Although originally formed at the confluence of the Red Rock River and Horse Prairie Creek, the Beaverhead River now begins its 69-mile-long journey at the outlet of Clark Canyon Reservoir, an irrigation storage facility constructed in 1964. It joins the Big Hole River at Twin Bridges, Montana, to form the Jefferson River. The river above Dillon is characterized by a tight channel meandering through densely covered willow banks. From Dillon, it flows through a broad open agricultural valley for 50 miles before reaching Twin Bridges. Its channel is restricted through a narrow canyon during part of its lower journey near Point of Rocks. The water in the Beaverhead River is comparatively cold, except in areas subject to dewatering, mainly located in the lower river. Fish cover primarily consists of submerged and overhanging bank vegetation, undercuts and long, deep pools.