2024 State of the Lakes

2024 key takeaways

• A return to wetter weather and increased runoff contributed to “fair” rankings for phosphorus levels and water clarity in most of the Yahara lakes. Lake Wingra, with its “good” status rankings, was the lone exception.
• LakeForecast monitors observed the highest number of nearshore cyanobacteria (blue-green algae) blooms compared to any other time in the program’s 10-year history, particularly on Lake Kegonsa.
• Despite 2024 seeing a temporary drop in most of the lakes, a history of rising chloride concentrations is an ongoing water quality concern, with the highest levels consistently measured in Lake Wingra.
• Per- and polyfluoroalkyl substances (PFAS) represent the latest contaminants of concern, with lakes Monona, Waubesa, and Kegonsa listed as impaired and under fish-consumption advisories for these “forever chemicals.”
• Continued progress is needed toward getting 100% of agricultural acres covered by nutrient management plans, achieving the wider adoption of land conservation practices, and developing more manure management facilities.

Lake Mendota

Lake Type: Drainage
Direct Drainage Area: 217 sq. miles
Total Drainage Area: 232.4 sq. miles
Surface Area: 9,847 acres
Shoreline Length: 22 miles
Mean Depth: 42 feet
Maximum Depth: 83 feet
Volume: 133,407 million gallons
Flushing Rate: 22% of volume/year

Lake Mendota sits at the top of the chain and is the largest of the five Yahara lakes by surface area, depth, and volume. Its direct drainage area consists predominantly of agricultural land uses. Inlet tributaries include Pheasant Branch Creek (west shore); and Sixmile Creek, Dorn Creek, Token Creek, and the Yahara River (north shore). The lake’s outlet (southeast shore) directs overflow water through the Yahara River and into Lake Monona. It has been listed as impaired under the Clean Water Act for phosphorus since 2011.

Lake Mendota Health Dashboard

Lake Monona

Lake Type: Drainage
Direct Drainage Area: 40.5 sq. miles
Total Drainage Area: 278 sq. miles
Surface Area: 3,277 acres
Shoreline Length: 13 miles
Mean Depth: 27 feet
Maximum Depth: 74 feet
Volume: 29,059 million gallons
Flushing Rate: 91% of volume/year

Lake Monona is the second largest of the five Yahara lakes by surface area, depth, and volume. It sits immediately downstream of Lake Mendota in the upper half of the chain. The lake’s direct drainage area consists mostly of urban land uses. Inlet tributaries flowing into the lake include Murphy’s (Wingra) Creek (west shore); the Yahara River (north shore); and Starkweather Creek (northeast shore). The lake’s outlet (south shore) directs overflow water through the Yahara River and into Upper Mud Lake and Lake Waubesa. It has been listed as impaired under the Clean Water Act for PCBs since 1998, total phosphorus since 2011, and PFAS since 2022.

Lake Monona Health Dashboard

Lake Wingra

Lake Type: Drainage
Direct Drainage Area: 5.4 sq. miles
Total Drainage Area: 5.4 sq. miles
Surface Area: 321 acres
Shoreline Length: 3.7 miles
Mean Depth: 9 feet
Maximum Depth: 14 feet
Volume: 1,585 million gallons
Flushing Rate: 77% of volume/year

Lake Wingra is the smallest of the five major lakes by surface area, depth, and volume. Originally a deep-water marsh, this dredged waterbody now flows into Lake Monona via Murphy’s (Wingra) Creek. The lake’s direct drainage area is located entirely within the city of Madison and dominated by urban land uses. Three minor, unnamed inlet tributaries flow into the lake at points along its west and southwest shore. It has been listed as impaired under the Clean Water Act for total phosphorus since 2011 and PCBs since 2012.

Lake Wingra Health Dashboard

Lake Waubesa

Lake Type: Drainage
Direct Drainage Area: 43.6 sq. miles
Total Drainage Area: 325 sq. miles
Surface Area: 2,083 acres
Shoreline Length: 9.4 miles
Mean Depth: 15 feet
Maximum Depth: 38 feet
Volume: 10,567 million gallons
Flushing Rate: 320% of volume/year

Lake Waubesa is the fourth largest of the Yahara lakes by surface area and volume. It sits immediately downstream of Upper Mud Lake and Lake Monona in the lower half of the chain. The lake’s direct drainage area is represented by a mix of urban and rural/agricultural land uses. Inlet tributaries that drain into the lake include Nine Springs Creek and Penitto Creek (flowing into Upper Mud Lake to the north); the Yahara River (north shore); as well as Swan Creek and Murphy’s Creek (southwest shore). The lake’s outlet (east shore) directs overflow water through the Yahara River and into Lake Kegonsa. It has been listed as impaired under the Clean Water Act for phosphorus since 2011 and PFAS since 2022.

