Pend Oreille River - Watershed

Watershed

Spreading across 25,792 square miles (66,800 km2), the Pend Oreille River watershed stretches across most of western Montana, northern Idaho and northeastern Washington, as well as tiny portions in southern British Columbia drained by the North Fork Flathead River and the lower Pend Oreille River. Much of the southern drainage divide of the watershed forms the border of Idaho and Montana, and a very short portion of the northeastern divide forms the border of British Columbia and Alberta. The river is sometimes considered as one with the Clark Fork, which is the primary river flowing into Lake Pend Oreille. For example, in Stewart Holbrook's book The Columbia, he repeatedly refers to the Pend Oreille River as the Clark Fork. If the lengths of the North Fork Flathead, main Flathead, Clark Fork and Pend Oreille are added together, the total is over 510 miles (820 km) stretching from the Rocky Mountains north of Glacier National Park to the Canada-U.S. border south of Montrose, British Columbia. This makes the Pend Oreille system the second longest tributary of the Columbia River (after the Snake River). It is the fourth largest by discharge (after the Snake, Kootenay and Willamette Rivers) and drains the second largest area (second only to the Snake).

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.