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<br />~~~) <br /> <br />f',~ <br />..,;z <br />o <br />CD <br /> <br />?ii.) <br /> <br />.~/., <br /> <br />TOPOGRAPHY, CLIMATE, AND VEGETATION <br /> <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 Wyoming Basin, <br />3) the Southern Rocky Mountain province, and 4) the Colorado Plateau. <br />Most of the Region is within 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 water <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 is characterized by severely eroded sedimentary rocks <br />forming flat-topp~d mesas, plateaus, and deep, nearly,vertical-walled <br />canyons. In contrast, the Wyoming Basin is a region of significant soil <br />deposition with low, isolated highlands. Because of the arid climate of <br />the Wyoming Basin, deflation hollows, alkali flats, sand dunes, playa, <br />and badland topography ar-e common. <br /> <br />The climate of the Region is characterized by dryness. The Region <br />is primarily in a "rain shadow,1I in that moisture-bearing western 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 lower <br />valleys to 200 or 300 inches per year in' the higher 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 winter with the extremely warm summer tempera- <br />tures at lower elevations. Relati',e humidity is 10'" cDmpared 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 ~ temperatures and annual sunshine <br />increase. w he.\J- <br /> <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 11,0.00. feet in eleva- <br />tion; 2) the forest zone, between 8,00.0 and 11,00.0 feet in elevation, <br />which is generally limited by 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 Dther <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, water and <br />barren lands. <br /> <br />2-5 <br />