Executive Summary – Diagnostic & Feasibility Study

John A. Downing, Jeff Kopaska et al.

Iowa State University

 

 

The purpose of this 2-year project was to study multiple aspects of Clear Lake, its watershed and community to determine water-quality related problems and their likely causes, and suggest a list of potential remedial measures.  The project was funded primarily by the Iowa Department of Natural Resources (IDNR), the City of Clear Lake, Cerro Gordo County, Hancock County and the Hancock and Cerro Gordo County offices of the Natural Resources Conservation Services (NRCS).  Iowa State University (ISU) also provided substantial cost-share.  The first part of this project, the diagnostic study, is a description of the principal findings of the analysis of the lake, its watershed, and the social landscape.

 

The study was performed by a large team of scientists and students with the help of dozens of citizen volunteers.  The work could not have been done without the generous contribution of time and effort by hundreds of willing citizens and the volunteer work of several ISU professors.  Over the course of the project, hundreds of thousands of meticulous measurements and analyses were made to study the lake, the watershed and the importance of Clear Lake to the regional social structure.

 

Lake and Watershed Characteristics 

Clear Lake is third largest of 34 natural, glacial lakes in Iowa, and is managed for water-based recreation and fishing.  It is shallow, with a maximum depth of 5.9 meters (19 ft) and an average depth of 2.9 meters (9.6 ft).  Water is supplied to the lake by small tributaries, rainfall, groundwater, and many areas of direct surface runoff.  Forty-seven percent of the water supply flows in from a large wetland complex (Ventura Marsh).  The lake’s watershed to lake-area ratio is only 2.3:1 and the watershed is composed of 59% cropland, 10% urban areas, 9% wetlands, 8% grasslands, 5% wooded lands, 5% roadways, 2% farmsteads, 1% pasture and 1% State Parks.

 

Clear Lake has a long history as a focal point for recreation in Iowa, and is currently intensively used.  Use is 44% camping, picnicking and other passive uses, 28% pleasure boating, 19% swimming, 7% fishing, 2% winter activities, and 0.2% hunting.  Total use of the two State parks on the lake (Clear Lake State Park, McIntosh Woods State Park) totals more than 660,000 person-days per year and is growing substantially.  In addition to the State Parks, the cities of Clear Lake and Ventura maintain recreational facilities on the lake.  There are 24 public access points on the lake and 15 of these have public docks.  Clear Lake is currently managed by the Iowa Department of Natural Resources for recreation and gamefish production.

 

Clear Lake is intensively used both by residents and by visitors from across Iowa and the region.  Clear Lake is located in Cerro Gordo and Hancock Counties, which have combined populations of around 60,000.  Much of the population of Cerro Gordo County is located quite near the lake (Mason City: population=29,000), and this population has a higher than average income for the State.  Agriculture and related industries are also important sources of income in this region (82%-91% of total land area in these counties).  Economic activity associated with the lake is intense.  Data on hotel/motel tax receipts indicate that Clear Lake has enjoyed an annual tourism impact of $30-$40 million annually for several years.  Estimates of willingness to pay for lake water quality maintenance or improvement suggest that citizens value the lake at between $20 and $40 million over a 5-year period, respectively.  Much of the valuation of the lake is expressed by local residents, although even visitors expressed a willingness to personally support water quality maintenance and improvement with substantial financial contributions.  Hunting revenues are quite small, but play an important social role.

 

Valuing Preservation and Improvements of Water Quality

One important indicator of social importance of a resource such as Clear Lake is the willingness that citizens have to pay for maintenance or improvement of the resource.  This is distinct from the amount that they pay to use a resource (an amount that is very large indeed, and approaches the GNP of the region associated with the resource), and indicates the unique economic value of the resource.  As part of this study, economists performed surveys to provide information on recreational usage of the lake, attitudes of recreators and local residents toward possible watershed management changes, as well as estimates of visitors’ and residents’ willingness to pay for water quality improvements.  Clear Lake is very important as a recreational resource, with visitors reporting high, persistent usage of the lake (an average of over 6 trips per annum). Both visitors and residents report a high willingness to pay to avoid further deterioration of the lake, about $100 for visitors and about $550 for residents. When asked about their willingness to pay for improvement from the current conditions, respondents indicate that they are willing to pay only moderate amounts for a low quality improvement, but substantial amounts for a more significant quality improvement, $215 and $600, respectively for visitors and residents.

