Jefferson River, one of the headstreams of the Missouri River. It is located in Montana and is 217 miles (349 km) long. From its source to Dillon, the river is called Red Rock Creek. From Dillon to Twin Bridges, it is called Beaverhead River. The name Jefferson River is given to the course from Twin Bridges to Three Forks, where the Jefferson, Madison, and Gallatin rivers join to form the Missouri.