segment of the northern Rocky Mountains, U.S., extending southward for 300 mi (480 km) along the Idaho–Montana border. Peaks average about 9,000 ft (2,700 m), with Scott Peak, in Idaho, the highest (11,394 ft). Owing to the inaccessibility of the mountains from the east, the explorers Meriwether Lewis and William Clark in 1805 were forced to travel northward more than 100 mi before finding a westward route through Lolo Pass (5,236 ft). Bitterroot National Forest extends across the centre of the range, which also passes through several other national forests. The region is drained by tributaries of the Columbia and Missouri rivers on the east and the Clearwater and Salmon rivers on the west.