The Ohio River is formed by the confluence of the Allegheny and Monongahela Rivers in Pittsburgh, Pa. (fig. 1). It flows 891 mi westward to the Mississippi River near Cairo, Ill. The U.S. Army Corps of Engineers (USACE) operates a series of 20 locks and dams along the Ohio River to maintain a minimum 9-ft depth of flow to support commercial navigation and recreational activities. The Ohio River is a source of drinking water for more than 3 million people (more than 25 million people live in the Ohio River Basin). The total drainage area of the Ohio River is 203,940 mi2 (Ohio River Valley Water Sanitation Commission, 2006).