The Clark Fork, or the Clark Fork of the Columbia River, is a river in the U.S. states of Montana and Idaho, approximately 310 miles 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. The river flows northwest through a long valley at the base of the Cabinet Mountains and empties into Lake Pend Oreille in the Idaho Panhandle. The Pend Oreille River in Idaho, Washington, and British Columbia, Canada which drains the lake to the Columbia in Washington, is sometimes included as part of the Clark Fork, giving it a total length of 479 miles, with a drainage area of 25,820 square miles. In its upper 20 miles 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 Saint Regis. The highest point within the river’s watershed is Mount Evans at 10,641 feet in Deer Lodge County, Montana along the Continental Divide. The Clark Fork is a Class I river for recreational purposes in Montana from Warm Springs Creek to the Idaho border.