The forest is 1.8 million acres (7,300 km²) and contains mountains and river drainage areas. The Lochsa and Clearwater Rivers flow through the forest region. Portions of the Selway-Bitterroot Wilderness are also located in the forest.
Lewis and Clark travelled through the region in their journey westward in 1805-1806. Upon reaching Lolo Pass at the eastern edge of the forest on September 15, 1805, William Clark was quoted as saying, "From this mountain I could observe high rugged mountains in every direction as far as I could see"