  We believe that there is no ongoing inspection program for septic systems in Cerro Gordo or Hancock County, although there are local regulations that regulate on-site wastewater treatment and disposal (e.g., Cerro Gordo County Ordinance 27).  These are important issues with respect to limiting phosphorus transport to Clear Lake.  It is suggested that local officials, both from the cities of Clear Lake and Ventura, as well as from Cerro Gordo and Hancock Counties, begin an assessment program to determine if there is a need to upgrade systems for the protection and preservation of Clear Lake.  Further, minimum setback from surface waters (lakes, ponds, streams) of open and closed portions of systems should be made ample to reduce chances of input from these sources (at least 100 ft). 

 

Two disturbing aspects of the nutrient budget of Clear Lake are (1) that rainfall is now so significantly phosphorus enriched that it represents a substantial fraction of the budget, and (2) that groundwater is now very phosphorus-enriched and represents a pool of nutrient that can be reduced only with great difficulty.  Although rainfall is likely a widespread problem with elements of local and long-range transport and cannot be remediated within the scope of this restoration project, groundwater is likely local in origin (see Chapter 8), but will decline very slowly over time, even long after surface inputs have been alleviated.  One new means of reducing concentrations of nutrients and other contaminants in groundwater is through phytoremediation.  In this approach, trees are planted that extend roots into the groundwater, removing target substances.  Research in Florida suggests that trees can remove up to 10 g of P per m2 each year (Dierberg & Brezonik 1983, Dolan et al. 1981).  Therefore, although not completely quantifiable at this time (and therefore not part of the cost analysis), establishing broad (e.g., 100 m or more), wooded buffer strips around Clear Lake, Ventura Marsh and their tributaries would engender a long-term benefit since groundwater is moving on the order of 50 feet linearly per year.  Groundwater would be helped most if such buffers were established along shores where groundwater flux is substantial (see Fig. 7, Chapter 8).  This would also provide an increased measure of shelter of the lake from wind.

 

2.  Reduction in bacteria inputs to the lake

 

a.  Bacteria sources.  Shallow, warm-water systems such as Clear Lake, frequently receive coliform bacteria from the surrounding watershed during rain events, and the very rich nutrient and sediment environments found in eutrophic and hypereutrophic lakes allow these bacteria and probably the pathogenic microbes to survive for relatively long periods.  Coliform bacteria reside in the intestines of warm-blooded creatures, which include humans, livestock, birds, raccoons, rabbits, deer, and many types of pets.  All of these creatures are potential sources of the bacteria that have been found in Clear Lake.  The locations around the watershed indicated as sources of bacteria in Clear Lake were discussed extensively in Chapter 6.  Bacteria and caffeine were found in outflows from all storm drains in the City of Clear Lake.  Additionally, lake-wide bacteria sampling indicated persistent sources of bacteria from Ventura.  But, all shoreline areas around Clear Lake were shown to be a potential source of bacteria to the lake.

 

Samples from some of Clear Lake’s sediments showed high populations of coliform bacteria.  These sediments, and also the bacteria, are often resuspended into the water column by wind and boat action.  The sediments do not produce coliform bacteria, but instead they are a place where these bacteria survive after being washed into the lake from its watershed.  Thus, the lake’s sediments can become an indirect source of bacteria at times. 

 

b.  Bacteria reductions.  Steps that could lead to the reduction of bacterial populations in Clear Lake include the following: controlling animal wastes in the watershed; addressing storm drain, sanitary and water management systems; addressing septic systems; and reducing the resuspension of lake sediments.  Controlling animal wastes is both an urban and rural issue.  In the agricultural sector, minimizing livestock contact with waterways is one important way to keep bacteria out of the lake.  Another would be to reduce or eliminate manure application near waterways or tile inlets.  Urban sources of animal wastes include pets and wildlife.  Wildlife-derived bacteria may be difficult to control, but this is likely an insignificant source.  Proper disposal of pet waste by owners could eliminate that potential bacteria source.  Citizens around the watershed can institute a number of measures to reduce bacterial inputs.

 

Issues involving storm drain, sanitary, septic and water management systems include overflows from the wastewater collection system of cities, urban residences not connected to the wastewater collection system and rural residences in the Clear Lake watershed.  We did not specifically measure or observe these problems, but we did find numerous occurrences of caffeine, a known human sewage tracer, in water from field tiles and storm drains in the Clear Lake watershed (Appendix 10).  It is assumed that all residences in the cities of Clear Lake and Ventura are connected to the wastewater system.  It is well known that the system is not adequate to prevent overflows during large precipitation events, but now overflows are pumped to areas that do not drain into Clear Lake.  In fact, raw sewage input to the wastewater treatment plant becomes very dilute during rainy periods (Kevin Moeller, pers. comm.), suggesting that there are connections between stormdrains and the sanitary sewage system that could be eliminated or investigated. Additionally, little is known about the condition of the septic systems in rural areas of the watershed.  There is little systematic inspection of septic systems in Cerro Gordo or Hancock County.  These are important issues with respect to limiting bacteria transport to Clear Lake.  It is suggested that local officials, both from the cities of Clear Lake and Ventura, as well as from Cerro Gordo and Hancock Counties, begin an assessment program to determine if there is a need to upgrade systems for the protection and preservation of Clear Lake from bacterial inputs.  

 

Places where intensive examination of systems is warranted are indicated on the bacteria distribution maps in Chapter 6 (Figs. 3-7).  In addition, small, dredged harbors are often anaerobic near the bottom and are thus likely areas where fecal bacteria can persist.  New dredging should be discouraged around this lake and all existing harbors should be aerated continuously and tested regularly for fecal bacteria.  There are some innovative filtration systems that have been reported to be efficient means of removing bacteria from storm drainage (Richard Brasch, Bonestroo Rosene Anderlik Assoc., St. Paul, MN, pers. comm.).  These systems should be installed where simpler means are found impractical.

 

Reducing bacteria sources from resuspended sediments will be addressed in the next section, concerning in-lake activities.  Any actions, which would reduce sediment disturbance in the lake, would act to reduce bacteria resuspension.

 

3.  In-lake activities

 

a.  In-lake problems.  Clear Lake is presently shallow and located in a relatively unprotected basin, thus it is very prone to wind-derived mixing of sediments.  The diagnostic study showed that the current rate of sediment delivery to Clear Lake is 650 yd3/yr (~ 1.6 million lbs/yr), but the rate of sediment deposition over the past 65 years has been over 71,200 yd3/yr.  The result of this sedimentation is that the lake has been reduced to 38% of its original (post-glaciation) volume, and its average depth has decreased by 6 feet.  Analyses of wind direction, duration, and velocity records indicate that prevailing winds are generally out of the SSE and NW during the open-water season (Fig. 7).  The physical impact of these winds on the lake was discussed in Chapter 7.  Wind- and boat-derived resuspension of sediments contribute greatly to turbidity, or lack of water clarity, in Clear Lake.  Additionally, these sediments are loaded with ammonia, phosphorus and bacteria, all of which contribute to water quality problems in Clear Lake. 

 

High densities of benthic fish populations have also been implicated as causes of increased turbidity and water quality degradation in Clear Lake. Two species of benthic fish that were historically present and remain in very high densities today are bullhead and carp.  Bailey and Harrison (1945) listed bullhead as very abundant, and carp as common.  Biomass estimates completed by DNR fisheries staff in 1999 and 2000 revealed bullhead density to be 150 to 300 lbs/acre, and carp at 100 to 200 lbs/acre.  Although these species were abundant in the 1940’s, it is unlikely that they dominated the total standing stock as they do today.  Despite the historical presence of bullhead and carp, aquatic vegetation flourished in Clear Lake.  Apparently the density of these bottom-feeding fishes was not great enough to have a severe impact on vegetation, especially under higher water clarity, lower nutrient conditions.  As water quality deteriorated in Clear Lake and water clarity became reduced the vegetation started to decline.  The loss of vegetation severely impacted populations of bass, bluegill and crappie.  With a void created by their absence, it is likely that bullhead and carp increased in numbers taking advantage of the degraded environment for which they were better suited.  Thus, the problem became additive, with increased numbers of bullhead and carp further degrading the environment, creating an environment suitable for bullhead and carp to increase their numbers.

