The Tygart Valley River — also known as the Tygart River — is a principal tributary of the Monongahela River, approximately 135 miles (217 km) long, in east-central West Virginia in the United States. Via the Monongahela and Ohio rivers, it is part of the watershed of the Mississippi River, draining an area of 1,329 square miles (3,440 km) in the Allegheny Mountains and the unglaciated portion of the Allegheny Plateau.
The Monongahela River (/məˌnɒŋɡəˈhiːlə/ mə-NONG-gə-HEE-lə, /-ˈheɪ-/ -HAY-), sometimes referred to locally as the Mon (/mɒn/), is a 130-mile-long (210 km) river on the Allegheny Plateau in north-central West Virginia and Southwestern Pennsylvania. The river flows from the confluence of its west and east forks in north-central West Virginia northeasterly into southwestern Pennsylvania, then northerly to Pittsburgh and its confluence with the Allegheny River to form the Ohio River. The river includes a series of locks and dams that makes it navigable.