Monongahela, river, 128 mi (206 km) long, formed at Fairmont, N W.Va., by the junction of the West Fork and Tygart rivers. It flows north, through a valley marked by a decline in heavy industry and coal mining, into SW Pennsylvania, where it joins the Allegheny River to form the Ohio River at Pittsburgh. The canalized river is navigable for most of its length. Iron, steel, and coal are the chief products moved on the river. The Monongahela River was the first river in the United States to be improved for navigation.