Great Lakes water resource region

The Great Lakes region, which is listed with a 2-digit hydrologic unit code (HUC) of 04, has an approximate size of 141,984 square miles (367,740 square kilometers), and consists of 15 subregions, which are designated with the 4-digit HUCs 0401 through 0415.

This region includes the drainage within the United States that ultimately discharges into: (a) the Great Lakes system, including the lake surfaces, bays, and islands; and (b) the St. Lawrence River to the Riviere Richelieu drainage boundary. It encompasses parts of Illinois, Indiana, Michigan, Minnesota, New York, Ohio, Pennsylvania, and Wisconsin.

List of water resource subregions