The Pend Oreille River watershed divide is formed on the east side by the Continental Divide. On the south, the Spokane River and Snake River drainage basins border on the Clark Fork. To the west smaller rivers such as the Colville River and tributaries of the Spokane drain the lands past the watershed divide. In the north is the Kootenay River, a similar-sized tributary of the Columbia. To the east, in Montana, is the Missouri River and tributaries such as the Marias River and Milk River. In the southwest the watershed borders on the Big Hole River and Jefferson River, headwater streams of the Missouri. The Pend Oreille/Clark Fork system is notable in that it cuts right between the Bitterroot Range and Selkirk Range, two major chains of the Rocky Mountains. The only other river to do so is the Kootenay, just to the north. The Rocky Mountain Trench runs across the northeast part of the watershed, through the Flathead Valley, and eventually terminating in mountains near the Clark Fork's confluence with the Flathead.