Lake Waubesa Health Dashboard

Lake Kegonsa

Lake Type: Drainage
Direct Drainage Area: 54.4 sq. miles
Total Drainage Area: 384.6 sq. miles
Surface Area: 3,210 acres
Shoreline Length: 9.6 miles
Mean Depth: 17 feet
Maximum Depth: 32 feet
Volume: 17,700 million gallons
Flushing Rate: 220% of volume/year

Lake Kegonsa is the third largest of the Yahara lakes by surface area and volume. It sits immediately downstream of Lake Waubesa and Lower Mud Lake in the lower half of the chain. The lake’s direct drainage area consists predominantly of rural/agricultural land uses. Inlet tributaries to the lake include the Yahara River and Door Creek (north shore) and two unnamed creeks (southwest and northeast shore). The lake’s outlet (east shore) directs overflow water through the Yahara River toward the Rock and Mississippi Rivers. It has been listed as impaired under the Clean Water Act for total phosphorus since 2011 and PFAS since 2022.

Watershed health indicators

The State of the Lakes assesses five areas of interest that represent vital, interconnected pieces of the larger water quality puzzle. Illustrated in Figure 2, they include both outputs (i.e., land-use actions taken) and outcomes (i.e., measured water quality responses). Each was chosen to illustrate critical cause-and-effect principles that play out as water interacts with and gets funneled through the watershed. 

The availability and movement of phosphorus are central themes given its dominant role in affecting overall lake conditions. Although a natural element essential for plant and animal growth, it can easily harm water quality due to excess supply and poor management. Sources of phosphorus can include eroded soil, fertilizer runoff, leaf debris in city streets, livestock manure, sewage releases, and improperly disposed pet waste. A common rule of thumb is that it takes only one pound of phosphorus to generate up to 500 pounds of algae growth.

1. Weather and climate drivers

Regardless of whether it is a change in average temperature, precipitation, ice cover, or some other factor, lakes dynamically respond to their environment. Regional heating and cooling patterns influence what types of aquatic organisms can thrive, how and when the water column mixes, and the timing and magnitude of annual freeze cycles. Meanwhile, the timing, intensity, and amount of rainfall over the watershed determine what can get moved from the land surface into our waters, including phosphorus-containing materials that fuel algal growth and turn the lakes green.    

Ice Cover

Winter ice conditions and the timing of ice-off influence everything from a lake’s thermal properties to the reproductive success of its aquatic life. Ice quality and overlying snow depth affect how much sunlight can penetrate to warm the water column and facilitate dissolved oxygen production through photosynthesis. Research by Dr. Zachary Feiner at UW-Madison’s Center for Limnology documents the impacts of early ice-off. As ice-off dates move earlier due to climate change, algae production gets an earlier start and then collapses right when certain types of zooplankton hatch and are looking for food. That means fewer zooplankton to graze on algae or feed juvenile fish. Resulting impacts can range from poorer walleye recruitment to more intense algal blooms. 

During the winter of 2023-24, Lake Mendota remained frozen for only 44 days, resulting in 54 fewer days of ice cover compared to the preceding year. This was the second shortest duration of ice cover for Lake Mendota based on the last 169 years of Wisconsin State Climatology Office recordkeeping. Historical evidence shows ice-cover durations in decline, with Lake Mendota having lost about a month of ice cover since recordkeeping began. 

Shorter and warmer winters lead to warmer lake temperatures earlier in the year, effectively creating a longer growing season for algae and aquatic plants. Warmer winters can also contribute to greater runoff and phosphorus delivery when wet precipitation falls on frozen soils. Rather than soak into the ground, rainfall more easily turns into runoff that can carry land-spread manure and other contaminants into the ditches and streams that drain to the lakes. It is estimated that, on average, nearly half of the total phosphorus loading through Lake Mendota’s monitored stream tributaries occurs from January to March, making late winter and early spring a vulnerable time for our lakes. 

Precipitation

A total of 46.45 inches of precipitation fell across the watershed during the 2024 water year (Oct. 1 – Sep. 30). This is a 20% increase over the 37.13-inch historic normal. As shown in Figure 3 below, May through September’s collective rainfall totaled 30.81 inches, compared to the normal 21.48 inches during this period over the last 30 years. The heavier precipitation led to higher lake levels, reversing what was experienced during the recent drought years that kept water levels low. 