 

Another in-lake problem is that the Little Lake no longer retains nutrients, a function that it originally performed.  During this analysis, 54% of the phosphorus load to Clear Lake passed through the Little Lake.  Perusal of past and present bathymetric maps (Figs. 35-39, Chapter 5) shows that the Little Lake basin was originally much deeper than it is today.  This deeper basin served as a nutrient and sediment retention site for the waters that flowed from west to east through the lake.  Thus, cleaner, clearer water passed into the main lake.  Through time, this basin filled with sediment and nutrients, and now it is full.  This is illustrated by the fact that the entire bottom of the Little Lake now tilts downward toward the main lake.  This results in increased nutrient flux to the main lake from the western watershed.

 

b.  Potential in-lake solutions.

 

·        Dredging.  One potential step that could decrease P-delivery to the main lake is dredging.  The diagnostic study showed that the current rate of sediment delivery to Clear Lake is 650 yd3/yr (~ 1.6 million lbs/yr), but the rate of sediment delivery over the past 65 years has been over 71,200 yd3/yr.  The lake has already been reduced to 38% of its original (post-glaciation) volume, and if no action is taken, it could completely fill in approximately 700-800 years and could become a wetland in a much shorter period.  This assumes that the trap efficiency of the lake will not change and current soil conservation measures in the watershed will continue.  The removal of some of the accumulated lake sediments will add to the life of the lake and provide better environment for aquatic life. 

 

Restoring Clear Lake to it's original volume would require the removal of 34.0 million yd3 (26.0 million m3) of sediment, and such a plan would be incredibly costly and difficult to execute especially because a huge spoil deposition area would be required.  Dredging selectively chosen areas of the lake could provide vast water quality benefits.  For instance, the PONDNET model was used to calculate potential phosphorus removal in the Little Lake.  If the Little Lake were dredged to its original volume, it would remove 64% of the phosphorus that flowed into it via overland flow.  When combined with watershed phosphorus reductions discussed earlier, this would result in a 50% reduction in Clear Lake’s phosphorus loading.  Also, by deepening the Little Lake, resuspension of sediments would be reduced in that portion of the lake.  That would likely result in increased water clarity, which would in turn promote the growth of aquatic macrophytes.  Increased growth of aquatic macrophytes would help to stabilize sediments, which again would help to increase water clarity and phosphorus retention.  One remedial option would therefore be the removal of about 9% of the total accumulated sediments in Clear Lake, by removing sediments only from the Little Lake basin of Clear Lake.

 

A realistic goal for the lake is to restore the Little Lake basin to near its original depth and shape, while minimizing any disturbance of existing vegetation beds in the Little Lake.  The proposed dredging areas are shown in Figure 8.  The dredging proposal for Clear Lake would involve the removal of over 2.3 million yd3 of sediment.  This would result in 102 acres (41 ha) having depths of 7-13 feet (2-4 m), 71 acres (29 ha) with depths of 13-20 feet (4-6 m) and 41 acres (17 ha) with depths of 20-27 feet (6-8 m).  This would be accomplished by dredging a polygon measuring 4400 feet (1340 m) by 3150 feet (960 m) at its widest dimensions (Fig. 8).  Following this action, the maximum depth of the Little Lake would be increased from 8 ft (2.4 m) to 27 ft (8.1 m).

           

The diagnostic study indicated that some sediment in Clear Lake may contain contaminants, therefore, this is an additional consideration with respect to dredging at Clear Lake.  Four sediment samples were collected from Clear Lake and analyzed at the University of Iowa Hygienic Laboratory for pesticides and heavy metals.  These analyses showed that cadmium, chromium, copper, lead and zinc were present in lake water and/or in sediment elutriates in concentrations that constitute violations of Iowa Water Quality Standards.  This indicates that these pollutants need to be addressed, particularly if the dredging option suggested here is exercised.  Additional samples should therefore be taken before dredging to determine the sources and the extent of these Water Quality Standard problems.  If further testing shows these pollutants to be widespread, it will likely impact lake dredging as a restoration option by requiring more stringent control over water quality and substantially increased costs.  Additionally, monitoring data from the Little Lake sampling site showed very elevated concentrations of ammonia-nitrogen in water near the lake bottom.  Hydraulic dredging would draw water from this area of elevated ammonia-nitrogen levels, and the elutriate water could pose another problem for water quality. 

 

After reviewing the information concerning heavy metals in sediments, Ralph Turkle, IDNR-EPD summarized his recommendations by saying “... I think the report should not dismiss the metals as background levels.  The pollutants should continue to be part of the total lake assessment with every attempt in the future to identify their likely source(s).  In addition, any recommended lake restoration effort (hydraulic dredging) will be faced with addressing the return water quality and its impact on the lake.”

 

There are two types of dredging, dry land and hydraulic dredging.  Dry land dredging would require that the lake be drained and the sediment removed after drying. It is not frequently used because of the considerable length of time the lake is empty.  It is very unlikely to be successful here since groundwater would tend to keep the lake wet.

           

Hydraulic dredging is more commonly used in this type of situation.  Hydraulic dredging requires a floating cutterhead dredge, a large centrifugal pump, a slurry pipe system, and a water return system.  The slurry that the hydraulic dredge creates is a mixture of lake water and lake sediment.

           

The disposal of dredged material is an important consideration associated with hydraulic lake dredging operations.  A containment area is needed which would allow the solids to separate out of the slurry. The containment area needs to have a larger capacity than the amount of sediment to be removed because of a swell factor.  This swell or bulking factor is the ratio of the volume that the dredge spoil will occupy in the containment area to the volume of sediment dredged.  The bulking factor is based on the type of sediment being dredged.  For this analysis a bulking factor of 1.5 was used (Ken Jackson, IDNR, personal communication).  It is important to note that flocculent, organic sediments can actually shrink to a volume less than that occupied on the lake bottom when they are adequately de-watered.  These sediments are abundant in Clear Lake, thus estimates of volume needed to hold dredge spoil may exceed the actual volume needed. 

           

It is important to locate the containment area as near the dredging site as possible.  This is because as the distance between the containment area and the dredging site increases the power requirements to pump the slurry increase dramatically.  This in turn increases the cost of the dredging operation. 

 

The suggested dredging plan indicates that over 2.3 million yd3 (1.8 million m3) of dredge spoil will be removed, and when a bulking factor of 1.5 is applied, this requires a containment site that could hold 3.5 million yd3 (2.7 million m3) of spoil.  This volume of dredge spoil would need to be stored in a large area.

 

The location of one potential containment site for the dredge spoil is shown in Figure 9.  The site would be located just to the northwest of the Ventura Marsh Wildlife Management Area.  This area is approximately 2.5 miles (4.0 km) from the center of the proposed dredge area.  A dam would be needed at both the east and west ends of this site, and small berms would be required along 2 property lines.  The east dam would be 6 ft high and 330 yds long, and the west dam would be 11 ft high and 1310 yds long.  The berm on the north side adjacent to the Davison property would be 3 ft high and 275 yds long, and the berm on the south side adjacent to the Woiwood property would be 3 ft high and 275 yds long.  In this configuration, 4.6 million yd3 (3.54 million m3) could be disposed at this site.  The area presently under consideration is located outside of the watershed, so water pumped out to this basin would not return to the lake.  This site is located on private land.  The cost of purchasing this land (a multiply divided 556 acre tract with separate parcels owned by Arthur Boehnke, Dolores Boehnke, Raymond Kassel etux, Ronald Knop etux, Robert Ollenburg, Marlys Pueggel Trust, and Kathleen Woiwood) could be $1,231,000, using a land value of $2,214 per acre (average value of agricultural land in Hancock county, November 1, 2000, from ISU Extension).  The cost of building the east dam could be $12,000, and the cost of building the west dam could be $95,000 for the dam and water control structure.  The cost of building the two berms would total $6,600.  These estimates are based on a cost of $3/ yd3 for dam construction (Dave Rohlf, NCRS, pers. comm.).  Engineering and permitting costs for this site might be $201,700.

 

            During the public meeting concerning the restoration of Clear Lake, there was substantial interest expressed in other potential containment sites for dredge spoil (see audio CD of hearing).  Six additional sites have been identified within a 5 mile (8 km) radius of the center of the dredging site in the Little Lake (Fig. 10).  No sites further away than 5 miles were considered, due to the expense incurred by pumping dredge spoil great distances.  It should be noted that none of these additional sites are large enough to contain all of the dredge spoil.  Also, the costs associated with installing pipes for pumping dredge spoil to more than one containment site would be extreme, and those costs are not estimated here. 

