The Ohio River originates at the confluence of the Allegheny and Monogahela Rivers at Pittsburgh (ORM 0) and generally flows southwest for 981 miles (1578 km) before entering the Mississippi River near Cairo, IL (ORM 981). After flowing from PA, the Ohio River delineates the geographical boundaries between OH and WV, OH and KY, IN and KY, and IL and KY (Figure 1). Most of the tributaries in the system drain water from these states including headwater tributaries, which flow from NY, MD, and VA.