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.

[citation needed] 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.

2008 rainfall replenished lake levels although the dredging operations have failed to stem the loss of reservoir storage capacity.

[2] In 2018, The City of Decatur completed a four-year long, $91 million dredging effort that increased the storage capacity of the lake by 30%.