 

The other potential spoil sites are numbered 1 through 6 in Figure 10, and are referred to as such in the following paragraph.  The important information for each potential site is shown in Table 5.  Each potential containment site would require at least one dike.  The dikes used in these estimates have a 12 foot wide top, 3:1 side slopes and estimated costs of $3/ yd3 for construction (Dave Rohlf, NRCS, pers. comm.).  Sites 1, 2, 4 and 5 would require only one dike each.  The dike for Site 1 would wrap from the north side all the way around to the east side, and would follow the curve of the land around the Lake Side Chapel.  The dike for Site 2 would stretch parallel to the road and the north and east sides of this site.  The dike for Site 4 would be parallel to the road along the north side of this site.  The dike for Site 5 would be located on the east side of this site.  Site 3 would require two dikes, one on the east and the other on the northwest corner.  Site 6 would require three dikes, one to the west, one to the north and one on the east side of this site.  A number of combinations of these sites could be used to contain the estimated 3.5 million yd3 of dredged material that would be removed from Clear Lake.  All of these areas are outside of the watershed, so water pumped to these basins would not return to the lake.

 

Another option for the containment of some dredge spoil is to construct fetch-foiling structures in Ventura Marsh.  The locations of the proposed structures, as well as a visual representation of these structures, are shown in Figure 5.  These are all areas where Geotubeä material could be used for structure construction.  These are flexible tubes into which sediment slurry is pumped and dewatering occurs through the pores in the membrane.  This is efficient because dredged material is being re-used to form the structure.  Further dredge spoil could then be pumped into the area behind the Geotubeä.  Using this material to construct the islands would allow the disposal of nearly 7,000 yd3 of dredged material per island.  Each structure is approximately 0.85 acres in size, and up to 13 of these structures could be constructed in the eastern cell of Ventura Marsh.  These structures could then hold just over 83,000 yd3 of dredged material.  These islands would be important because they would provide vegetated windbreaks within the marsh, and vegetative plantings are necessary to help hold the dredged material in place.  Additionally, these structures would provide excellent habitat for waterfowl breeding, as well as increased hunting opportunities within the marsh.  The cost of the island structures would be as follows: Geotubeä materials, filling, and installation, $135,000 per island; construction mobilization and deployment, $20,000; obtaining the dredge material to fill the tubes is considered in the dredging cost. 

 

The combination of these dredge material containment sites would contain all of the dredge spoil that is proposed to be removed.  Using these locations would alter the present configuration of Ventura Marsh.  Figure 11 provides a glimpse of what Clear Lake would look like bathymetrically following dredging.  Implementation of this option would increase lake volume from its present 42,054,700 m3 to 43,822,600 m3 post-restoration.

 

·        No-wake zones.  Another step which would help with the problem of sediment resuspension would be to enforce existing no-wake zones in the lake, and perhaps expand the no-wake zones in some of the more vulnerable areas of the lake.  Motorized vessels should proceed at speeds that minimize the depth of penetration of motor backwash in shallow waters.  Lake monitoring, particularly the work summarized in Chapter 7 of the diagnostic study, showed that no-wake zones are often ignored by recreational boaters.  No-wake zones presently extend 300 feet out from shore throughout Clear Lake.  These near-shore areas are often shallow, and are vulnerable to sediment resuspension by all forms of motorized watercraft.  Areas particularly vulnerable to sediment resuspension are shown in Figures 4-11 of Chapter 7 in the diagnostic study.  These figures could be used as a guide to determine where no-wake zone expansion might be most beneficial, if that is employed as a restoration option.  Limiting sediment resuspension is important because resuspension inhibits water clarity and macrophyte growth.  Additionally, sediment disturbance in shallow areas inhibits the establishment of rooted aquatic macrophytes.  These macrophytes consolidate sediments, limiting resuspension, and use nutrients, helping water clarity. 

 

·        Aeration.  Aeration presently is an ongoing winter activity at Clear Lake.  If the Little Lake is dredged, year around aeration of the Little Lake should be considered.  The Little Lake will contain the deepest area of the lake, so aerating that basin would avoid anoxia and provide important wintering habitat for fish.  Hypolimnetic aeration is a possible restoration activity that might be quite helpful in limiting phosphorus remobilization from lake sediments.  Hypolimnetic aeration differs from the present method of aeration because it pumps water to the shore to be aerated, rather than pumping air to a deep spot in the lake.  This allows the water to remain stratified, which could be important in the Little Lake following dredging.  Maintaining stratification in the Little Lake will be important for limiting remobilization of phosphorus from the sediments.  Additionally, hypolimnetic aeration could provide an opportunity for in situ alum treatment of the aerated basin to limit phosphorus remobilization and lengthen springtime clear-water periods throughout the lake.

 

·        Breakwater structures.  The construction of breakwater structures has great potential to improve fisheries habitat and improve water quality at the same time.  The diagnostic study showed that wind resuspension is a major problem for Clear Lake water quality.  Breakwaters placed parallel to existing bulrush beds would protect them from the pounding forces of the wind and waves.  These areas would then grow more vigorously and provide quiet water that would enhance the growth of submergent vegetation within the bulrush.

 

Potential sites for breakwaters include:  State Dock, Baptist Camp, McIntosh Woods State Park, Farmers Beach, Lekwa Marsh.  All of these sites are either publicly owned or undeveloped shorelines, which would improve the likelihood of public acceptance.  Placing these structures in 5 to 6 feet of water would dampen the wind resuspension of nutrients, reduce wind disturbance to nearshore vegetation, reduce turbidity, and create excellent fish habitat.  The riprapped portion of the breakwater would attract small and large fishes and the quiet water on the backside would provide quality spawning and nursery habitat with the mixed growth of submergent and emergent vegetation.  Constructing these structures from the shoreline out in a T or L configuration would allow shore anglers to access the main arm of the breakwater.  The sociological and economic surveys reported on in Chapters 2 and 3 underlined the public’s expressed need for more public access to the lake.  These structures would address one aspect of the perceived public need.

 

Other potential locations for breakwater structure would be near storm drain outfalls along residential shorelines.  Suggested locations for all of the breakwater structures are shown in Figure 12.  The breakwater structures would provide the vegetation and fisheries benefits listed above, and would serve as additional filters for the water draining off of the watershed and entering the lake.  By placing breakwaters near storm drains, the vegetation growth promoted by the quiet water area would serve to filter nutrients out of the water entering the lake.  This would be an effective way to further cleanse storm drain water on its way to the lake proper. 

 

·        Fish population management.  To address the problems with benthic fish populations in Clear Lake, it will be necessary to improve two critical areas.  First, it would be necessary to improve water quality through the nutrient mitigation outlined above.  This would help reestablish aquatic vegetation, which so many other fish species are dependant upon.  Second, it would be necessary to reduce and control populations of bottom-feeding fishes, primarily carp and bullhead.  This would improve water clarity and also enhance the recovery of aquatic vegetation.  Doing one without the other may not bring the intended results, so combining the two appears to be the best plan. 

 

Water quality improvements are the intended results of many of the watershed alterations discussed earlier.  If the watershed and in-lake structures that are suggested achieve the desired results, increases in water quality and water clarity would result.  This should then result increased aquatic vegetation, which would provide critical habitat for fish.

 

Effective management of benthic fish should be designed to hold carp populations below 50 lb/acre.  Some removal of benthic fish from Clear Lake has occurred since the early 1900’s.  Although past removal of carp from Clear Lake may appear impressive, it has not been adequate to have had major impact on the fish community or water quality.  Currently, contract commercial fishermen have been taking the surplus and not making a substantial dent in the population.  To increase the harvest, a monetary incentive could be considered.  The fisherman would continue to receive payment for the sale of fish, but they could also receive an additional payment (so many cents per pound) from the DNR.

 

Bullheads are not currently available to harvest under the present contract commercial fishing program.  Some neighboring states do allow for the commercial harvest of bullheads.  This could be considered for Clear Lake.  Population estimates of bullhead during 1999 and 2000 indicate a population of 1.5 to 3 million bullheads in Clear Lake.  Despite this dense population, only about 36,000 bullheads were harvested by sport anglers during those two years combined.  These fish were considered to be angler acceptable size, averaging 9 inches long and 0.4 pounds.  A review of creel surveys on Clear Lake shows a downward trend in bullhead harvest over the past 15 years.  Two hundred thousand bullhead were taken by anglers in both 1986 and 1987, but have never approached these levels since.  Angler attitudes have changed over the past two decades.  Between 20% and 30% of Clear Lake fishermen were specifically targeting bullheads during the mid-1980’s, while in recent years, less than 5% target bullheads.  It is unlikely that sport angler harvest will have any impact on reducing bullhead numbers in the future.  It may also be socially acceptable to allow for a commercial harvest since so few anglers desire to catch these fish. 

