Lake Decatur is a 2,800-acre (11 km2) reservoir located in the city of Decatur, Illinois, USA, 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 and has a watershed of 925 square miles (2,400 km2) or 592,000 acres (240,000 ha) reaching into parts of seven counties. The largest lake on the Sangamon River, it was created in 1920–1922.