Surface water is considered to be those open bodies of water that are larger that one-tenth of an acre. According to the Illinois Department of Natural Resources, there are four classes of surface water that contribute to the more than 1.6 million acres of surface water in Illinois: Lake Michigan, flowing waters, reservoirs and impoundments. An impoundment is a structure such as a weir or dam that raises the level of a body of water above its natural level.
Lake Michigan is one of the nation's five Great Lakes, and it is the only one that is entirely within the U.S. Some 976,640 acres, or seven percent of the lake, is within Illinois, bordering Lake and Cook counties, and accounting for 61 percent of the state's surface water.
Rivers, canals, streams and creeks account for 325,000 acres, or 20 percent of the state's surface water. Illinois is bordered by the Mississippi, Ohio and Wabash Rivers, as well as Lake Michigan.
Manmade impoundments account for 259,400 acres, or 16 percent of the state's surface water. This does not include the three reservoirs of Lakes Carlyle, Rend and Shelbyville, which total 54,580 acres, or 3 percent of the state's surface water. Most of the state's impoundments are in the southern and western areas of the state, and only 32 percent are publicly owned.
Groundwater occurs as a result of precipitation. Illinois receives 35 to 42 inches of annual rainfall. This seeps deep into the ground, where it can remain dormant, or flow into manmade wells and natural springs. Approximately, 50 percent of the state's population drinks groundwater from wells, of which there are more than 410,000 tapping into groundwater supplies.