 

Attitudes and Perceptions Regarding Water Quality and Community

Because substantial improvements to lake water quality often require social change to take place, the beliefs, attitudes and values of residents are an important part of planned landscape change in communities.  A critical goal of this study was to propose feasible restoration alternatives toward the improvement of water quality in Clear Lake.  Therefore, part of this study summarized interviews with residents reflecting a broad range of experiences and connections with the lake and community, while balancing income, gender, education, occupation and years lived in the area.  Interviews were conducted individually using photographs as discussion points.  Analyses focused on (1) residents’ relationship to the lake, (2) organizational and social aspects of the community, (3) perceptions of water quality, and (4) perceived community needs as they relate to water quality. 

 

Clear Lake is the focal point of the community and the region.  Visual changes are watched in detail, including water clarity, water level, fish populations, etc. 

The lake has a strong personal importance to the community but is also important as part of the community’s link with the external world through tourism.  Although residents expressed concern about Iowa’s water quality in general, specific views about Clear Lake are optimistic, although there is concern for the cost of remediation.  Residents expressed interest in additional lake-centered facilities including a bike trail around the lake, more public docks, additional boat trailer and vehicle parking, and small pocket parks on the lakeshore.  The community is extremely well educated about water quality and communicates very well within itself and without.  There is a large degree of tolerance for the visual aspects of water-quality enhancing structures (e.g., wetlands, filter strips).  Considerations for future community action indirectly related to water quality emerged from this part of the study: community sense of place, interpretation of local history and perceptions of public/private ownership.

 

Historical Changes in the Waterscape

Dr. Ken Carlander, a well-known emeritus scientist at Iowa State University and a long-time analyst of the Clear Lake ecosystem, began to chronicle changes in the Clear Lake shoreline in the early 1950s by establishing a photographic record.  One part of this study was to repeat his photographs from the same vantage points to see how Clear Lake has changed.  Comparisons show striking changes in the extent and density of emergent and submergent rooted plants in Clear Lake.  Reduced water clarity has resulted in a reduction in the extent and biodiversity of rooted plants.  These plants are important as fish and wildlife habitat as well as the stabilization of bottom sediments and shorelines.

 

Limnology and Ecology of Clear Lake

Clear Lake is a formerly oligotrophic to mesotrophic lake that has increased in total phosphorus concentration from around 60 ppb in the early 1970s to around 190 ppb in 2000.  Total phosphorus appears to be increasing in Clear Lake at an average rate of about 4 ppb/year.  At this rate, Clear Lake would move from a eutrophic/hyper-eutrophic lake to a hyper-eutrophic lake, attaining 340 ppb by 2040.  Total phosphorus has therefore already tripled in the last 30 years and appears to be climbing under current watershed management scenarios.  Concurrently, water clarity has been cut to nearly a third of what it was in the early 1970s, and probably around 10% of the clarity the lake had near the turn of the century.  Water clarity in 1974 was about 0.9 meters (nearly 3 ft.), but is now around 0.35 meters (about 1 ft.).

