Lake Mendota is located approximately 120 km west from Milwaukee, a city on the western shore of Lake Michigan. Although the lake is comparatively small (surface area 40 km2) and shallow (mean depth 12.2 m), it is significant as a birthplace of modern limnology and an early instance of artificially accelerated eutrophication.

The lake has a freezing period of about three and a half months annually, and the concentration of dissolved oxygen in the water is generally high in the epiliminion throughout the year, though it falls to 1-2 mg/l in the bottom water during the summer.

Owing to the influx of domestic wastewater, the lake has suffered from eutrophication since the beginning of this century. From 1912 to 1958, copper sulphate was applied over the lake to reduce the overgrowth of algae. The algal bloom was effectively suppressed but the lake became polluted with copper, which still remains accumulated in the bottom sediments as insoluble copper carbonate.

The diversion project for wastewater is particularly noteworthy in the lake's water treatment plan. When the main lake became heavily eutrophicated, inflowing wastewater was diverted to the three small lakes downstream, but these also became eutrophicated over time. Therefore, the wastewater flowing into the four lakes was collectively diverted to the Yahara River to cope with the situation. The history of wastewater treatment in L. Mendota and the nearby city of Madison tells of a bitter struggle for an effective control of eutrophication (Q).