Great Lakes water resource region

The Great Lakes water resource region is one of 21 major geographic areas, or regions, in the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.

The Great Lakes region, which is listed with a 2-digit hydrologic unit code (HUC) of 04, has an approximate size of 141,984 sqmi, and consists of 15 subregions, which are listed 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. Includes parts of Illinois, Indiana, Michigan, Minnesota, New York, Ohio, Pennsylvania, and Wisconsin.