 

Biological control of carp and bullhead could provide another opportunity for reducing the populations of these species in Clear Lake.  Flathead catfish appear to be the best predator for controlling undesirable species.  Flatheads have been used successfully in Minnesota and Iowa on small lakes to reduce overabundant bullhead.  A small number of flatheads were stocked in the fall of 2000 in Clear Lake.  Additional fish are scheduled to be released this summer.  A stocking strategy needs to be developed, analyzed and refined as work continues with this species.  Besides being a very effective predator, they could also provide a unique opportunity to catch a trophy-sized fish in the future.  Care should be taken however to avoid replacing one problem species with another.

 

Other predators that might be considered include largemouth bass and walleye.  Although largemouth bass are an effective predator of bullhead, previous stockings have not done well in Clear Lake probably due to lack of abundant suitable habitat.  This is likely to self-correct as water quality improves.  Walleye will also readily consume bullhead, however large numbers of walleye are already stocked.  Walleye density could be improved through the use of large fingerlings (>8 inches), in years when fry stockings produce a weak year class.

 

Any significant reduction in bullhead or carp populations, whether it be through mechanical removal or biological control, must be accompanied with a strategy to fill the void created with desirable sportfish.  Sufficient predators must be available to control increased bullhead and carp reproduction.  In addition, adequate panfish brood stock must be available to fill the void created.

 

4.  Role identification

 

There are many activities suggested in the sections above which all can help to improve the water quality of Clear Lake.  No one agency, person or institution can work independently and achieve the goal of cleaning up Clear Lake.  Rather, collectively people, agencies and institutions will need to work together to achieve this goal.  To aid in this endeavor, many tasks and roles are identified below.

 

a.  The role of citizens, residents and visitors.  Every person can help to maintain and improve water quality at Clear Lake.  The community at Clear Lake has been involved with the alteration and degradation of the lake over the last 100 years.  Now the community has identified remediation as a major goal and is profoundly engaged in this pursuit (see Chapters 2 and 3).  Personal citizen action can be very beneficial.  Taking responsibility for reducing nutrient loads to the lake is one important step.  This can include supporting riparian and watershed regulations and guidelines, reducing nutrient input through “good housekeeping” and awareness that everything that hits the ground goes to the lake, supporting watershed cleanliness, inspecting plumbing and real estate for potential sources of nutrients, being aware of and using low P products (washing products, fertilizers, chemicals) and using good old-fashioned common sense Citizens should be encouraged to ask themselves “would this help the lake if it got into it?”  Additionally, it is important to keep the watershed free of bacteria-laden materials.  This can be done by picking up after pets, controlling all types of human and animal wastes, and supporting structures that filter urban runoff.

 

b.  The role of cities.  The cities of Clear Lake and Ventura are provided many amenities by their proximity to Clear Lake.  This close proximity also allows them to impact the lake in many ways, and some of them are clearly detrimental to the lake.  Other cities have involved themselves in many activities to protect their lakes.  One potential activity is to adopt comprehensive riparian management and development plans.  Aspects of these plans could include increasing set-backs and buffers, decreasing shore impacts and erosion, maintaining high levels of cleanliness of pavement and land, minimizing impervious structures, decreasing construction-related impacts and routing effluents and nutrients out of the watershed and treating them to reduce harm elsewhere.  If the plans outlined in this report were to go into place, it would also be important to promote and enforce riparian and nutrient management regulations.  Inspecting, repairing and/or upgrading storm drains, sanitary systems and other water management systems, with the objective of lake restoration and minimizing impacts on the lake, is another important suggestion.  Installing and maintaining bacteria- and sediment-trapping storm drain filters can be effective in removing these pollutants before they enter lakes.  There also may be abandoned septic systems or other waste disposal systems that are still leaking into the lake.  Addressing these issues within communities can be important in promoting a healthy lake. 

 

c.  The role of counties.  Cerro Gordo and Hancock Counties both contain areas of the Clear Lake watershed.  The lake’s watershed provides most of the nutrients which enter Clear Lake.  In particular, the unincorporated residential and agricultural areas which are under the respective counties’ jurisdictions provide the majority of these nutrients.  These counties receive many benefits from the lake, and they could also be involved in numerous activities that would, in turn, benefit the lake.  One potential activity is to adopt comprehensive riparian management and development plans.  Aspects of these plans could include increasing set-backs and buffers; decreasing shore impacts and erosion to tributaries, wetlands, shorelands and the lake; maintaining high levels of cleanliness; decreasing construction-related impacts; and, routing effluents and nutrients out of the watershed and treating them to reduce harm elsewhere.  In relation to these plans, it is also important to promote responsible riparian and nutrient management.  Rural waste management systems may also be contributing to the degradation of the lake.  Activities that would reduce these impacts might include inspecting and repairing tile-line, storm-drain, sanitary and other water management systems; addressing old, abandoned septic and waste disposal systems; inspecting septic systems; upgrading septic systems to efficient designs that immobilize nutrients; and, enforcing regulations that prohibit disposal of “black” water to tile-lines and surface drainages.  Adopting regulations that limits unplanned proliferation of casual drainage to the lake and its tributaries, and instead favors water-retention in wetlands, is a potential county activity.  Counties could also promote moderate nutrient use in agricultural lands and seek ways to help producers keep nutrients within fields.  Along the lines of general cleanliness and good housekeeping, counties could employ frequent street and road cleaning, as well as strive to control dust from construction sites and unpaved roads.

 

d.  The role of the agricultural community.  Agricultural areas surrounding Clear Lake provide 43% of the phosphorus that enters the lake (or perhaps more if one considers phosphorus moving as dust and groundwater).  In order to restore Clear Lake, this phosphorus needs to be kept on the fields, where it is vitally important, and out of the lake.  Thus, it is important that the agricultural community be receptive to initiatives that would decrease agricultural nutrient inputs to the lake.  One such initiative would be voluntary agreement to changing 5-10% of the row-cropped land in the watershed from crops to permanent vegetation.  This would result in a 10-20% reduction of the phosphorus export from all agricultural lands.  Use of assessment tools, such as the NRCS Iowa P-Index, help farmers to determine their risk of phosphorus loss from the cropland.  Expanded use of best management practices could decrease erosion and increase nutrient efficiency.  Related to nutrient efficiency, the diagnostic study showed that over 50% of the crop fields in the Clear Lake watershed had levels of soil phosphorus much higher than levels optimal for crop production (see Chapter 11).  Reductions in phosphorus applications and improved timing and methods of application could decrease field losses and decrease levels of phosphorus in groundwater.  Consideration of Conservation Reserve Program (CRP), Wetlands Reserve Program (WRP) or other conservation easements of sensitive lands could ease nutrient delivery to streams and the lake.  Similarly, construction and restoration of wetlands as retention and detention structures could decrease soil and nutrient delivery to the lake. 

 

e.  The role of government agencies.  IDNR has indicated that government agencies will likely play a leadership role in the restoration of Clear Lake.  There are many restoration alternatives which will help these agencies work with the community, the lake, and the watershed to lower phosphorus, sediment and bacteria inputs and decrease remobilization of these substances within the lake.  One set of alternatives is simply the application of basic watershed restoration techniques, such as:

·        promoting permanent vegetation on 5-10% of the most erosion-prone land to reduce P flux from the western watershed,

·        installing and restoring wetland systems to retain nutrients,

·        installing P retention and detention ponds in urban and agricultural areas,

·        “Daylighting” some sections of tile lines and restoring streams and wetlands.

Working with the agricultural community to increase phosphorus retention on agricultural lands is an important restoration alternative.  Ventura Marsh can and should act as a nutrient filter for waters that enter Clear Lake.  An important restoration activity might be to restore and maintain Ventura Marsh, so that it acts as a nutrient retention basin.  This might involve the structural changes listed earlier, as well as fish population management.  Nutrient retention is a common theme in this restoration process, and another alternative which addresses nutrient retention is dredging the Little Lake.  If the Little Lake were dredged to its original depth, it would act as a nutrient retention basin, holding back 64% of the phosphorus that enters it from passing on to the main body of the lake.  Benthic fish population management and maintenance is another restoration option that has great potential for improving water quality in Clear Lake.  Expanding and enforcing no-wake zones could have a similar positive impact.  Finally, expanding and maintaining hypolimnetic aeration could address phosphorus resuspension issues.

