The Thornton Reservoir significantly reduces overland and basement flooding by protecting 35,090 structures. It also improves the quality of area waterways by significantly reducing the backflow of untreated sewage into Lake Michigan and by storing combined sewer overflow during floods before release to the Calumet wastewater treatment plant. The original 14,600 acre-feet reservoir was increased by 9,600 acre-feet to a total volume of 24,200 acre-feet (7.9 billion gallons) for the new “composite” reservoir.