If the amount flowing in to the impoundment is more than is released, the impoundment level rises. Most dams that are built for water supply or electricity production vary the amount of water released to the river below the dam. Thus, these impoundments only store water, rather than regulate the flow of water. Most of the rivers in Wisconsin receive their water from some combination of natural lakes, rainfall, overland flow, and groundwater. Many rivers in Wisconsin begin as springs , where groundwater rises to the land surface.
Rivers also receive groundwater flow directly into the bed of the river.
In many parts of Wisconsin the water table is above the level of rivers and lakes. Because groundwater flows from high to low just as surface water does, groundwater flows into most rivers. The groundwater that flows into a stream is called the stream s baseflow. Discharge is the amount of water a river is carrying. Hydrologists measure discharge in units of volume per time, usually in cubic feet per second.