           

Analysis of alternatives

 

            The management approaches discussed above compliment each other; in-lake activities help to restore the basin by increasing water depths and stabilizing sediments, while watershed restoration helps improve water quality and decelerate the rate at which the lake will degrade in the future.  We suggest that both approaches would be most effective if adopted together; for it will do little good to remove the sediments from the lake if soil erosion and nutrient loads rapidly return sediment and phosphorus to the lake, while watershed restoration would only restore the water quality of the supply to a lake of short life and dubious aquatic potential. 


Benefits of restoration project

 

Clear Lake is an important recreational resource.  Survey results and data analyses conducted by Azevedo, Herriges and Kling (Chapter 2) estimate that 178,650 households visit Clear Lake every year. It is a well-used lake in this region due to its location near Clear Lake and Mason City, and it's proximity to Interstate 35.  Interviews conducted by Wagner (Chapter 3) indicate the major social role that the lake plays in the region.  Additionally, few other public lakes are available for recreation in close proximity to Clear Lake.  Over the years, multiple public entities have made investments in facilities in the parks around Clear Lake.  It is important to note that water quality is likely to degrade substantially if no restoration is attempted.  The major benefit of this project is to improve this recreational resource, by reversing degradation due to eutrophication and sedimentation, thus improving water quality within the lake for the benefit of water-based recreation.

 

Respondents to the Clear Lake survey indicated a willingness to pay of $19.5 million to avoid the deterioration of Clear Lake.  Alternatively, respondents expressed a willingness to pay of about $40 million for quality improvements at the lake.  These numbers represent the value to visitors and residents of water quality maintenance and improvement.  In considering whether investments to clean up the lake are worth the costs, these value estimates provide the appropriate baseline for comparison.  These large values associated with water quality improvements at the lake are consistent with the lack of good substitute resources and the potential quality of this unique resource.

 

Local residents and others may also be interested in the amount of economic activity generated locally as a result of water quality improvements.  Households that responded to the survey on average made 6.6 visits to Clear Lake per year, so an estimate of annual total visits to Clear Lake would be 178,650 X 6.6 = 1,179,100 visits per year.  Survey respondents reported spending an average of $51 in or near the City of Clear Lake on a typical visit, for a present expenditure level of just over $60 million annually.  Following restoration, survey visitors indicated they would increase the number of trips they took to Clear Lake from 6.6 to 10.32 trips per year.  If no additional households visited Clear Lake and these estimates accurately reflect future behavior, the number of visits would increase to 178,650 X 10.32 = 1,843,700.  If spending remained the same, the level of visitors’ expenditures in Clear Lake would then increase to over $94 million annually.  The number of households that visit Clear Lake should increase following restoration, so these numbers may underestimate results following restoration.  It should be noted that the numbers used above are estimates, and do not account for residents’ expenditures, but yield an indication of the great economic value of Clear Lake.

 

It is important to note that from a societal perspective, this economic activity could occur elsewhere if Clear Lake's water quality is not improved and acceptable alternatives are available.  Thus, it does not necessarily reflect a net increase in economic activity to the region.  Therefore, although of significant importance to city and regional managers, measures of economic activity do not necessarily measure the inherent value of a resource such as water quality in Clear Lake. Rather, the willingness to pay represents this value.

 

If none of the restoration options are employed, the lake will become more and more eutrophic, and thus become a less attractive recreation resource.  Additionally, in time, the lake will fill to the point that its value as a recreational lake will likely decline dramatically.  Long before that time, however, severe water quality problems will be encountered.  The combination of watershed improvements, management approaches and in-lake activities would enhance the recreational value of the lake and greatly prolong its useful life.  Watershed restoration and lake deepening would act to reduce nutrient inputs and dilute their effect in the lake, however, the lake would still be likely to have some water quality problems due to algae blooms and low transparency.  This is due to the limits on restoration imposed by phosphorus-rich precipitation and groundwater.

 

How long will it take?  It should be noted that the time-course of response to nutrient abatement is likely to be quite long.  Cooke et al. (1993) have analyzed a number of cases in which external nutrient loads were reduced substantially.  They found that short-term (<5 years) improvements were only noted in about half the lakes and that most lakes take more than 5 years for changes to begin to be detected.  In shallow lakes, the problem is exacerbated by internal loading, so changes may be very slow.  Lakes may improve over a decade or two before the equilibrium level is approached.  The time-course of restoration should therefore be expected to be 5-30 years depending upon the speed and degree to which restoration activities are undertaken.

 

 

Post-restoration monitoring plan

 

Once the dredging and watershed improvement work has been undertaken, a monitoring program should be established to determine the rate of recovery of the lake.  Such a plan is outlined here.  Three sampling stations should be established at the points in the lake shown in Figure 13.  For at least a five‑year period, samples should be taken monthly during the months of September through April and biweekly during May through August at the water surface, 0.5 meters deep and at 1 meter depth intervals to the lake bottom (maximum depth, 8 m).  Samples would be collected between 0800 and 1600 hours.  All samples should be analyzed for total and soluble reactive phosphorus; nitrite plus nitrate nitrogen, ammonium, unionized ammonia (the toxic form) and total nitrogen; total suspended solids; silicate; pH; temperature; and dissolved oxygen. Representative alkalinities should also be determined. Samples collected from the upper mixing zone should be analyzed for chlorophyll a. Algal biomass in the upper mixing zone should be determined through algal genera identification, cell counts and cell volumes, and reported in terms of biomass of each genus identified. Secchi disk transparency should also be determined at each sampling period. The surface area of the lake covered by macrophytes at mid-summer should be determined, and the predominant species should be identified and their distribution shown on a map.  Water samples should also be taken from the main tributaries (9 sampling sites, Fig. 13) on the same days that the lake is sampled (when they are flowing) for analyses of total phosphorus, total nitrogen, total suspended solids and inorganic suspended solids.

 

Cost analysis

 

            The preliminary cost analysis (Table 6) contains several assumptions and is for planning purposes only.  More detailed cost estimates should be included as part of the engineering design of the project.  The following items are addressed in this cost estimate: land conservation by planting permanent vegetation, pond and wetland installation, Ventura Marsh renovations, water control structure renovations, dredging, fish barrier construction and post-restoration lake monitoring.  Several items that should be implemented but that are not addressed in this cost estimate are (1) costs for encouraging the adoption of Integrated Crop Management (ICM) strategies and other BMP practices throughout the watershed, (2) costs for improvements upon septic systems, sanitary sewers and storm drain systems in the watershed, (3) costs associated with planting of woodlands along lake shores, streams and wetlands to mitigate groundwater P, (4) other costs incurred by citizens, cities, counties and agricultural producers to mitigate P, sediment and bacteria fluxes from individual properties and (5) costs associated with various policy changes outlined in Table 4.  These are also useful remedial measures the cost of which would vary greatly with the scope of application. 

 

            The costs of converting cropland to WRP permanent easements, assuming there are willing landowners and the land is accepted by NRCS, might be assumed by NRCS.  This includes legal costs, survey costs, tile breakage and wetland construction and wetland and upland seedings (Dan Selky, Hancock Co. NRCS District Conservationist, pers. comm.).  The landowner is also paid 100% of the appraised agricultural land value in return for the permanent conservation easement.  The average value of agricultural land in Hancock County as of November 1, 2000, was $2,214, and the average value of agricultural land in Cerro Gordo County at the same time was $2,189 (ISU Extension 2000).  For the purpose of this estimate, all land was valued at $2,200.  There are two types of land conversions proposed.  The first is converting cropland to permanent vegetation in areas with a high potential for phosphorus losses.  If the top 5% of phosphorus exporting lands were idled by the purchase of permanent conservation easements that would idle 276 acres of cropland, and an estimate of the cost of the permanent easements would then be $607,200.  If the top 10% of phosphorus exporting lands were idled by the purchase of permanent conservation easements that would idle 544 acres of cropland, and an estimate of the cost of the permanent easements would then be $1,196,200.  The cost of converting cropland to permanent vegetation was based upon a study performed by Colleti (1996), Department of Forestry, ISU.  The cost of site preparation, and purchasing and planting the seed to convert cropland to switchgrass, was a total of $77/acre.  The modeling effort suggested that 276 or 544 acres be placed into permanent vegetation, which would translate to a cost of $21,250 or $41,900, respectively. 

