Rocky Mountain Trench, narrow valley that runs parallel to the western face of the Rocky Mountains in Canada and the northern United States. It extends about 1,400 km (about 870 mi) in a north-northwestern direction from just south of Flathead Lake in western Montana to the source of the Yukon River near the border between British Columbia and the Yukon Territory. The width of the valley floor ranges from approximately 3 to 20 km (approximately 2 to 12 mi), and the elevation of the trench ranges from approximately 600 to 1,000 m (approximately 2,000 to 3,000 ft). Hydroelectric dams in eastern British Columbia and northwestern Montana have altered the flow of many of the trench's rivers, which include the Fraser, Kechika, Kootenay (spelled Kootenai in the United States), Peace, Columbia, and Liard. Chief economic activities in the southern portion of the trench include transportation, mining, forestry, agriculture, and recreation; the northern portion is largely uninhabited.