Other major lakes include Whitefish Lake in the Flathead Valley and Lake Mc Donald and St. The largest reservoir in the state is Fort Peck Reservoir on the Missouri river, which is contained by the second largest earthen dam and largest hydraulically filled dam in the world.Other major reservoirs include Hungry Horse on the Flathead River; Lake Koocanusa on the Kootenai River; Lake Elwell on the Marias River; Clark Canyon on the Beaverhead River; Yellowtail on the Bighorn River, Canyon Ferry, Hauser, Holter, Rainbow; and Black Eagle on the Missouri River.
Approximately 31,300,000 acres (127,000 km) of wilderness in 12 separate wilderness areas that are part of the National Wilderness Preservation System established by the Wilderness Act of 1964. Montana is a large state with considerable variation in geography, topography and altitude, and the climate is, therefore, equally varied.
The Pend Oreille River joined the Columbia River, which flows to the Pacific Ocean—making the 579-mile (932 km) long Clark Fork/Pend Oreille (considered a single river system) the longest river in the Rocky Mountains.
The Northern Divide turns east in Montana at Triple Divide Peak, causing the Waterton River, Belly, and Saint Mary rivers to flow north into Alberta.
See also: List of monocotyledons of Montana, List of coniferous plants of Montana, List of lichens of Montana, List of amphibians and reptiles of Montana, List of birds of Montana, Fish of Montana, Mammals of Montana, and List of taxa described from Montana Vegetation of the state includes lodgepole pine, ponderosa pine; Douglas fir, larch, spruce; aspen, birch, red cedar, hemlock, ash, alder; rocky mountain maple and cottonwood trees.
Forests cover approximately 25 percent of the state.
Montana's water resources provide for recreation, hydropower, crop and forage irrigation, mining, and water for human consumption.