Lake Decatur is a  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 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.