May, June, and July each registered several days with rainfall greater than one inch. July experienced significant runoff events, with July 2nd dropping 2.49 inches of rain and July 13th-14th recording 4.69 inches in less than 48 hours. Large, “gully washing” rain events like these were more frequent, contributing to increased soil erosion and flooding throughout the watershed. Wetter years like 2024 also speak to the impacts that runoff has on phosphorus loading, water clarity response, and algal bloom frequency. More stormwater moving across the landscape means more opportunity for pollutants to wash into our lakes. Because of an unusually warm winter followed by high spring rainfall totals, the lakes received much higher phosphorus inputs compared to the recent drought years.

2. Watershed phosphorus mass balance

Calculating the difference between the mass of phosphorus entering (imported into) and leaving (exported from) the watershed tells us whether the net balance is trending in the right direction. The movement and fate of livestock, feed, fertilizer, harvested crops, animal waste, and other phosphorus sources are factored into the analysis. The goal is to attain a negative balance, indicating more phosphorus is being exported than imported on an annual basis. This situation reduces the overall availability of phosphorus from being able to reach area waterways. 

Conversely, a positive balance signals an annual net accumulation of phosphorus in the watershed, usually leading to its gradual buildup in area soils. Phosphorus-saturated soils subject to erosion from disturbance or a lack of protective, year-round plant cover can eventually end up at the bottom of nearby lakes and streams. Phosphorus is also more easily “leached” (or released in dissolved form) from these soils when they encounter rainwater and snowmelt. 

Past improvements to the overall mass balance are attributed to multiple factors. They include decreases in imported commercial fertilizer, less phosphorus-containing feed supplements consumed by livestock, exported byproducts of manure digestion, and advanced phosphorus-management strategies implemented by Madison Metropolitan Sewerage District (among others). Examples of mass balance detractors include increases in livestock numbers and milk production that lead to more manure. 

3. Land conservation practices

The implementation of land conservation practices is critical in our efforts to improve water quality. Practices such as perennial grasslands, vegetative buffers, cover crops, and rotational grazing are prime examples of solutions that reduce the amount of soil and nutrient runoff reaching nearby surface waters. Another critical conservation practice aiding in reducing these losses is the implementation of nutrient management plans.

Nutrient management plans are a tool that allows farmers to evaluate different cropping, tillage, and nutrient applications (i.e., fertilizer and manure) to optimize crop production while minimizing runoff risks. Records filed with Dane County Land & Water Resources in 2024 indicate that 44,830 out of 84,321 total agricultural acres within the Yahara lakes watershed (53%) were mapped as having a nutrient management plan (Figure 4). This was roughly the same amount reported in 2023. Since Dane County does not receive plans from every farm on an annual basis, the reported percentage of agricultural land having a nutrient management plan is likely to be considerably higher but still short of the goal.

One of the key environmental metrics within nutrient management plans is the Rotational Average Phosphorus Index (PI), representing estimated pounds of phosphorus loss per acre per year. The higher the PI value the greater the risk for phosphorus to reach surface waters. State standards require all agricultural fields to have a PI of 6 or less. However, based on modeling summarized in RENEW THE BLUE: A Community Guide for Cleaner Lakes & Beaches in the Yahara Watershed (2022), a Rotational Average PI of 2.1 or less is needed to reach phosphorus loading targets and water quality goals. Based on information from nutrient management plans submitted in 2024, the average PI for fields within the Yahara lakes watershed was 2.4.

4. Phosphorus delivery to the lakes

Most phosphorus is delivered to the Yahara chain of lakes through tributary streams that collect and funnel upland-generated runoff as it moves downhill. How much is transported depends on the seasonal timing and intensity of runoff events, the location and availability of major phosphorus sources, and measures taken to contain those sources and manage runoff. Over the past 12 years, 43% of the total phosphorus entering Lake Mendota through its monitored tributary streams occurred during the three months of January, February, and March.

Stream monitoring helps evaluate the effectiveness of conservation practices by tracking changes in phosphorus loading. Loading describes the total mass of phosphorus entering a water body over a specific period of time. In our case, we characterize loading in pounds of phosphorus (calculated by multiplying in-stream concentrations by streamflow) delivered to Lake Mendota through its monitored stream tributaries in a given water year. 

