Jefferson River

Jefferson River
River
Confluence of Beaverhead and Big Hole Rivers forming the Jefferson near Twin Bridges, Montana
Country	United States
State	Montana
Tributaries
- left	Beaverhead River
- right	Big Hole River, Boulder River (southwestern Montana)
Source
- location	Twin Bridges, Montana
- coordinates	45°34′05″N 112°20′21″W / 45.56806°N 112.33917°W / 45.56806; -112.33917
Mouth	Missouri River
- location	Three Forks, Montana
- coordinates	45°55′39″N 111°30′29″W / 45.92750°N 111.50806°W / 45.92750; -111.50806Coordinates: 45°55′39″N 111°30′29″W / 45.92750°N 111.50806°W / 45.92750; -111.50806
Length	83 mi (134 km)
Basin	9,532 sq mi (24,688 km2)
Discharge	for Three Forks
- average	1,899 cu ft/s (54 m3/s)
- max	17,000 cu ft/s (481 m3/s)
- min	43 cu ft/s (1 m3/s)
Montana rivers. The Jefferson–Beaverhead–Red Rock is in the southwest corner.

The Jefferson River is a tributary of the Missouri River, approximately 83 miles (134 km) long, in the U.S. state of Montana. The Jefferson River and the Madison River form the official beginning of the Missouri at Missouri Headwaters State Park near Three Forks. It is joined 0.6 miles (1.0 km) downstream (northeast) by the Gallatin.

From broad valleys to a narrow canyon, the Jefferson River passes through a region of significant geological diversity, with some of the oldest and youngest rocks of North America and a diversity of igneous, metamorphic, and sedimentary formations.

The region was only intermittently inhabited by Native Americans until relatively recent times, and no single tribe had exclusive use of the Jefferson River when the Lewis and Clark Expedition first ascended the river in 1805. Today, the Jefferson River retains much of its scenic beauty and wildlife diversity from the days of Lewis and Clark, yet is threatened by water use issues and encroaching development. The Jefferson is a segment of the Lewis and Clark National Historic Trail, administered by the National Park Service.

From the Rocky Mountains of southwestern Montana, three small rivers converge to form the headwaters of the Jefferson River. The longest begins at Brower's Spring, 9,030 feet (2,750 m) above sea level, on the northern flank of the Centennial Mountains. The site is marked by pile of rocks. The water flows west then north as Hell Roaring Creek before merging with Rock Creek and flowing west through Upper and Lower Red Rock Lakes. Here it becomes the Red Rock River, flowing west through Lima Reservoir and then northwest into Clark Canyon Reservoir near Dillon. Below the dam, the river is known as the Beaverhead River. It is joined by the Ruby River above the town of Twin Bridges and converges with the Big Hole River to form the Jefferson about two miles downstream from town.