The North Fork of the Sun River begins high up in the Bob Marshall Wilderness and flows generally Southward for about 20 miles (32 km) 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.
The Sun is a Class I river from Gibson Dam to its confluence with the Missouri River for public access for recreational purposes.[9]
A sign located 3 miles south of Great Falls gives information on the history of the Sun River:[13]
This river was called "The Medicine" by the Indians. On the return trip from the coast Capt. Lewis of the Lewis & Clark Expedition struck this river approximately 50 miles west of here. He followed it down to the Missouri passing near this point July 11, 1806. In his journal under that date he said:
"when I arrived in sight of the white-bear Islands the missouri bottoms on both sides of the river were crowded with buffaloe I sincerely believe that there were not less than 10 thousand buffaloe within a circle of 2 miles around that place"
The City of Great Falls covers a portion of the plain across which the Expedition made their difficult 18 mile portage around the falls of the Missouri in June 1805