Perched at the top of the chain and receiving most of the drainage from the Yahara lakes watershed, the condition of Lake Mendota offers a good indicator for how the downstream lakes will be impacted. Lake Mendota is also the largest lake with the greatest number of monitored streams and the most complete long-term dataset. Most of the phosphorus received by the lower lakes in the chain is through the outlets of the upper lakes as it cascades through the system.

Figure 5 shows the change in stream-monitored phosphorus loading since 2013. Total precipitation is also plotted in orange to distinguish between wet and dry years. From 2021-2023, average annual phosphorus loading to Lake Mendota significantly declined and even managed to dip slightly below target levels. This was largely due to the three-year period of drier weather that followed years of above-average precipitation, reducing the amount of runoff able to transport phosphorus to the lakes. Scientists estimate a doubling of summer days when the lakes are clear and free of algal blooms if these lower levels can be maintained. Unfortunately, a wetter-than-normal 2024 helped erase many of those earlier gains, highlighting the value of land practices that reduce runoff and the sources of excess phosphorus.

5. In-lake water quality responses

Several in-lake metrics are used to assess overall lake health and track changes over time. Those metrics include water clarity, phosphorus concentration, presence of cyanobacteria (blue-green algae) blooms, and beach closures. Each is summarized in the following sections. Four of the five lakes dropped to a “fair” ranking for phosphorus and clarity in 2024. Lake Wingra, which ranked “good” on both measures, was the lone exception.

Phosphorus

After falling precipitously during the recent drought years of 2021-2023, in-lake phosphorus concentrations jumped in all the lakes except for Lake Wingra (Figure 6). No obvious trends are evident in any of the lakes when looking at longer time horizons. When averaged across individual decades beginning in 1980, lakes Monona and Waubesa show very slight downward trends while those for lakes Mendota and Kegonsa are not evident. This speaks to the reality that more work is needed to reduce phosphorus loading, the main culprit that is restricting sustained water quality improvements.

After Lake Mendota’s phosphorus concentrations following fall turnover hit record lows in 2022 and 2023, a much wetter 2024 saw these very positive, low-runoff effects start to disappear (Figure 7). Turnover occurs when deeper lakes cool to the point where the water column can completely mix. Higher phosphorus concentrations that have built up throughout the summer in the lake’s bottom waters are then mixed throughout the lake. When less phosphorus enters the lake from the watershed, the bottom-water buildup of phosphorus is reduced, thereby reducing internal (in-lake) sources that can fuel algal growth the following year.

Fall-turnover phosphorus concentrations were especially low in 1988, 2012, and 2023 following those extended drought periods. The findings confirm that Lake Mendota’s phosphorus levels decline when watershed sources are reduced, resulting in significant and relatively quick water quality improvements.

Water Clarity

Offshore water clarity measured in all five lakes dropped in 2024, giving up the earlier gains made from the low-runoff years of 2021-2023 (Figure 8). When looking at longer time horizons, no clear trends are readily discernable. When averaged across individual decades beginning in 1980, lakes Mendota and Waubesa show very slight downward trends while those for lakes Monona and Kegonsa are not apparent.

Because water clarity often varies across a given lake, Clean Lakes Alliance uses a network of trained monitors to collect additional data through its LakeForecast program. From Memorial Day through Labor Day, monitors submit at least twice-weekly reports on water clarity, water temperature, and the severity of observed green algae and cyanobacteria blooms (among other variables). Collected information can be seen in real-time on LakeForecast.org, allowing the public to stay up to date on current lake conditions. These reports are also used to raise awareness about changing water quality conditions, advocate for improvement projects, and to prepare the annual State of the Lakes.

To measure nearshore clarity, monitors use a sampling device called a turbidity tube to report conditions at 81 sites around all five lakes (Figure 9). Figure 10 on the following page shows the median summer (Jul-Aug) clarity readings for each lake from 2014-2024. Values under 50 are considered “murky,” between 50-80 “fair,” and between 80-120 “good.” Values for 2024 fell within the fair to good range. Lake Waubesa was reported as having the highest nearshore clarity while Lake Kegonsa had conditions bordering on murky. Comparing nearshore to offshore findings highlights how clarity can vary over both time and space within individual lakes, and why both assessments are provided in this report.

Cyanobacteria Blooms

To determine cyanobacteria bloom frequency, the number of days on each lake with at least one report of a strong cyanobacteria bloom observed within the monitoring sites was counted. By dividing the number of “cyanobacteria bloom days” by the total number of sampling days for each lake, a percentage is generated representing how often the monitors observed at least one major bloom within their nearshore sampling area. This method lessens overreporting in situations when different monitors report the same cyanobacteria bloom.

