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Lake Mendota

Lake Mendota

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.

Maple Bluff Beach on Lake Mendota
Maple Bluff Beach on Lake Mendota

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 Watershed
Lake Mendota Watershed

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
  • Public beaches: 8
  • Maximum depth: 82 feet 
  • Mean depth: 42 feet
  • Flushing rate: 4.4 years

How was the water (2019)?

A reproducing population of zebra mussels was first confirmed in the lake in 2015. Zebra mussels filter out small forms of free-floating algae (phytoplankton), making the water clearer at the center of the lake, but prefer not to eat cyanobacteria, helping this organism proliferate near the shoreline. They are most likely contributing to increases in cyanobacteria blooms, beach closures, and the accumulation of algal mats that wash up on downwind shorelines.

Zebra mussels
Zebra mussels

2019 average (July – August) water clarity ranked “good” and phosphorus concentrations ranked “fair,” according to Wisconsin DNR criteria for deep lakes.

Lake Mendota Total Phosphorus at Fall Turnover 2019

Lake Mendota Total Phosphorus at Fall Turnover (surface measurement, 1975-2019). Fall turnover is when the lake mixes from top to bottom due to temperature changes in the water. This is the time when phosphorus concentrations are most uniform in the lake. Concentrations were low during 2010-2017. Concentrations went up in both 2018 and 2019 due to high phosphorus loading from the lake’s major tributaries.
Lake Mendota 2019 Beach Closures

Lake Mendota beaches were closed 63 times during the summer of 2019, primarily due to cyanobacteria blooms or both cyanobacteria and high E. coli (64%). A total of eight Lake Mendota beaches are monitored.

2019 Community projects & initiatives

  • City of Middleton repaired damage to the Donna Drive Pond, including reinforcing the embankment, improving the outlet system, and enlarging the pond to provide more flood-storage capacity.
  • Friends of Pheasant Branch Conservancy is continuing to work with the City of Middleton and Dane County to repair and restore the Pheasant Branch Creek corridor after it was impacted by the devastating floods of August 20-21, 2018.
  • Dane County provided funding for a new “Continuous Cover Program” to offer small family farms an incentive to convert lands from annual row crops into perennial grasses to improve water quality and reduce flooding.
  • Town of Westport completed a project to control bank erosion along Six Mile Creek, a Lake Mendota tributary stream.
  • Village of DeForest constructed five bio-retention basins at the Conservancy Place Athletic Complex. Wetland restorations continued at the Marvin and Marie Schweer’s Natural Area and along the Upper Yahara River Trail.

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!

Read about the other lakes of the Yahara Watershed

Here’s how Lake Mendota faired in previous years: