river, most westerly of the Missouri River's three headstreams, rising in the Gravelly Range in southwestern Montana, U.S., near the Continental Divide and Yellowstone National Park (where it is known as Red Rock River). It flows west through Red Rock Pass and Upper and Lower Red Rock lakes, then north past Dillon (where it becomes the Beaverhead River) to Twin Bridges, where it is joined by the Big Hole and Ruby rivers; it then continues northeast as the Jefferson River. After a course of 225 miles (362 km) to a point just northeast of Three Forks, the Jefferson joins the Madison (Madison River) and Gallatin (Gallatin River) rivers to form the Missouri River. It was named for President Thomas Jefferson (Jefferson, Thomas).