Blackfoot River, watercourse, southeastern Idaho, U.S., formed by the confluence of Slug and Lanes creeks, near the Caribou-Targhee National Forest in Caribou county. It flows northwestward through Blackfoot River Reservoir (used for irrigation) and then west to join the Snake River in Bingham county after a course of about 95 miles (153 km). The river is named for the Blackfoot Indians.