 

Clear Lake is typical of large, shallow, corn-belt kettle lakes.  Clear Lake receives a very elevated rate of supply of nutrients (most notably phosphorus) from its watershed, rainfall and groundwater, resulting in a volume-weighted average spring phosphorus concentration of 186 ppb.  Much of the watershed is at the western end of the lake, thus nutrient supply and concentrations are higher in the west end than the east end of the lake.  The very shallow depth (maximum 5.9 m or 19 ft) means that wind mixing returns nutrients from the sediments into the water column during the warm, summer season.  This large input of nutrients from the watershed and the remobilization of sediment nutrients gives Clear Lake a very high concentration of nutrients such as nitrogen and phosphorus.  The mixed agricultural and urban watershed furnishes very high nutrient loads to the lake, some of which has been deposited into the sediment layers.  These high nutrient inputs, coupled with the fish-, boat- and wind-induced mixing of sediments, are significant impediments to water quality, since they have now turned Clear Lake into a eutrophic to hyper-eutrophic lake.  The impact of these nutrient loads is exacerbated by a greater-than-average concentration of suspended silt that likely arises due to wind resuspending watershed-derived silt, carp and other benthic fish digging in sediment deposits, and power-boat wakes disturbing sediments in shallow waters.  Further exacerbating water quality problems is a declining population and biodiversity of rooted water plants which formerly held bottom sediments and protected shores and shallow waters from wave erosion.

 

Very high nutrient concentrations in Clear Lake have fueled over-abundant growth of algae, resulting in green water and frequent algae blooms.  Phytoplankton in Clear Lake follow a seasonal pattern that is typical of temperate, shallow, hypereutrophic lakes.  Algal biomass is generally highest in mid-summer when it forms conspicuous “blooms” of algae coloring water an intense green color.  Cyanobacteria (“bluegreen algae”) and diatoms make up the majority of the algae.  Cyanobacteria usually dominate the algae and make up >80% of the algae in mid- to late-summer.  As is frequently the case for eutrophic lakes, the types of Cyanobacteria present in blooms include some of the groups that can produce toxins under certain conditions.  These groups compose an average of 35% of the algae.  Because such toxins could become harmful to invertebrates, fish, wildlife, livestock, and humans, reduction of nutrient levels to eliminate Cyanobacterial dominance would be welcome.

 

Declines in water quality have reduced fish and wildlife habitat substantially in Clear Lake.  The number of species of aquatic plant species found in Clear Lake has declined from 35 species in 1952 to 21 species in 1981 to 12 species in 1999.  More than 80% of the species currently present in Clear Lake have declined significantly since 1981.  Aquatic plants cover about half the area of the lake that they covered in 1981.  Rooted aquatic plants are important wildlife and fish habitat and stabilize bottom sediments and shore zones.

 

Bacteria were studied intensively in Clear Lake and were usually found at low levels, especially in the open waters away from shore.  Concentrations of fecal coliforms, E. coli, and fecal enterococci were highest near shore and showed patterns that should allow remedial measures to trace bacteria sources and eliminate or reduce these inputs.  Bacteria were found to enter the lake around much of the shore.  Since much of the shore is in residential development, significant amounts of bacteria likely result from urban activities.  In spite of this, parkland and agricultural lands also appear to contribute substantial bacterial inputs.  Concentrations of bacteria were found to be highest during the warmth of mid-summer, especially following rainfall events.

 

Sedimentation has resulted in substantial changes in the bottom of Clear Lake.  During the first 10,000 years of its life, we calculate that the lake filled-in about 38% of its original basin.  Almost ¼ of this volume was filled-in since 1935.  Agricultural, urban and construction activities around the basin have reduced the average depth of the lake by one foot since 1935.  Around 85,000 tons of sediment are added to the lake each year causing the lake to lose depth at a rate of about 4 mm/year.  Assuming a constant rate of sediment addition to the lake, Clear Lake would be completely filled-in in 700-800 years.  Normally, however, these processes usually accelerate as lakes become shallower, so this life-time may be over-estimated.