Figure 11 shows the percentage of sampling days when strong evidence of a cyanobacteria bloom was observed on each lake (2014-2024). Overall, monitors reported comparatively high cyanobacteria blooms during the 2024 sampling period (Jun-Aug). Monitors on Lake Wingra did not report a single significant bloom, while those on Lake Kegonsa reported a program-record number of sightings. 

Beach Closures

Beach closures are another useful indicator of general lake health. Clean Lakes Alliance looks at closure data provided by Public Health Madison & Dane County for 17 beaches (Figure 12). Covering four of the five Yahara lakes, these tested public beaches were selected for analysis due to the consistency of tracking data over the reporting period. Results are reported as total closure days recorded for each season, roughly running from Memorial Day to Labor Day. For example, if two beaches on a given lake are closed for a total of five days each, 10 closure days would be reported for that lake. 

Closures are most often the result of high cyanobacteria and/or E. coli bacteria levels, with closure rates strongly influenced by timing and frequency of testing. Most beaches are tested once per week and then daily for beaches with a closure in effect. Cyanobacteria blooms, which are generally a product of high lake fertility, can be dangerous due to their potential to release toxins that can harm people, pets, and wildlife. High E. coli bacteria concentrations can also be harmful as they are an indicator of human or animal fecal matter in the water that may contain dangerous pathogens. 

In 2024, there were 133 closure days among the beaches tracked for this analysis, surpassing the long-term median. E. coli was the leading cause of closures (68%) followed by cyanobacteria (32%). Tenney Park Beach on Lake Mendota was not subject to testing due to being closed for a construction project during the 2024 season.

Call to action

“Filthy water cannot be washed.” – West African proverb

Water is life, and Greater Madison is a special place because of it. The water surrounding us has sustained thriving lakeshore communities for thousands of years. Long considered sacred by Indigenous cultures, the lakes reflect the stewardship legacies we choose to pass down from generation to generation.  

As today’s land and water caretakers, we are each called to build upon the best of these legacies and contribute to something larger than ourselves. Clean Lakes Alliance is merely a catalyst, and we recognize that the power for change resides within each of us as individuals. As our alliance grows, so does the impact of each individual action that shapes the world in which we live. This belief is fundamental to why we are here and what kind of community we aspire to become. 

To learn more, be sure to check out “Renew the Blue” and “Top 10 Ways to Help the Lakes at Home” for guidance on how to improve your stewardship credentials.

Stewardship is about making choices you can feel good about, and that are cumulative in impact. Water quality is then the result of many individual decisions occurring across a watershed.

About the State of the Lakes

The annual State of the Lakes is released each year as part of the Greater Madison Lake Guide. In it, we report out to the community on the state of water quality in our lakes. The report also looks at our collective progress toward our phosphorus reduction goal.

The report highlights information from many partners to share the most up-to-date science on water quality in our lakes. We feature local projects, including work in urban areas to protect stormwater quality and progress on farms to keep nutrients on the fields and out of our lakes.

In addition, we provide more information about Clean Lakes Alliance and our efforts to engage the community and advocate for the lakes. This report serves as a reference and a resource, highlighting community progress toward cleaner, healthier lakes for all.

Learn more about our lakes

Learn more about lakes Mendota, Monona, Wingra, Waubesa, and Kegonsa.

Past reports:

• 2023 State of the Lakes
• 2022 State of the Lakes
• 2021 State of the Lakes
• 2020 State of the Lakes Report
• 2019 State of the Lakes Report
• 2018 State of the Lakes Annual Report
  • State of the Lakes – 2018 Summary
• 2018 Progress and Challenges video – shown at the 2019 Community Breakfast “Join the Wave”
  • Message from Clean Lakes Alliance
  • A Watershed Effort (messages from our partners)
  • The Science of Our Lakes in 2018
  • Lake by Lake Water Quality Report Card
  • Plan 2020: A Clear Path Forward
  • Back to the Beach Initiative
  • Financials
• 2017 State of the Lakes Annual Report
  • 2017 State of the Lakes Progress and Challenges video
• 2016 State of the Lakes Annual Report
• 2015 State of the Lakes Annual Report
• 2014 State of the Lakes Annual Report
• 2013 State of the Lakes Annual Report
• 2012 Annual Report / 2012 State of the Lakes Report
• 2011 State of the Lakes Report

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