Lake Mendota is the largest and deepest lake in the Yahara chain of lakes. The lake is surrounded by mostly agricultural land with areas of rapid urban growth.
The north end of the Yahara Watershed, to the north of Lake Mendota, is known for fertile soils and multi-generational dairy farms. The University of Wisconsin-Madison and the Madison downtown lie along the southern shore of Lake Mendota, although much of the isthmus and downtown area drains into Lake Monona or Lake Wingra.
Major tributaries feeding into Lake Mendota include, from west to east, Pheasant Branch Creek, Dorn Creek, Sixmile Creek, the Yahara River, and Token Creek. Nutrients that flow into the later are carried down the chain of lakes via the Yahara River and represent the largest source of excess nutrients in the lower lakes.
In urban areas of the Mendota Watershed, most of the phosphorus reductions will come from better controls on construction erosion, leaf management, and stabilizing waterway banks to reduce erosion. In rural areas, most phosphorus reduction will come from improved agricultural practices including cropping and nutrient management.
To improve water quality in Lake Mendota, the Yahara CLEAN plan calls for reducing phosphorus runoff from both urban and rural areas. Improvements to reduce phosphorus loading into the lae will help improve water quality in the other three lakes in the chain (Monona, Waubesa, and Kegonsa) by reducing the amount of phosphorus flowing to the lower lakes via the Yahara River.
Lake Mendota by the numbers
The lake is 82 feet at its deepest, known as the “deep hole” between Picnic Point and Maple Bluff. Water in Lake Mendota moves slowly, replenishing every 4.4 years.
- Watershed area: 72,094 acres
- Surface area: 9,842 acres
- Shoreline: 22 miles
- Maximum depth: 82 feet
- Mean depth: 42 feet
- Flushing rate: 4.4 years
How was the water?
In 2018, total phosphorus levels and water clarity were “good” in the middle of the lake, according to DNR criteria. However, the 18 citizen monitor reports of cyanobacteria blooms were above the 4-year summer median of 7.5.
Phosphorus levels (2018 data)
- 0.025 mg/L or “good” according to DNR criteria for deep lakes
Beaches (2018 data)
- Closed 23% of the time and open 77% of the time
- 60 closure days due to cyanobacteria (blue-green algae)
- 7 closure days due to E. coli bacteria
- 11 closure days due to cyanobacteria and E. coli
- 104 closure days due to flooding
In contrast to the conditions in the middle of the lake, a large number of cyanobacteria blooms closed beaches located on the nearshore of the lake. Based on data collected by Public Health Madison & Dane County, Lake Mendota beaches were closed a record number of 78 days from Memorial Day to August 20th, compared to the summer 2010-17 median of 35 days for the whole summer. After August 20th, all the beaches were closed for the remainder of the season (August 21 – Labor Day) due to flood conditions.
What’s new on Lake Mendota?
Lake Mendota summer water quality conditions are now linked to the recent invasion of zebra mussels. The full effect of zebra mussels in the Yahara lakes is still uncertain. Results from other infested lakes suggest increases in water clarity in the middle of the lakes will occur. However, shoreline management problems could get worse from increased aquatic plant growth, and possible increases in filamentous algae and scums of cyanobacteria. Summer water quality in 2018 was also impacted by the unusual amount of flow into and out of the lake resulting from above normal precipitation in the region.
2018 Lake Mendota projects
- Dane County, Yahara WINS, and Clean Lakes Alliance continued support for a pilot project at nine farms testing manure composting. The project by Yahara Pride Farms will reduce pathogens and help farmers manage manure in the winter by composting instead of winter spreading.
- Yahara Pride Farms continued to implement cost-effective, farmer-led practices to protect soil and water quality. YPF also piloted innovative practices like manure composting.
- Dane County
- Worked with farmers to implement practices that divert phosphorus from the lake
- Granted funds to help permanently protect 95.5 wetland acres near the Fishery Area and to help purchase a 100.9 acre farm, with 10.9 acres purchased and easements on 90 acres for the Cherokee Marsh Natural Resource Area
- Permanently protected 11 acres in Token Creek County Park and Natural Resource Area
- Removed 2.7 miles of phosphorus-laden legacy sediment from Dorn Creek as part of the “Suck the Muck” project
- Began construction on a $1.6 million nutrient concentration system at the Middleton digester. The digester will further treat and filter manure utilizing reverse osmosis technology, and reduce the risk of phosphorus runoff
- City of Madison approved $2.36 million to purchase a 31-acre addition to Cherokee Marsh
- Village of DeForest
- Constructed 12 public/private stormwater facilities, maintained a 40-acre wetland in the Marvin & Marie Scheweers Natural Area, and restored wetlands in Reigstand Park
- Maintained a high-quality sedge meadow along the Yahara River Trail
Friend group projects for the watershed
- Friends of Cherokee Marsh maintained 95 acres with a prescribed burn at Yahara Heights Park and continued to collect critical information on stream health as part of the Yahara WINS volunteer monitoring program.
- Friends of Pheasant Branch Conservancy evaluated the feasibility of a carp bubble barrier system in Pheasant Branch Creek through a Clean Lakes Grant.
- The UW-Madison Lakeshore Nature Preserve worked with staff to remove woody invasive plants from 17 acres, sow native seed on 11 acres, and control invasive garlic mustard plants on more than 50 acres of the 300-acre reserve.
Organizations supporting Lake Mendota
The following groups work on water quality issues in the Lake Mendota watershed. Consider contacting an organization near you to get involved!
- City of Madison
- City of Middleton
- Dane County
- Friends of Cherokee Marsh
- Friends of Pheasant Branch
- Madison Water Utility
- Public Health Madison & Dane County
- UW-Madison Center for Limnology
- Village of DeForest
- Yahara Pride Farms
- Yahara Watershed Improvement Network
- Friends of the Yahara River Headwaters