A watershed is an area of land that water flows across as it moves toward a common body of water, such as a stream, river, lake or coast. Green Lake is part of the Big Green Lake Watershed, which is primarily located in Green Lake County and the remaining 40% extending into Fond du Lac County. A portion of the watershed also edges along the southwest corner of Winnebago County. Green Lake’s outlet is the Puchyan River, which drains into the Fox River and ultimately into Lake Michigan at Green Bay. The Big Green Lake Watershed also includes Spring Lake, Little Twin Lake and Big Twin Lake.
Eight tributaries flow into Big Green Lake, including Silver Creek, the largest stream that drains 52% of the watershed’s area, and the Southwest Inlet, the confluence of Roy, Wuerches and Spring Creeks that drains 17% of the watershed. While vastly different in size, these two areas contribute nearly identical sediment loading to Green Lake (30% each) and the majority of phosphorus loading (45% from Silver Creek and 28% from the Southwest Inlet).
The remaining tributaries include: Dakin Creek, Hill Creek, White Creek, and Assembly Creek.
In 2014, the Department of Natural Resources (DNR) classified Green Lake as an impaired waterbody because it fails to meet water quality standards due to low dissolved oxygen at certain depths, the likely but unproven culprit being excessive phosphorus loading. Dissolved oxygen in water bodies is essential for the survival of organisms important in lake ecosystems, from small zooplankton to large trophy fish.
For Green Lake, the obvious signs of water quality decline are increased weed and algae growth. These visible nuisances are symptoms of much larger watershed-wide cause: Excessive phosphorus loading.
One pound of phosphorus can fuel the growth of 500 pounds of algae. Any attempts to increase water clarity or to reduce weeds and algae in the lake will require watershed-wide improvements on the landscape that keep phosphorus where it falls, on the land and out of the lake.
Phosphorus ends up in waterways from many sources. It is a naturally-occurring element found in leaves, for example, which is why urban street cleaning operations and proper leaf management is important. It is found in human and pet waste, from dogs to ducks to cattle. It is applied through chemical or manure applications on crops. Since sediment particles have a natural electric charge, phosphorus readily binds to them, which means that eroded fields, stream banks and construction sites are visible signs of invisible phosphorus loading.
On average, a single drop of water remains in Green Lake for 21 years before flowing out the Puchyan River. With such a long retention time, sediment, phosphorus and other nutrients can exist or accumulate within Green Lake for long periods of time, making water quality improvements a slow and steady process. With such a long retention time, sediment, phosphorus and other nutrients can exist or accumulate within Green Lake for long periods of time, making water quality improvements a slow and steady, but also critical, process.