 

Internal nutrient loading via the resuspension of benthic sediments by wind-induced waves and recreational boat traffic is a common problem facing the managers of shallow lakes like Clear Lake.  Benthic sediment resuspension may contribute to the suppression of fish and macrophyte communities, domination of the phytoplankton community by potentially toxic cyanobacteria, suspension of toxic ammonia and increased restoration time-scales.  In Clear Lake, resuspension by wind-induced waves and recreational boat traffic may contribute to daily, often substantial, nutrient flux with total phosphorus concentrations increasing by over 100% and ammonia concentrations reaching levels toxic to fish.  When wind speeds exceed 10 m·s-1 (22 mph), a large proportion of the lake’s sediments may become mobile.  Sediments in Clear Lake are most susceptible to turbulence by wind waves and boats in the lake’s shallow western basin and around the lake’s margins.  Here, it is likely that violations of the lake’s no wake zones may exacerbate wind-induced resuspension and may slow the resettlement of resuspended sediments.  Additionally, the frequently observed sediment plumes passing from the western basin into the larger basins to the east suggest that prevailing currents may transport large loads of sediments and nutrients throughout the lake.  Unless measures are taken to suppress the impacts of wind and boats on the lake’s sediments, we may expect problems associated with sediment resuspension, including increased restoration time-scales, may become more severe as the lake’s depth continues to decline, exposing more sediments to turbulence.

 

Increased phosphorus concentrations in Clear Lake have resulted in decreases in many aspects of the quality of the Clear Lake ecosystem.  Judging from trends in water clarity, Clear Lake was likely oligotrophic-mesotrophic at the turn of the century, mesotrophic until the mid 1970s, then moving from eutrophic in the mid-1970s to near hyper-eutrophic in the late 1990s.  Phosphorus concentrations of the magnitude seen in Clear Lake during this study are very poor for continued quality of recreational use.  If trends continue in this vein, users of Clear Lake should expect further degradation of water clarity, reduced oxygen levels, frequent blooms of toxic algae, increased survival and persistence of fecal and potentially pathogenic bacteria, accelerated filling and siltation, mobile toxins, increased impacts of ammonia on the quality of fish and other aquatic organisms, continued declines in biodiversity and year-to-year stability, degraded fish and wildlife habitat, decreased fish production and a fish community more highly dominated by rough fish.

 

The increase in total phosphorus concentration in the lake has yielded a profound increase in algal abundance.  The dense algae that have bloomed in Clear Lake have decreased water clarity to the point that rooted aquatic vegetation has declined substantially.  Turbid waters with toxic algae favor the growth of resistant fishes like carp and bullhead that perturb sediments and uproot vegetation.  Sediment resuspended by fish and increased wind mixing in the absence of rooted vegetation further decreases water clarity, further reducing the ability of aquatic plants to cleanse waters and stabilize sediments.  Resuspended sediments lead to increased phosphorus concentrations that have favored even more algae growth.  Projected increases in phosphorus concentrations indicate that, in the absence of remedial measures, Clear Lake will continue to decline in quality and utility as a recreational resource.

 

In order to improve the limnological aspects of Clear Lake, three fundamental changes would need to take place:

  • Reductions in phosphorus loading to the lake.
  • Reductions in silt input and resuspension by fish, wind and boat action.
  • Reductions in inputs of bacteria from the watershed surrounding the lake.

Such changes would give rise to gradual improvements in the lake, the course of which is likely to span 5-30 years before substantial improvements would be achieved.

 

Knowledge of the hydraulic and nutrient budgets as well as various limnological details allow computation of future water quality under various scenarios of improved watershed characteristics.  One can thus calculate the expected change in water quality (i.e., phosphorus concentration) by calculating the impact of a reduction in phosphorus input.  We examined the fit of more than a dozen such models and found that current phosphorus concentration at spring circulation could be predicted within 2% of the actual phosphorus concentration.  This model is thus likely to predict the phosphorus concentration under future remedial states.  Application of these equations indicates that it would take around a 60% reduction in total phosphorus inputs to bring the lake back to the total phosphorus concentrations that were seen in the late 1970s and early 1980s.  This analysis suggests that a 60% reduction in total phosphorus loading to Clear Lake should bring water clarity to the 0.8-1.2 m. level, once lake conditions equilibrate.  This water clarity level is somewhat conservative because increased water clarity and carp management taken together would greatly reduce suspended solids in the water column, affording even greater increases in water clarity.  It is likely, therefore, that such a management scenario could bring water clarity in Clear Lake back to pre-1970 levels, allowing marked increases in the entire lake as an ecosystem and recreational resource.

