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<br />TOPOGRAPHY, CLI~~TE, fu~D VEGETATION
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<br />The topography of the Region is characterized by mountains, plateaus,
<br />and valley basins, The Region includes parts of four physiographic
<br />provinces: 1) the Middle Rocky Mountain province, 2) the (,yarning Basin,
<br />3) the Southern Rocky Mountain province, and 4) the Colorado Plateau,
<br />Most of the Region is ~ithin the Colorado Plateau, while the Southern
<br />Rocky Mountain province occupies the eastern fringe and the remaining
<br />two provinces cover the northern areas.
<br />
<br />Stream erosion has been the basic geomorphic agent, Erosion by ~ater
<br />shaped the land following mountain formation in the early Tertiary period,
<br />although glaciers and wind have contributed to some land formation, The
<br />Colorado Plateau province is characterized by severely eroded sedimentary
<br />rocks forming flat-topped mesas, plateaus, and deep, nearly vertical-walled
<br />canyons. In contrast, the Wyoming Basin is a region of significant soil
<br />deposition with lo~, isolated highlands, Because of the arid climate of
<br />the .,yoming Basin, deflation hollows, alkali flats, sand dunes, playa,
<br />and badland topography are common.
<br />
<br />The climate of the Region is characterized by dryness, The Region
<br />is primarily in a llrain shadow,1I in that moisture-bearing ~vestern air
<br />masses must cross several mountain ranges before reaching the area. Pre-
<br />cipitation is heaviest in the winter and spring, except in the extreme
<br />southern portion where most of the moisture is in the form of summer
<br />thunderstorms, Snowfall varies from 5 inches per year in the lo~er
<br />valleys to 200 or 300 inches per year in the highe, mountains, which are
<br />located around the Region's periphery, Total precipitation varies from
<br />6 inches in the valleys to 50 inches in the mountains, although most of
<br />the Region receives from 10 to 20 inches of precipitation annually.
<br />
<br />Temperature variations are extreme when one compares temperatures
<br />at the higher elevations in ~yinter with the extremely warm summer tempera-
<br />tures at lower elevations. Relative humidity is low compared with the
<br />rest of the United States. Readings are generally from 10 to 60 percent
<br />throughout the year. The combination of wind, temperature, and humidity
<br />accounts for a high rate of evaporation, Evaporation generally increases
<br />toward the southwestern portion as temperatures and annual sunshine
<br />increase.
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<br />Topography and climate are two major influences on vegetation zones
<br />in the Upper Colorado River Region. Three distinct vegetation zones are
<br />found here: 1) the alpine system, generally above 11,000 feet in eleva-
<br />tion; 2) the forest zone, between 8,000 and 11,000 feet in elevation,
<br />~hich is generally limited by adequate precipitation; and 3) the lower
<br />pinyon-juniper zone. Forests cover approximately 38 percent of the
<br />Region's area, with about half of that in the forest zone and the other
<br />half in the lower pinyon-juniper zone. About one-half of the area in the
<br />Upper Colorado River Region is occupied by desert rangeland. Here,
<br />grasses, sagebrush, and saltbush-greasewood predominate, The remainder
<br />of the Region is occupied by cropland, pasture, urban land, ~acer and
<br />barren lands,
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<br />2-5
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