What Kind of Lakes are the Yahara Lakes?

The Yahara lakes are classified as eutrophic lakes. Eutrophic lakes have an ample supply of nutrients and usually an abundant crop of weeds and algae. Natural eutrophication is a slow process in which sediment and nutrients enter a lake from runoff, causing an increase in plant life and a gradual filling in of the lake. The time required for this filling or "aging" depends greatly on the surrounding landscape and on the nature of the lake itself. The rate of aging can be speeded up by human input of sewage and polluted runoff from farms and cities. Through this process of "cultural" eutrophication, blue lakes can quickly become green lakes.

The Yahara lakes are certainly victims of cultural eutrophication. Problems with algae growth were first reported in the 1880's, possibly caused in part by sewage discharging into Lake Monona from an expanding urban population. Although Lake Mendota never received large quantities of sewage, Lake Monona, Waubesa and Kegonsa were all heavily affected by the discharge of treated sewage from the Madison area. Most sewage was diverted from Lake Monona in 1936 and from Lakes Waubesa and Kegonsa in 1958. Following these diversions, the lower three lakes improved greatly. In 1971, remaining treatment plant discharges from small communities upstream from Lake Mendota were diverted around the lakes; and in the 1980's, remaining wastewater discharges tributary to any of the Yahara lakes were also diverted, so that none of the lakes now receives any significant impact from sewage or "point" sources of pollution. However, the lakes continue to receive sediment, nutrients and other types of pollutants from runoff from the surrounding farmlands and cities.