Tygart Valley River  The Tygart Valley River — also known as the Tygart River — is a principal tributary of the Monongahela River, approximately 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 in the Allegheny Mountains and the unglaciated portion of the Allegheny Plateau. The Tygart Valley River rises in the Allegheny Mountains in Pocahontas County and flows generally north-northwestwardly through Randolph, Barbour, Taylor and Marion counties, past the towns of Huttonsville, Mill Creek, Beverly, Elkins, Junior, Belington, Philippi, Arden, and Grafton, to Fairmont, where it joins the West Fork River to form the Monongahela River. (The Tygart is thus the "East Fork" of the Monongahela.) Downstream of Elkins, the Tygart passes through a gap between Rich Mountain and Laurel Mountain, which are considered to be part of the westernmost ridge of the Allegheny Mountains and the boundary between the Alleghenies and the Allegheny Plateau. Along its course the river collects Leading Creek at Elkins; the Middle Fork River and the Buckhannon River (its largest tributary) in Barbour County; and Sandy Creek and Three Fork Creek in Taylor County. Just upstream of Grafton, the river was impounded by a U.S. Army Corps of Engineers dam in 1938 to form Tygart Lake. Valley Falls State Park is along the river between Grafton and Fairmont. At the United States Geological Survey's stream gauge in Philippi, the annual mean flow of the river between 1940 and 2005 was 1,922 ft³/s (54 m³/s). The river's highest flow during the period was estimated at 61,000 ft³/s (1,727 m³/s) on November 5, 1985. The lowest recorded flow was 4.9 ft³/s (0.1 m³/s) on several days in October 1953. At an upstream gauge near the community of Dailey in Randolph County, the annual mean flow of the river between 1915 and 2005 was 358 ft³/s (10 m³/s). The highest recorded flow during the period was 19,900 ft³/s (564 m³/s) on May 17, 1996.