The name Montana is derived from the Spanish montaña, meaning mountain. The State, third largest in the Union, is bounded on the north by Saskatchewan, Alberta, and British Columbia; on the east by the Dakotas; on the south by Wyoming and Idaho; and on the west by Idaho. Its area is 146,997 square miles, of which 866 square miles is water surface.
Two-thirds of the surface of the State is a plain broken by a network of valleys, many of the smaller ones carrying no water except during rare floods, and by isolated groups of low mountains. The western mountainous section, roughly 200 miles wide, is composed of generally parallel ranges on a northwest-southeast axis, but the Continental Divide follows a meandering course north and south. In the north the main range of the Rockies fronts the eastern plain, but farther south an increasing spread of ranges lies east of the Divide, comprising the sources of the Missouri River and its tributaries.
The highest peaks are east of the Divide rather than along its crest. Granite Peak (12,990 alt.), near the southern boundary, is the highest point in the State. Fairview (1,902 alt.), on the Dakota boundary, and Troy (1,892 alt.), in the northwest corner, have the lowest altitudes. Montana is generally lower than other Rocky Mountain States. In eastern Montana, along the Yellowstone River and other streams where erosion has been too rapid to allow vegetation to gain foothold, grotesque badlands formations in vivid colors extend for many miles.
Montana's most important eastern rivers are the Missouri and Yellowstone. As evidenced by its broad alluvial plain, the Yellowstone is the older; it also has the most direct course. Its valley, one of the most productive agricultural districts in the State, has terraced landscapes shaped by long processes of land elevation and erosion. The Missouri is the larger river, formed by the junction of the Madison, Jefferson, and Gallatin at Three Forks; it describes a huge, irregular northerly arc on its course eastward. Its valley bottom is narrower, the sides generally lower and smoother than those of the Yellowstone, but more rugged than those of the Milk, an important northern tributary. Between the Missouri and the Yellowstone lies the vast expanse of prairie, cut by tributary streams into grassy uplands and rocky hills.
Clark Fork of the Columbia is the master river of western Montana. Coursing westward from its source near Butte, it is joined by the Blackfoot, the Bitterroot, and the Flathead. Generally slow-running, it becomes turbulent in places. The Kootenai River, which joins it in British Columbia, makes only a brief dip into the northwestern corner of the State. In volume of water, the Kootenai compares with the Missouri.
The Continental Divide separates Montana into distinct climatic divisions, partially protecting the area to the west from severe southwardsweeping cold waves and forcing condensation of much of the moisture carried by westerly winds. Winters west of the Rockies are therefore more moderate, summers cooler, and rainfall more plentiful than in subarid eastern Montana. The State's climate as a whole shows great changeability. In January and February, fierce unpredictable storms may be followed suddenly by warm chinook winds and sunshine. Late freezes and snowfalls may delay spring locally until June, and are not unknown even in July. In any of the mountain areas excessive daytime heat is sure to be relieved by cool nights. Westerly winds prevail.