Water exchange is fairly slow. If streams are the major source of lake water, nutrient levels are often high and water exchange takes place more rapidly. Managing the watershed to control nutrients and soil that enter the lake is essential to protecting water quality. Controlling water that runs from the land's surface into the lake is important for drainage lakes and impoundments, and some seepage and groundwater lakes. Protecting groundwater quality is particularly important for seepage and groundwater drainage lakes.
Watershed management becomes especially critical in impoundment lakes.
If a stream is dammed the natural movement of water will be restricted, causing soil and nutrients to collect in the impoundment. Lake managers measure inflow and outflow to determine a lake's water budget. As shown in the formula below, a water budget consists of several elements. Precipitation in Wisconsin averages 30 inches per year. Evaporation depends on the type of summer weather, but is usually about 21 inches.