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topography. Most of the precipitation is provided by Pacific air masses that move inland from the west. Summer cyclonic thunderstorms sweep into the southern portion of the region from the Gulf of Mexico. Canadian arctic air can occupy the northern portion of the region during winter months. Since the region is distant from major sources of moisture and the air masses cross numerous mountain ranges en route to the area, precipitation is sparse except in high mountain areas. Average annual precipitation varies from less than 6 inches in the lowest valleys to more than 50 inches in the higher mountains. Average precipitation in the valleys and agricultural areas is from 10 to 20 inches per year. nprecipitation from late October through mid-April consists primarily of snow, particularly at higher elevations. Annual snowfall ranges from about 5 inches in the lower valleys and plateaus to 200 to 300 inches in the higher mountains. Snow accumulations occasionally exceed 100 inches at the higher elevations and do not completely melt until late summer.1I [6J The frost-free period varies greatly with elevation from 20 days or less at elevations above 8,500 feet to more than 180 days at elevations below 5,000 feet. Approximately 60 percent of the Upper Basin land area is under Federal administration, 15 percent is Indian Trust land, and 25 percent of the area is State, corporate, and individual land. The Bureau of Land Management and the Forest Service administer about 58 percent of the public land, most of it under the multiple-use doctrine. Grazing and crop production are two of the most important land uses. Forests cover about one-third of the area. Extensive outdoor recreation use is made of both public and private lands. Historically, the Basin has been sparsely populated. However, the population began to increase significantly about 1965. Since the 1980 census, the 14