Youghiogheny River, river rising in Preston county, W.Va., U.S., at Backbone Mountain, near the western edge of Maryland. It flows past Connellsville, Pa., to enter the Monongahela River at McKeesport, Pa., after a course of 135 miles (217 km). The Youghiogheny is the only river in western Maryland that does not flow south into the Potomac River. Its name is derived from the Algonquian word meaning “contrary stream.” The former borough of Somerfield, Pa., the site of a pioneer ford, was inundated by the reservoir formed by a dam (completed 1948) on the Youghiogheny (there joined by the Casselman River) at Confluence, Pa.