High Quality Content by WIKIPEDIA articles! High Quality Content by WIKIPEDIA articles! High Quality Content by WIKIPEDIA articles! High Quality Content by WIKIPEDIA articles! The Clark Fork is a river in the U.S. states of Montana and Idaho, approximately 360 mi long. The largest river by volume in Montana, it drains an extensive region of the Rocky Mountains in western Montana and northern Idaho in the watershed of the Columbia River, flowing northwest through a long mountain valley and emptying into Lake Pend Oreille in northern Idaho. The Pend Oreille River, which drains the lake to the Columbia, is sometimes included as part of the Clark Fork, giving it a total length of 479 mi, with a drainage area of 25,820 sq mi. In its upper 20 mi in Montana near Butte, it is known as Silver Bow Creek. Interstate 90 follows much of the upper course of the river from Butte to northwest of Missoula. The Clark Fork is a Class I river for recreational purposes in Montana from Warm Springs Creek to the Idaho border.