Clarkes Fork, Or River, a branch of the Columbia, formed at the "Horse Plain," near the centre of Missoula co., Montana, hit. 47° 21' N., lon. 114° 38' W., by the junction of the Bitter Root river from the south and the Flathead from the north. It Hows in a general N. W. direction, and joins the Columbia in Washington territory, lat. 48° 50' N., lon. 117° 45' W. At its mouth there is a fall of 15 ft., and a quarter of a mile above it passes through a deep gorge, where there is a further fall of 3 ft. About 130 m. above its mouth the river expands into a lake, 45 m. long and 10 or 15 m. wide, called Pend d'Oreille or Kalispelum. The Flathead river rises in the Rocky mountains, in British Columbia, about lat. 49° 30' X., flows S. into Montana, and for a short distance before joining Clarke's river pursues a W. course. About 50 m. above the junction is Flathead lake. The Bitter Root rises in the Big Hole mountains in the S. W. part of Montana, and flows first X., then X. W., and finally E. Its principal tributary is the Hell Gate, which rises in the Rocky mountains, about lat. 46° N., lon. 112° 30' W., and pursues a N. W. course.

The total length of Clarke's river from the source of either tributary is about 650 m.