The Bull Run River is a 21.9-mile (35.2 km) tributary of the Sandy River in the U.S. state of Oregon. Beginning at the lower end of Bull Run Lake in the Cascade Range, it flows generally west through the Bull Run Watershed Management Unit (BRWMU), a restricted area meant to protect the river and its tributaries from contamination. The river, impounded by two artificial storage reservoirs as well as the lake, is the primary source of drinking water for the city of Portland, Oregon.