 

The second type of land conversion proposed is to restore and create wetlands for nutrient retention in the landscape.  The locations and types of wetlands (restored or newly constructed) are shown in Figure 3.  The costs associated with each particular wetland are shown in Table 7.  The total cost for constructing or restoring these wetlands is estimated to be $971,050.  These costs include purchasing the land or a conservation easement for the land in the wetland and a 100 foot upland buffer surrounding each wetland, building any required dikes, building or upgrading water control structures and planting upland buffers to native vegetation.  All dikes estimated in this exercise had a 12 foot wide top, 3:1 slopes on the sides and cost $3/ yd3 for construction (Dave Rohlf, NCRS, pers. comm.).  Legal costs, survey costs and the cost of tile breakage are not included in this cost estimate.  If federal funds are not available or are not sufficient, the total cost of permanently idling the cropland, creating or restoring the wetlands, establishing plantings and placing them under permanent easement could be $2,209,150.  It should be noted that USDA programs are short-term allocations and should not be considered a permanent fix.

 

            Restoring Ventura Marsh is a many-faceted endeavor.  One aspect of the restoration is building a dike across the upper end of the marsh, to facilitate differing water level management scenarios in the eastern and western basins of the marsh.  To build a dike that would tie into existing slopes and curve with the existing marsh, the dike would be approximately 1260 yards long.  For this estimate, a dike height of 21 feet was used.  This allows for significant excavation to get down to a solid substrate to tie the dike into, and allows the dike to be high enough to pool significant amounts of water in the western basin, if future land purchases allow expansion of the western basin.  The dike would be 12 feet wide across the top, have 3:1 slopes on the sides, and cost $3/ yd3 construction (Dave Rohlf, NCRS, pers. comm.).  This structure would cost approximately $226,800.  One of two types of water control structures could be used in this dike.  One would be a three bay concrete stop-log structure, and the other would be a set of 10 Wisconsin tubes.  Both would cost in the neighborhood of $30,000.  Thus, the total cost of the Ventura Marsh dike would be around $256,800.

 

            In addition to building a dike between the eastern and western basins of Ventura Marsh, it is suggested that wind- and wave-breaking islands be installed in the eastern basin of the Marsh.  These islands would enhance waterfowl production, expand hunting opportunities, limit wind-resuspension of sediments and encourage the growth of aquatic macrophytes.  Islands formed out of Geotubes placed side-by-side would be around 40 feet wide and 500 feet long.  Each tube, and the interior of each island (if they were deployed such that there was an interior area) could be filled with material dredged from the Little Lake.  The tubes for each island would hold 3,300 yd3 of dredged material.  The storage capacity of the interior would vary for each island, so it is not estimated here.  The cost of island construction was based on dredge spoil island construction at Grass Lake, Illinois.  Each island would cost approximately $85,000 for the Geotube materials and filling.  There would be an additional charge of approximately $20,000 for mobilization and deployment by the construction firm.  Vegetating the island with a common wetland species such as Reed canarygrass would cost approximately $100/island.  If thirteen such islands were constructed in Ventura Marsh (Fig. 5), the total cost of this restoration activity would be $1,126,300.

 

            Improving the fish barrier and water control structure between Ventura Marsh and Clear Lake, as well as widening the grade, is another facet of Ventura Marsh restoration.  Expanding the width of the grade to 2 or 3 times its present width would be beneficial in many respects, as discussed earlier (Fig. 6).  Expanding the grade width by 25 yards, and having a face with 3:1 slopes would require around 26,550 yd3 of rock fill.  At a cost of $3/ yd3 for construction (Dave Rohlf, NCRS, pers. comm.), this structure would cost approximately $79,650.  It is possible that material dredged from the Little Lake could be used for some of this fill.  The cost of planting trees and grasses on this structure is estimated at $1,000 (Coletti 1996).  The cost of constructing a goose-proof fence on the west side of the expanded grade is not included in this cost estimate. 

 

To control water levels in the marsh, a pumping station could be installed that would move water from the marsh to the Little Lake.  IDNR personnel estimate a pump capable of removing 1/10th foot of water from the marsh in 24 hours would be adequate.  This pump would the need to be able to pump 5,660 gallons per minute.  For this exercise, the cost of a pump and pumphouse is estimated at $50,000 (Guy Zenner and Doug Jahnke, IDNR, pers. comm.).  In addition to the pump, an overflow structure is necessary.  The overflow structure could be a three-tiered, two “pond” fish trap that would replace the existing structure in the grade.  For this estimate, three, 4-bay stop-log structures would be placed in series between the marsh and the lake.  The areas between the structures could be used as fish traps if so desired.  The cost of one 4-bay stop-log structure was estimated to be $75,000, so the cost for this structure is estimated to be $250,000.  The total cost for the new and expanded grade could be $380,850.  The total cost for all structural work on Ventura Marsh then would be $1,763,950.

 

            The cost of further sampling and analysis of sediments for heavy metals and pesticides was estimated on current per-sample and field rates of the ISU Limnology Laboratory and the University of Iowa Hygienic Laboratory (Table 6).  We recommend that thirty core-type sediment samples be taken from the proposed dredge area, and that they be located throughout to represent the entire dredge area.

 

The cost of dredging was based upon the cost for a recently completed dredging project, that of Upper and Lower Pine Lakes in Hardin County.  The cost of dredging at Pine Lakes was $1.53/yd3.  Lake Ahquabi was also recently dredged, with a dredging cost of $2.05/yd3.  The table below indicates the estimate of cost for dredging for each of the projected dredge options at Clear Lake.  It should be noted that these costs are based upon smaller, easier dredging projects.  In particular, at Clear Lake, the main spoil containment site is located 2.5 miles from the center of the dredging site.  This will probably require the installation of a booster pump, which may significantly increase the cost of the dredging operation.

 

Dredge Volume (yd3)

Cost 1

($/ yd3)

Cost 2 ($/ yd3)

Total Cost 1 ($)

Total Cost 2 ($)

2,312,450

1.53

2.05

3,538,050

4,740,500

 

            The costs of spoil site preparation, land purchase, engineering costs and site preparations for the spoil site originally identified were outlined in a previous section.  The cost for the land purchase and site preparation would be $1,344,600, while permitting and engineering costs might be $201,700, for a total cost of $1,546,300.  If it is determined that spoil containment should occur at a combination of the additional sites, the combination of Sites 3 and 5 seems to be the most economical and would allow enough freeboard above the stored dredge spoil for an adequate margin of safety.  The total cost for the combination of these two sites would be $2,093,600.  This combination is nearly $550,000 more expensive than the originally identified containment site.  Additional expenses would be incurred by having to run pipe to two containment sites instead of one.  This would add over $1,000,000 to the total cost of the project.  The total cost for dredging, assuming the $2.05/ yd3 cost, would be $4,740,500.  The total cost for dredging and spoil containment, if the originally identified spoil site is used, would be $6,286,800.

 

            The cost of adding an aerator similar to those presently installed was assumed to be similar to the cost of a recent installation of an aerator at Rice Lake (Jim Wahl, IDNR, pers. comm.).  That cost was $50,000.  The annual cost of operation for the aerators presently at Clear Lake is approximately $700 per year (Jim Wahl, IDNR, pers. comm.).  The cost of a hypolimnetic for the Little Lake would be on the order of $125,000 and would have several important benefits.

 

The cost of benthic fish population management can involve many different aspects.  One aspect that is not estimated here would be the cost of stocking flathead catfish in the lake.  Flathead catfish are not presently cultured by IDNR, so the cost of stocking them depends upon purchase price from commercial fishermen, personnel costs for capturing and transporting them from other locations, or starting a culturing operation.  Another aspect would be the cost of benthic fish removal.  The number of fish removed from the lake could be based on many things, but the most scientifically valid is probably standing stock.  Standing stock estimates conducted by the IDNR showed that carp biomass ranged from 110 to 240 lbs/acre during 1999 and 2000.  If standing stock estimates were continued in the future, the IDNR could target a pre-determined poundage of carp to be removed and budget for that total.  For example, if the standing stock was 100 lbs/acre, a 50% or 50 lbs/acre removal could be requested.  Fifty lbs/acre would equal about 180,000 pounds.  If IDNR paid 10 cents/lb, then $18,000 would need to be budgeted for carp removal.

