The Powder River's tributaries arise in the southern Blue Mountains in the Umatilla National Forest. The river's main stem begins in Sumpter, where McCully Fork, Cracker Creek and several smaller tributaries join, and flows east-southeast through the tailings of past dredge mining and into Phillips Reservoir. After exiting Phillips Reservoir, the river continues east for about before turning sharply north through the Bowen Valley and Baker City, Oregon. From here the river meanders the floor of the Baker Valley and passes by the cities of Haines and North Powder, where it is joined by the North Powder River. Here the river turns again sharply east-southeast, flowing through Thief Valley Reservoir, in a valley along the southern edge of the Wallowa Mountains. The river then transits the Lower Powder Valley and enters the Snake River on the Idaho-Oregon state line from the west, upstream from the Brownlee Dam at the Powder Arm of Brownlee Reservoir downstream from Richland.