The Sun River is a tributary of the Missouri River in the Great Plains, approximately 130 mi long, in Montana in the United States.It rises in the Rocky Mountains in two forks, the North Fork and South Fork, which join in the Flathead National Forest above Gibson Reservoir along the county line between Teton and Lewis and Clark counties. It flows E, SE, and E away from the mountains, past Simms, Sun River, and Vaughn and joins the Missouri at Great Falls.The water of the river is used extensively for irrigation, through the Sun River Project of the United States Bureau of Reclamation. The irrigation area covers approximately 92,000 acres .The North Fork of the Sun River begins high up in the Bob Marshall Wilderness and flows generally Southward for about 20mi until it meets up with the South Fork of the Sun River. Almost immediately the two forks flow into Gibson Reservoir, impounded by Gibson Dam. From when the water leaves the reservoir until it meets the Missouri River in Great Falls, the flowage is known as the Sun River.
