High Quality Content by WIKIPEDIA articles! 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, USA. 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 km2) in the Allegheny Mountains and the unglaciated portion of the Allegheny Plateau.