 

The cost of building breakwater structures is based upon the average bid price for a similar project conducted on the Okoboji lakes in 1996 (Ken Jackson, IDNR, pers. comm.).  The structures estimated here would be built in a T or L shape out from shore, have a 10 ft wide top and be situated parallel to shore approximately 300 feet out and in 6 feet of water.  The length of the breakwater parallel to the shore will vary by structure, but for this estimate, a standard 300 ft length was used.  These structures would rise 2 ft above water level at normal pool, which is assumed to be spillway elevation.  Each structure described above would require 4,500 tons of rock.  If the cost of acquiring and placing the rock were $45/ton, each structure would cost $202,500.  Sixteen such structures are suggested, with 10 located near the City of Clear Lake, two at McIntosh Woods, and one each at the City of Ventura, the Baptist Camp, Farmer’s Beach and Lekwa Marsh.  Additional structures could be placed near storm drains from the residential area on the south side of the lake.  The cost of installing these breakwater structures could be $3,240,000.

 

            The cost of the post restoration monitoring study was estimated on current per-sample and field rates of the ISU Limnology Laboratory and the University of Iowa Hygienic Laboratory.

 

References:

 

Bailey, R. M. and H. H. Harrison.  1945.  The fishes of Clear Lake, Iowa.  Iowa State College Journal of Science 20:57-77.

 

Canfield, D. E. and R. W. Bachmann. 1981. Prediction of total phosphorus concentrations, chlorophyll-a, and Secchi depths in natural and artificial lakes. Can. J. Fish. Aquat. Sci. 38: 414-423.

 

Coletti, J.  1996.  Progress report to Leopold Center for Sustainable Agriculture.  Agroecology Issue Team, Iowa State University, Ames.

 

Cooke, G.D., E.B. Welch, S.A. Peterson and P.R. Newroth.  1993.  Restoration and management of lakes and reservoirs.  Lewis, Boca Raton.

 

Dolan, T.J., S.E. Bayley, J. Zoltek and A.J. Hermann.  1981. Phosphorus dynamics of a Florida marsh receiving treated wastewater.  J. Applied Ecology 18: 205-220.

 

Downing, J. A., J. Kopaska, and D. Bonneau.  2000. Rock Creek Lake restoration, diagnostic/ feasibility study.  Iowa Department of Natural Resources, final report.

 

Dierberg, F.E. and P.L. Brezonik.  1983. Nitrogen and phosphorus mass balances in natural and sewage-enriched cypress domes.  J. Applied Ecology 20: 323-337.

 

Dillon, P. J., and F. H. Rigler, 1974. A test of a simple nutrient budget model predicting the phosphorus concentration in lake water. J. Fish. Res. Board Can. 31: 1771-1778.

 

Kirchner, W B. and P J. Dillon, 1975. An empirical method of estimating the retention of phosphorus in lakes. Water Resources Research. 11: 182-183.

 

Larsen D. P. and H. T. Mercier. 1976. Phosphorus retention capacity of lakes. J. Fish.Res. Board Can. 33: 1742-1750.

 

National Climatic Data Center.  2001. On Line Climate Data.  http://www.ncdc.noaa.gov/ol/climate/climatedata.html

Nurnberg, Gertrud K. 1984. The prediction of internal phosphorus load in lakes with anoxic hypolimnia.  Limnol. Oceanogr., 29 (1) 111-124.

 

Organisation for Economic Cooperation and Development (OECD) 1982.  Eutrophication of waters: monitoring, assessment and control, Paris. 154p.

 

Reckhow, K. H., 1977. Phosphorus models for lake management. Ph.D. dissertation, Harvard University, Cambridge, Massachusetts. Catalog No. 7731778, University Microfilms International, Ann Arbor, Michigan.

 

Reckhow, K. H., 1979. Uncertainty applied to Vollenweider's phosphorus criterion. J. Water Poll. Cont. Fed. 51: 2123-2128.

 

Reckhow, K. H., and M. N. Beaulac, and J.T Simpson, 1980. Modeling phosphorus loading in lake response under uncertainty: A manual and compilation of export coefficients.  U.S. Environ. Prot. Agency. EPA-440/5-80-011.

 

Reckhow, K. H., and S. C. Chapra, 1983. Engineering Approaches for Lake Management - Volume 1: Data Analysis and Empirical Modeling, 340p.

 

Reckhow, K. H., and J. T. Simpson, 1980. A procedure using modeling and error analysis for the prediction of lake phosphorus concentration from land use information. Can. J. Fish. Aquat. Sci. 37: 1439-1448.

 

United States Department of Agriculture.  1978.  Agricultural Handbook 537.  Washington, D.C.

 

Vierbicher Associates, Inc.  2000.  Clear Lake Storm Water Management Plan.  162 pp.

 

Walker, W.W. Jr., 1985.  Empirical methods for predicting eutrophication in impoundments.  Report No. 3.  Phase II:  Model refinements.  USCOE waterways experiment station technical report No. E-81-9.  Vicksburg, Mississippi.  300p.

 

Walker, W.W. Jr., 1987.  PONDNET - Flow & Phosphorus Routing in Pond Networks.  Software package available at http://www.nalms.org/.

 

Young, R. A., C. A. Onstad, D. D. Bosch, and W. P. Anderson.  1994.  AGricultural Non-Point Source pollution model, version 4.03, user’s guide.  United States Department of Agriculture-Agricultural Research Service, North Central Soil Conservation Research Laboratory, Morris, Minnesota.

 


Table 1.  Calculated sediment and nutrient inputs to Clear Lake following a two-inch rain event.  The table shows the results for all model iterations and the percent reductions from present conditions for each iteration.

 

Total Phosphorus

Total Nitrogen

Sediment

 

Metric (kg)

Imperial (lbs)

Metric (kg)

Imperial (lbs)

Metric (kg)

Imperial (tons)

Present Conditions

2042

4502

7248

15979

261274

288

 

 

 

 

 

 

 

1% Land Idling

2008

4426

3702

8162

246758

272

  % change

-2%

-2%

-49%

-49%

-6%

-6%

 

 

 

 

 

 

 

5% Land Idling

1829

4033

3407

7512

219542

242

  % change

-10%

-10%

-53%

-53%

-16%

-16%

 

 

 

 

 

 

 

10% Land Idling

1667

3675

3121

6880

199584

220

  % change

-18%

-18%

-57%

-57%

-24%

-24%

 

 

 

 

 

 

 

5% Land Idling and Wetlands

873

1925

N/A

N/A

N/A

N/A

  % change

-57%

-57%

N/A

N/A

N/A

N/A

 

 

 

 

 

 

 

10% Land Idling and Wetlands

834

1839

N/A

N/A

N/A

N/A

  % change

-59%

-59%

N/A

N/A

N/A

N/A

 

 


TABLE 2.  Results from the Wisconsin Lake Modeling Suite 3.1.1.

Phosphorus Prediction and Uncertainty Analysis Module

Wisconsin Lake Modeling Suite 3.1.1 (1999)

Panuska, J. and J. Kreider. Wisconsin Department of Natural Resources. Madison, Wisc.

Observed spring overturn total phosphorus (SPO): 185.0 mg/m^3

Observed growing season mean phosphorus (GSM): 166.0 mg/m^3

% Confidence Range: 70%

 

           Lake Phosphorus Model              Low   Most Likely   High     Predicted  % Dif.

                                            Total P   Total P    Total P   -Observed        

                                            (mg/m^3) (mg/m^3)   (mg/m^3)   (mg/m^3)         

 Walker, 1987 Reservoir                        106      166        199        -19       -10

 Canfield-Bachmann, 1981 Natural Lake          140      189        213          4         2

 Canfield-Bachmann, 1981 Artificial Lake        82      100        108        -85       -46

 Reckhow, 1979 General                           48       75         90        -91       -55

 Reckhow, 1977 Anoxic                           421      658        789        492       296

 Reckhow, 1977 water load<50m/year              121      189        227         23        14

 Walker, 1977 General                          296      463        555        278       150

 Vollenweider, 1982 Combined OECD              145      209        243         34        19

 Dillon-Rigler-Kirchner                        173      271        325         86        46

 Vollenweider, 1982 Shallow Lake/Res.          133      197        231         22        13

 Larsen-Mercier, 1976                          253      395        474        210       114

 Nurnberg, 1984 Oxic                           149      233        279         67        40

 

         Lake Phosphorus Model          Confidence Confidence  Parameter    Back       Model  

                                           Lower      Upper      Fit?    Calculation   Type   

                                           Bound      Bound               (kg/year)           

 Walker, 1987 Reservoir                      107        229      Tw Pin         0       GSM

 Canfield-Bachmann, 1981 Natural Lake         59        544         FIT         1       SPO

 Canfield-Bachmann, 1981 Artificial Lake      31        288         FIT         1       SPO

 Reckhow, 1979 General                         46        108         FIT         0       GSM

 Reckhow, 1977 Anoxic                         434        891       P Pin         0       GSM

 Reckhow, 1977 water load<50m/year            119        268       P Pin         0       GSM

 Walker, 1977 General                        253        741         Pin         0       SPO

 Vollenweider, 1982 Combined OECD            110        352         FIT         0       ANN

 Dillon-Rigler-Kirchner                      177        370        P qs         0       SPO

 Vollenweider, 1982 Shallow Lake/Res.        105        326         FIT         0       ANN

 Larsen-Mercier, 1976                        268        521       P Pin         0       SPO

 Nurnberg, 1984 Oxic                         134        358           P         0       ANN


 

TABLE 3:  Estimated sizes and phosphorus reduction capacity (from PONDNET) for nutrient retention wetlands in the Clear Lake watershed.

