The Sun River watershed extends from the Continental Divide in the Bob Marshall Wilderness over 130 miles to where the Sun River joins the Missouri River at Great Falls, Montana. Elevations range from 9,392 feet at the highest point in the drainage to 3,350 feet at the mouth. Average annual precipitation varies from more than 70 inches in the headwaters (mostly snow) to approximately 12 inches in the Simms area. The watershed includes portions of Lewis & Clark, Teton, and Cascade counties. This description of the watershed begins at the headwaters and progresses in a downstream direction.
The North Fork and South forks of the Sun River meet in a canyon at the upper end of Gibson Reservoir and form the headwaters of the main Sun River. Both forks skirt the eastern boundary of the Bob Marshall Wilderness. The North Fork begins along the continental divide near Sun River Pass and flows in a southerly direction for approximately 29 miles through a mostly broad, timbered mountain valley. The North Fork drainage covers more than 258 square miles, about 7% of which was burned by forest fires in 1988. The average streamflow for the North Fork for the years 1911-1993 was 386 cubic feet per second. The highest flow recorded in the North Fork was 51,100 cubic feet per second during the record 1964 flood. The peak flow for the driest year (1992) recorded by the U.S. Geological Survey was only 1,470 cubic feet per second.
The South Fork of the Sun River, a steeper gradient river, begins along the continental divide in the Scapegoat Wilderness near Flint Mountain and flows north about 25 miles in a rocky, timbered canyon to meet the North Fork. There is a scenic set of rapids near the mouth of the South Fork that migratory fish are able to navigate. The South Fork drainage encompasses about 252 square miles.