 

Groundwater Hydrology 

A potentially important part of the nutrient budget of Clear Lake is groundwater inflow and outflow.  An understanding of the geology and hydrogeology of the Clear Lake region is thus needed to understand lake-groundwater interactions.  The following objectives were investigated:

 

·        ·        determine the thickness of Quaternary units underlying the lake and overlying the regional bedrock aquifer;

·        ·        estimate hydraulic heads in the regional aquifer and their relationship to the lake elevation and shallow groundwater flow;

·        ·        determine the nature and types geologic units affecting flow to and from the lake.

 

Estimation of groundwater discharge (or seepage) to lakes is necessary to determine nutrient load, but it is difficult task and generally involves the extrapolation of small-scale measurements to a much larger lake area.  In cases where the geology beneath the lake is not well known and where discharge may vary, large errors are involved in the measurement and extrapolation steps.

 

A geochemical investigation of groundwater was undertaken in order to understand the presence and absence of nutrients and contaminants in groundwater and their potential to enter Clear Lake.  Groundwater samples from the 32 out of 33 piezometers were analyzed for total P, total N, Si, alkalinity, electrical conductivity and pH. Additional parameters (major cations and anions, trace elements, dissolved O2, dissolved organic carbon) were measured in order to understand the geochemical environment in which the nutrients occur.  Geochemical speciation models and soil P measurements were used to determine potential sources of P.  Selected samples were analyzed for fecal coliform bacteria and caffeine, in order to determine potential sources of nutrients and Cl.  A radioactive isotope of hydrogen, tritium (3H), was used to determine the relative age of the groundwater.  Nutrient and contaminant loads from groundwater to Clear Lake were calculated from estimates of groundwater inflow and outflow and estimates of the concentrations of nutrients (primarily P, N and Si) and Cl in groundwater.  Nutrient load per time was calculated by multiplying discharge (L3/T) times concentration (M/L3).  Because of Clear Lake’s nature as a flow-through lake, nutrients will be added to the lake in areas of inflow and lost from the lake in areas of outflow.

 

Ventura Marsh Biology, Ecology and Biomanipulation

Early in the study, we found that Ventura Marsh (a large wetland that processes 49% of the water budget) was not removing nutrients from the water but was a significant source of nutrients.  Experience in other shallow water bodies in Iowa and elsewhere indicated that this was due to impact of non-native fish (carp) on the sediments and vegetation, creating a nutrient supply rather than a sink.  We therefore evaluated the effects of a benthivorous fish reduction.  After a substantial fish removal was obtained, water clarity increased as a result of decreased suspended sediment and phytoplankton biomass. Water column total phosphorus declined by about 25% from 147 ppb to 115 ppb.   Prior to the clear water phase, phytoplankton was phosphorus limited.  Zooplankton grazing reduced phytoplankton biomass during the clear-water phase.  The biomass of Daphnia and Ceriodaphnia increased following fish removal.  During this period, grazing pressure was high and standing phytoplankton biomass remained low.  Phytoplankton appeared to be regulated by top-down control for approximately two months before reverting back to bottom-up control.  Changes in water quality due to wind and/or return of juvenile carp may account for the switch back to bottom up control.  Macrophyte diversity and density increased after the initiation of the clear water phase.  We therefore concluded that restoration of Ventura Marsh and carp control could be one potentially viable remedial measure for decreasing nutrient flux into Clear Lake.

 

Watershed Loads, Tributaries and Nutrient Budgets 

Various tributaries to Clear Lake were sampled to identify areas contributing greatest nutrient loads.  A total of 37 sampling stations were established across the watershed.  Inputs of various elements were calculated multiplying concentrations by the water (hydraulic) loading rate at each point.  The study spanned one very wet year and one very dry year.  The amount of precipitation has a large impact on the overall nutrient loading rate as well as the distribution of the nutrient loads among the many potential sources.