Wetland

(number from Fig. 3)

Wetland/water body area (ac)

Volume (ac/ft)

Potential P Reduction

Flows to …

1

65.5

48.6

33

2

2

12.3

13.7

30

Ventura Marsh

3

67.5

342

62

4

4

17.2

32.7

49

14

5

24.3

78.9

50

4

6

3.4

5

39

Clear Lake

7

7.6

29.2

29

Clear Lake

8

18.6

55.3

29

Clear Lake

9

4

7.3

43

Clear Lake

10

11.7

22.2

44

Clear Lake

11

50.6

354.7

73

Clear Lake

12

22.8

41.8

56

Clear Lake

13

5.6

3.6

41

Ventura Marsh

14

12.3

16.4

54

15

15

10.8

15.5

37

Little Lake

Ventura Marsh

187.4

614.6

50

Little Lake

Little Lake

319.8

3381

64

Clear Lake

 


TABLE 4.  Suggested activities for lake restoration.

Citizens, Residents & Visitors

Cities and Municipalities

Regional Governments (i.e., Counties)

Agricultural Communities

·          Support riparian and watershed regulations and guidelines

·          Reduce nutrient input through “good housekeeping” and awareness that everything that hits the ground goes to the lake

·          Support watershed cleanliness

·          Inspect your plumbing and real estate for potential sources of nutrients

·          Be aware of and use low P products (washing products, fertilizers, chemicals)

·          Use old fashioned good sense: ask yourself “would this help the lake if it got into it?”

 

 

·          Comprehensive riparian management and development plans keep sediments and nutrients in their place

o              Increase set-backs and buffers

o              Decrease shore impacts and erosion

o              Maintain high levels of cleanliness of pavement and lands

o              Minimize impermeable structures

o              Decrease construction-related impacts

o              Route effluents and nutrients out of the watershed and treat them to reduce harm elsewhere

·          Inspection, repair and/or upgrades of storm-drain, sanitary and water management systems can be of great help

·          Installation of bacteria-trapping storm drain filters reduces bacterial tracers

·          “Abandoning” septic and waste disposal systems reduces unwanted leakage

·          Promotion of riparian and nutrient management regulations improves nutrient, sediment and bacteria fluxes

 

·          Adopt comprehensive riparian management and development plans

o              Increased set-backs and buffers

o              Decreased shore impacts and erosion (tributaries, wetlands & shorelands)

o              Maintenance of high levels of watershed cleanliness

o              Decreased construction-related impacts (erosion and dust control)

o              Routing effluents and nutrients out of the watershed and / or treating them to reduce harm

·          Inspection and repair of tile-line, storm-drain, sanitary and water management systems helps reduce unnecessary losses

·          Employment of frequent street and road cleaning reduces sediments and nutrients

·          Adoption of policies concerning storm- and land- drainage (favoring water-retention)

·           “Abandoning” old septic and waste disposal systems

·          Inspection and promotion of the upgrading of septic systems to efficient designs that immobilize nutrients

·          Reduction or elimination of tile-line or surface disposal of black water

·          Promotion of responsible riparian and nutrient management

·          Promotion of moderate nutrient use in agricultural lands and seeking ways of helping producers keep nutrients out of waterways

·          Control dust from construction and unpaved roads as it is part of the airshed

 

 

·          Use approaches to decrease agricultural nutrient losses to waters

·          Use BMPs to decrease erosion and increase nutrient efficiency

·          Reduce P application to decrease field-losses and help groundwater

·          Minimize livestock contact with waterways to help decrease P delivery

·          Adopt voluntary land management practices to ease nutrient delivery to streams and lakes

 

 

 

 


Table 5.  Additional spoil containment site options. 

Site

1

2

3

4

5

6

Distance from dredging area

4.2 mi

3.5 mi

3.4 mi

2.6 mi

2.5 mi

1.9 mi

Volume (yd3)

1,312,800

2,525,600

2,706,200

1,447,400

1,383,100

788,500

Dike Length (yd)

1260

1640

950

1310

650

700

Mean Dike Height (ft)

10

10

11

8

12

10

Max. Dike Height (ft)

20

15

15

10

15

25

Dike Cost

$92,400

$120,270

$80,120

$69,870

$62,400

$53,340

Water Control Structure Cost

$3,000

$3,000

$3,000

$3,000

$3,000

$3,000

Land Area (ac)

320

640

440

320

160

320

Land Cost

$704,000

$1,408,000

$968,000

$704,000

$704,000

$352,000

Permitting and Engineering Costs

$119,990

$229,690

$157,670

$116,530

$115,410

$61,250

Total Cost

$919,390

$1,760,960

$1,208,790

$893,400

$884,810

$469,590

 

TABLE 6.  Preliminary estimate of the total cost of lake restoration.

Restoration Alternative

Cost (with land under

permanent easements)

Land idling (if no NRCS funding)

$1,238,100

Wetland restoration and construction

$971,050

Ventura Marsh restoration

$1,763,950

Sediment sampling and analyses

$26,000

Lake dredging

$6,286,800

Hypolimnetic aerator installation

$125,000

Breakwater structure construction

$3,240,000

Contracted fish removal (annual cost)

$18,000

Post restoration monitoring

     $472,275

Subtotal

$14,141,175

10% increase for unknown costs

   $1,414,118

TOTAL COST

$15,555,293

 

TABLE 7.  Estimated costs of land purchased, site preparation, vegetative plantings, and water control structure for nutrient retention wetlands in the Clear Lake watershed.

Wetland

(number from Fig. 3)

Land Cost

Dike Cost

Planting Costs

(upland buffer strips)

Water Control Structure Cost

Total Cost

1

$176,526

$9,186

$1,446

$3,000

$190,158

2

$47,683

$4,724

$928

$2,400

$55,736

3

$61,300

$0

$1,064

$600

$62,964

4

$58,354

$4,593

$223

$1,200

$64,370

5

$14,932

$2,362

$329

$1,200

$18,823

6

$16,900

$615

$201

$1,800

$19,516

7

$38,845

$4,265

$530

$2,400

$46,040

8

$38,015

$6,603

$622

$1,800

$47,039

9

$0

$0

$0

$10,000

$10,000

10

$124,075

$0

$556

$10,000

$134,631

11

$16,655

$3,150

$351

$600

$20,756

12

$25,727

$0

$0

$600

$26,327

13

$171,583

$4,183

$1,045

$600

$177,411

14

$28,717

$3,875

$544

$1,200

$34,337

15

$45,369

$14,348

$199

$3,000

$62,916


 

 

 

FIGURE 1. Present conditions modeling results from AGNPS model for the Clear Lake watershed.


FIGURE 2. Top 1%, 5%, and 10% of phosphorus exporting cropland cells determined by AGNPS model.


FIGURE 3. Proposed wetland restoration and construction in the Clear Lake watershed.

 

FIGURE 4.  Schematic diagram of nutrient retention wetlands and their nutrient retention rates.


 

FIGURE 5.  Aerial view of Ventura Marsh at the present time, and a visual representation of it in the future.


FIGURE 6.  Aerial view of Ventura Grade at the present time, and a visual representation of it in the future.

 

FIGURE 7.  Wind rose for Clear Lake.  Wind data from April 1998 – August 1999, Mason City Airport, source National Climatic Data Center, (http://www.ncdc.noaa.gov/ol/climate/climatedata.html).

 

FIGURE 8. Proposed area of dredging and dredging depths in the Little Lake.

 


FIGURE 9. Containment site for dredge spoil.

 


Figure 10.  Additional potential dredge spoil containment sites.

 


FIGURE 11. Bathymetric map of the Clear Lake after dredging.

 

Figure 12.  Potential locations of breakwater structures in Clear Lake.

 

FIGURE 13. Post restoration water quality monitoring sites.