The Musselshell River begins with the headwaters of the North and South forks in the Little Belt, Castle, and Crazy mountains in central Montana. Converging near Martinsdale, Montana, they form the mainstem, which flows east and north through a landscape varying from mountains to rolling plains and badlands. The upper, coldwater region of the Musselshell supports five species of gamefish, including native cutthroat and mountain whitefish. Abundant brush cover, good gravel bars, and clarity characterize the habitat of the South Fork. The North Fork's excellent willow cover and undercut banks provide favorable habitat for brown, rainbow, and brook trout. Averaging 60 feet wide at the juncture of the forks, the Musselshell draws extra water downstream for irrigation. Three storage reservoirs add to its flows, Bair Reservoir on the North Fork, Martinsdale Reservoir on the South Fork, and Deadman's Basin, a mainstem diversion. For the 50 miles from Barber to Roundup, the Musselshell widens, a number of irrigation diversions reduce streamflows, and the water becomes warmer. Riparian vegetation is commonly honeysuckle, wild rose, willow, and isolated groves of cottonwood trees. For its last 90 miles, the river broadens to 100 feet, accompanied by a rapid increase in turbidity. Vegetation adjacent to the river thins due to the arid climate, livestock grazing, and unstable and saline soils. However, the warmwater portion of the Musselshell supports a substantial resident fish population and provides spawning areas for sauger and channel catfish from Fort Peck Reservoir. The Musselshell River empties into Fort Peck Reservoir north of Mosby, 342 miles from its sources.