 

Much of the watershed lies in the agricultural region to the west of the lake, thus much of the phosphorus entering the lake comes from agricultural lands.  The average phosphorus budget for the lake indicates that 43% derives from the agricultural watershed, 7% from groundwater, 6% from the City of Clear Lake, 2% from the City of Ventura, and 2% from unconsolidated county urban lands.  An average of 31% of the phosphorus budget derives from direct rainfall on the lake, since Iowa’s rainfall phosphorus has been enriched 10-fold with airborne phosphorus over the past 30 years.  The large amount of phosphorus deriving from direct rainfall was somewhat surprising and ironically makes remediation more difficult because of the small watershed to lake area ratio.  Another surprise is the high phosphorus concentration of groundwater.  Groundwater concentrations were quite high, suggesting that phosphorus has moved down through the soil profile enriching the groundwater.  Another surprising result was that 9% of the lake’s overall phosphorus budget derives from internal loading from Ventura Marsh.  Ventura Marsh provides much of the water flowing into Clear Lake and concentrations of major nutrients in this water are very high (average 350-400 ppb of phosphorus).  Because of carp activities and poor aquatic plant development in the marsh, however, somewhat more phosphorus leaves the marsh than enters, indicating that the marsh is a net source of sediments and does not cleanse water as large wetlands usually do.  Nutrient loading to the lake should generally be much higher in wet years than dry ones, and the fraction of the nutrient budget derived from rain and groundwater declines substantially under wet conditions.

 

Although the predominance of agricultural lands in the watershed makes them a major overall nutrient source, nutrient losses per unit land area indicate areas where nutrient losses are most severe.  In general, phosphorus losses were somewhat higher (per unit area) from urban lands than agricultural lands.  This indicates that substantial reductions in phosphorus input could be achieved by both urban and agricultural communities.  Sediment losses from lands varied markedly, indicating broad differences in land use management.  Sediments tend to cause water quality impairment on their own, but also carry large amounts of phosphorus with them.  Not surprisingly, agricultural areas supply the largest amounts of nitrogen per unit area, likely owing to the prevalent use of pure N fertilizer in Iowa agriculture.  Nitrogen is not a large problem for Clear Lake, since phosphorus is generally the production-limiting element.

 

Because of the importance of agriculture in the watershed, extensive analyses of soil phosphorus and management practices were performed.  Eighty-nine percent of the agricultural watershed was planted in corn and soybean rotation, while 7% was planted in continuous corn and the remaining land was under CRP, alfalfa or pasture.  Forty-eight percent of the land was managed with chisel plowing in combination with disking and/or field cultivation.  Twenty-one percent of the remaining land was in no-till, 18% in ridge-till, 7% moldboard plow and 6% was V-ripped.  Forty-four percent of the fertilizer application is incorporated by fall plowing, disking or injecting, which is the environmentally preferred method.  On average, P fertilizer was applied to fields 2.2 times over the last 5 years.  P fertilizer was typically applied at 65 lb P2O5/acre which is a rate that is lower than the recommendation of Iowa State University.  Eleven percent of the farmland had received manure in the previous five years.  Manured fields in the watershed were very similar in P concentration to those receiving high and/or frequent applications of inorganic fertilizer.

 

Soil phosphorus was tested using 3 commonly used agronomic soil P tests and two environmental P tests.  Although these methods measured different amounts of P, most yielded similar overall results concerning identifying high-testing areas.  The survey of soil P status and P management practices of the Clear Lake agricultural watershed was useful to identify areas that may be sources of large P loads to the lake and to identify priority areas where changes in P management practices would be desirable.  Approximately one third of the area of the watershed had soil-test P values that were twice to five times higher than levels needed to maximize crop production.  Soil test summaries since the 1960s for the two counties surrounding the lake and our survey data from the lake watershed shows that P management practices have markedly increased soil P tests over time.  The highest soil-test P values were found in a very small number of fields that received either P fertilizer or manure, which likely are the source of the major proportion of the P being transported to the lake.

