Lake Mendota

About Lake Mendota

Lake Mendota is the largest and deepest lake in the Yahara chain. The land surrounding the lake is primarily agricultural and is experiencing 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 Lake Mendota 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.

Lake Mendota by the numbers

Lake Mendota 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: 72,092 acres
  • Surface area: 9,842 acres
  • Shoreline: 22 miles
  • Maximum depth: 82 feet 
  • Mean depth: 42 feet
  • Flushing rate: 4.4 years

Water clarity

  • 4.9 feet or “good” according to DNR criteria for deep lakes (2017)

Beaches

  • Open 96% of the time and closed 4% of the time (2017)
  • 7 closures days due to E.coli bacteria (2017)
  • 24 closures due to blue-green algae (2017)
After an excellent 2016 beach season, Lake Mendota’s beaches lost 31 out of a total 792 possible beach days in 2017 (99 days multiplied by 8 beaches), which is a return to the long-term median. Closures were primarily due to blue-green algae; Tenney Beach and the Memorial Union Pier lost the most days.

Maple Bluff Beach on Lake Mendota

Phosphorus levels

  • 0.036 mg/L or “fair” according to DNR criteria for deep lakes (2017)
For an update on phosphorus reduction projects, see the State of the Lakes Annual Report.

What’s new on Lake Mendota?

The explosion of zebra mussels is the big news for Lake Mendota, which has the potential to drastically change the lake and its shoreline. According to scientists at the UW Center for Limnology, we can likely expect more green, filamentous algae to grow in thick mats along the shoreline (read “Zebra mussel invasion in full swing in Lake Mendota”; “Zebra mussels transforming depths of Lake Mendota”)

Other big news was the 2017 blue-green algal bloom that colored Lake Mendota from Picnic Point to the Yahara River and into Lake Monona on Father’s Day weekend. According to the Center for Limnology, the warm, calm conditions and phosphorus levels created the perfect conditions for the bloom and resulting fish kill (read “Madison in Bloom: Blue-green algae hits Lake Mendota.”)

With these changes in lake ecology, managing runoff will be more important than ever to mitigate the impact of zebra mussels and blue-green algae.

Cyanobacteria Lake Mendota

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 Friends of the Yahara River Headwaters Madison Water Utility Public Health Madison & Dane County UW-Madison Center for Limnology Village of DeForest Yahara Pride Farms Yahara Watershed Improvement Network