The Rocky Mountain Trench, also known as the Valley of a Thousand Peaks or simply the Trench, is a large valley on the western side of the northern part of North America's Rocky Mountains. The Trench is both visually and cartographically a striking physiographic feature extending approximately from Flathead Lake, Montana, to the Liard River, just south of the British Columbia–Yukon border near Watson Lake, Yukon. The trench bottom is wide and is above sea level. The general orientation of the Trench is an almost straight 150/330° geographic north vector and has become convenient as a visual guide for aviators heading north or south. Although some of its topography has been carved into U-shaped glacial valleys, it is primarily a byproduct of geologic faulting. The Trench separates the Rocky Mountains on its east from the Columbia Mountains and the Cassiar Mountains on its west. It also skirts part of the McGregor Plateau area of the Nechako Plateau sub-area of the Interior Plateau of British Columbia. It is up to wide, if measured peak-to-peak, and varies in valley relief, but is clearly visible by air and satellite/remote sensing and is easily discernible to those ascending any of the mountains or ridges lining it. The Trench is drained by four major river basins: the Columbia, Fraser, Peace and Liard. Two reservoirs of the Columbia River Treaty fill much of its length today - Lake Koocanusa and Lake Kinbasket. A further British Columbia power initiative created Lake Williston. Rivers that follow the Trench, at least in part, are the Kootenay River, the Columbia River, the Canoe River, the Flathead River, the Fraser River, the Parsnip River, the Finlay River, the Fox River, and the Kechika River. The North Fork of the Flathead River, flowing into Flathead Lake with the other branches of the Flathead River, is part of the Columbia River system. The Kechika is part of the Liard River system, and the Fox, Parsnip and Finlay Rivers are part of the Peace River system. The Canoe River is a short tributary of the Columbia system, draining into Kinbasket Lake, a reservoir on the Columbia River. The Kootenai River, however, does not fully follow the Trench but exits Canada southwest via Lake Koocanusa reservoir to the Libby Dam. The Kootenay River (Canadian spelling) is a tributary of the Columbia, joining the Columbia at Castlegar, BC after a meander through the United States as