 

Analysis of Clear Lake Fisheries

The fisheries of Clear Lake are an important component of recreation as well as an important indicator of water quality.  Because of their importance, fish have been studied in Clear Lake since the early 1940s.  The most striking change in the fishery has been the near disappearance of the sunfish family (bluegill, crappie & largemouth bass).  These fish are still present but are only occasionally caught.  The loss of these important fish is probably due to water-quality mediated declines in aquatic vegetation which are necessary as spawning and nursery cover.  Although bullhead and carp have been common in the lake for 50 years, they are now the dominant fish, existing at densities of 150-300 lb/acre and 100-200 lb/acre, respectively.  They have probably filled the void left by the bass, crappie and bluegill because of their great tolerance of degraded water quality conditions (e.g., sediment, Cyanobacteria, low oxygen, ammonia).  Since resistant fish like carp degrade water quality, successful improvements will need to include management of fish populations to reduce carp dominance.  This can be accomplished by (1) improving water quality to enhance vegetation and decrease substances degrading fish habitat, and (2) managing bottom-feeding fish (primarily carp and bullhead) to reduce their abundance. 

 

Point Sources and Potential Pollutants

Part of the diagnostic study examined the point-source input of materials to Clear Lake and the potential for toxic substances to be concentrated in lake sediments.  The latter was done in case sediment dredging should be employed as a restoration option.  There are currently no permitted point-source dischargers of effluents into Clear Lake.  On three occasions over the past five years, however, past exigencies have resulted in the discharge of some pre-treatment sewage effluent into the lake.  The largest of these was the discharge of 250,000 gallons of diluted pre-treatment sewage into the lake on June 20, 1998.  Total P in this sewage was probably about 1.5 mg/L, meaning that sewage bypasses such as this, although certainly to be avoided, only would supply about 0.02% of the lake’s phosphorus budget.  This is surprising, but the sheer magnitude of watershed inputs and rainfall are of massive proportions.  Sediments contain some materials of concern, notably cadmium, chromium, copper, lead and zinc.  Potential sources of these elements are batteries not properly disposed of, leaching from chrome-plated metals, trace sources in agricultural fertilizers, building materials, leaded fuels, lead shot, fishing weights, and leaching from plated steel.

 

Summary of Diagnostic Study

 

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 of the Diagnostic Report) 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.

 

Feasibility of Lake Restoration

 

            Clear Lake is an excellent lake of outstanding potential for recreation and is of very great economic importance.  Improvement of water quality could benefit from the activities of many at all levels, including citizens and municipal, county, state and federal government agencies.  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 measures suggested are:

  • reduction in phosphorus inputs to the lake,
  • reduction in bacteria inputs to the lake,
  • improved 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 can be reduced by 50-60% by implementing practices designed to address these issues.  This will lead to a substantial increase in water clarity and improved biological function.

 

Improving watershed management and increasing the depth of certain parts of the lake by dredging are two important aspects of this lake restoration plan.  These management approaches compliment each other; dredging helps to restore the basin by increasing water depths and 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. 

 

Watershed management activities identified that would benefit Clear Lake include: 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.  The total estimated cost for these activities is $15,555,300.  Respondents to the Clear Lake survey indicated a willingness to pay of $19.5 million to avoid the deterioration of Clear Lake.  Alternatively, respondents are willing to pay about $40 million for quality improvements at the lake.  These numbers represent the value to visitors and residents of water quality improvements.  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 substitutes and the potential quality of this unique resource.

 

If no 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 declines dramatically.  Long before that time, however, severe water quality problems will be encountered.  The combination of watershed improvements and lake dredging will enhance the recreational value of the lake and greatly prolong its useful life.  Watershed restoration and lake deepening will act to reduce nutrient inputs and dilute their effect in the lake, however, the lake is still 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.  Researchers 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.