Lake Decatur and the Sangamon River

Lake Decatur is a 2,800-acre (11 km2) reservoir located in the city of Decatur, Illinois, east of downtown. The city and lake both share the name of U.S. Navy Commodore Stephen Decatur. The lake is 613 feet (187 m) above sea level. Lake Decatur has a watershed of 925 square miles (2,396 km² or 592,000 acres) spanning 7 counties. The largest lake on the Sangamon River, it was created in 1920-1922.[1]

In the years since its completion in 1922, the aging Lake Decatur has suffered from extensive siltation problems. It was designed to sit atop relatively flat prairie land, and is vulnerable to deposits of silt eroded from farm fields in the 925-square-mile (2,396 km2) upper Sangamon River drainage area. From 1923 through 1983, Lake Decatur lost an estimated 35% of its designed storage volume.

Although this trend has since been slowed by ongoing dredging, as of 2004 Lake Decatur was described as having an average depth of only 8 feet (2.4 m). 2008 rainfall replenished lake levels although the dredging operations have failed to stem the loss of reservoir storage capacity; city officials considered, but ultimately decided against, constructing a second reservoir for water storage purposes.[2]

The Sangamon River is a principal tributary of the Illinois River, approximately 250 miles (402 km) long, in central Illinois in the United States. It drains a mostly rural agricultural area between Peoria and Springfield.

The Sangamon is impounded in Decatur to form Lake Decatur, constructed in 1920–1922 to provide a water supply for Decatur. This lake, formed by damming the main stem of the river, with no control over upstream land uses, has had major problems with siltation and agricultural pollution. The lake often has excessive nitrate levels from agricultural runoff.