river rising in the hilly plateau region of Potter county, Pennsylvania, U.S., and flowing generally northward for about 80 miles (130 km). The river enters New York state where the Allegheny Reservoir is impounded at Allegany State Park; turning southwest, it continues for 120 miles (190 km), meandering to the southeast and again southwest and eventually joining the Monongahela River at Pittsburgh to form the Ohio River. In its total length (325 miles 【523 km】), it drains an area of 11,700 square miles (30,300 square km). Its chief tributaries are the Kiskiminetas, Clarion, and Conemaugh rivers and Red Bank, Oil, and French creeks. The Allegheny was important for keelboat navigation before the beginning of railway competition in the mid-19th century. Several dams were built (1903–38) to make the river navigable from Pittsburgh to East Brady. Flood-control dams have been built on many of its major tributaries.