The Trough is a narrow canyon where the South Branch of the Potomac River runs between Sawmill Ridge and River Ridge, northeast of Moorefield on the boundary of Hampshire and Hardy counties. George Washington gave as good a description as anyone, writing in his journal in 1748: "The Trough is [a] couple of Ledges of Mountains, Impassable, running side and side together for above 7 or 8 miles and ye River down between them." Washington, visiting as a young surveyor, is supposed to have raced through in a canoe. The Trough remains popular with canoeists and kayakers today, with fishermen, and with sightseeing tourists on the Potomac Eagle excursion train.

Less than a decade after Washington's visit, a bloody battle ensued at the Trough in 1756, during the French and Indian War. A large band of Shawnees defeated 16 to 18 settlers in the Battle of the Trough, killing seven of them.

Historically accessible only by water or on foot, the Trough was penetrated by a branch of the Baltimore & Ohio Railroad about 1910. The B&O's successor, the Chessie System, turned the branch line over to the West Virginia State Rail Authority in 1978. The authority continues to operate the rail lines, now known as the South Branch Valley Railroad. The Potomac Eagle operates over these rails. Passengers routinely see the train's namesake, since bald eagles reestablished themselves in the Trough in the early 1980s.

The great flood of 1985 ripped through the Trough, doing great damage to railroad facilities. Water reportedly backed up to a depth of 90 feet at the head of the canyon.