The headwaters of the Tongue River rise in the Bighorn Mountains of Wyoming. From these sources the river flows northeast to its confluence with the Yellowstone River at Miles City. The river's flows in Montana are controlled by the 3,500-acre Tongue River Dam. Above the reservoir, the river meanders through a broad open valley. Here its main features are turbid water, slow velocity, muddy bottoms, and warm temperatures. Downstream from the dam, the river flows for 10 miles through a narrow, restrictive canyon with increasing gradient and accompanying cooler water temperatures and gravel bottoms. The Tongue River again becomes a slow, meandering valley stream for its last 10 miles.