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<br />o <br />'-l fans, Most soils in the study unit are deep, loamy, <br />'.0 well-drained soils that are alkaline (Heil and others, <br />~ 1977), <br />Soils in the mountainous regions are (I) Cryolr <br />oralf.~-rock outcrop soils, which are light-colored, <br />wcll-drained soils that formed from weathering ofa <br />variety of crystalline and sedimentary rocks and <br />(2) Cryumbrepts-Cryochrepts-rock outcrop soils, <br />which are dark- to light-colored, well-drained soils <br />in the alpine areas straddling the Continental Divide <br />(table 2) (Heil and others, 1977), Soils in the highest <br />mountainous areas generally are medium to very <br />strongly acidic, <br />[n the nonmountainous areas, the most widely <br />distributed soil association is the Cryaquolls- <br />Argiboro Ils-Calciboro Ils-Cryboral fs-Cryoboro Ils- <br />Torriorthentic-Haploborolls-rock outcrop soils, which <br />generally are dark-colored, well-drained, moderately <br />deep to deep soils, The Haplargids-Camborthids- <br />Torriorthent~ association, which nmges in depth from <br />shallow to deep, has light-colored, well-drained soil, <br />and has fonned from weathering of in-place or locally <br />transported sandstones and shales, The Haplargids- <br />Natrargids-Calciorthids- Torriorthents association is <br />deep, light-colored, well-drained soil formed from the <br />weathering of sedimentary rocks and from eolian <br />material. The soil association present at the lower <br />altitudes is the Torriorthents, which ha.~ depths <br />ranging from shallow to deep and is a light-colored, <br />well-drained soil that fonned from [ocally weathered <br />sedimentary rocks such as shales and sandstones, <br />Soils in the nonmountainous areas generally are <br />alkaline in nature, <br /> <br />Ecoregions <br /> <br />Ecological regions (ecoregions) have bcen <br />established to characterize homogeneous areas on the <br />basis of landscape features such as vegetation, soils, <br />geology, physiography, and land use (Gallant and <br />others, 1989), The study unit has been subdivided into <br />five ecoregions: high-altitude tundra; low- to high- <br />altitude forests; low- to middle-altitude semidesert <br />shrublands; semidesert shtublands and wooded <br />uplands; and salt deserts (fig, 6), The distribution of <br />vegetation is determined primarily by mineral avail- <br />ability /Tom soils and geologic materials and by <br />amount of precipitation (Chaney and others, 1987), <br />High-altitude tundra is characterized by a cold, <br />humid to arid climate with vegetation above treeline <br />containing low-growth shrubs, cushion plants, and <br />forbs, Along the forest-tundra interface, there are <br />sparse stands of Englemann spruce, subalpine fir, <br />Limber pine, and Bristlecone pine, The middle- to <br /> <br />high-altitude forests consist of Englemann <br />spruce and subalpine fir; some areas are locally <br />dominated by aspen, These forests vary from <br />cool humid to warm dry climates depending on <br />the altitude, In the low-to middle-altitude for, <br />esrs, vegetation consists of aspen, Douglas fir, <br />Ponderosa pine, Gambel oak, and Pinon pine- <br />juniper woodlands, <br /> <br />Low- to middle-altitude semidesert <br />shrublands have a semiarid climate with vegeta- <br />tion consisting of greasewood, four-winged salt, <br />brush, shadscale, and sagebrush often <br />interspersed with grasses, Semidesert shru- <br />blands and wooded uplands are in a semiarid to <br />arid climate, Sagebrush, rabbitbrush, and <br />grasses grow in the semidesert shrublands, In the <br />wooded uplands, vegetation consists of juniper <br />and Pinon pine, grasses, and sagebrush, Salt <br />deserts are arid and have vegetation consisting of <br />saltbrush, greasewood, rabbitbrush, horse brush, <br />and desert grasses, In the ecoregions classified <br />as salt deserts and semidesert shrublands, non- <br />native vegetation grows a.~ a result of agricultural <br />activities in the area, Irrigated crops include <br />barley, com, frui~ hay, oats, vegetables, and <br />wheat (Chaney and others, 1987), <br /> <br />Population <br /> <br />The 1990 population in the largely rural <br />Upper Colomdo River Basin was about 234,000, <br />less than 10 percent of the total population of the <br />State of Colorado (fig, 7) (Bureau of Census, <br />1990), The largest population center in the study <br />unit is Grand Junction (30,209), which is located <br />at the confluence of the Colorado and Gunnison <br />Rivers, Some other principal cities in the area <br />include Delta (3,952), Glenwood Springs <br />(6,841), Gunnison (4,762), and Montrose (9,306) <br />(Bureau of Census, 1992), The larger cities in <br />the basin are located predominantly near agricul- <br />tural lands or in mountain recreational communi. <br />ties, Population of many counties in the basin <br />increased about 5 percent from 1990 to 1992, <br />Eagle and Summit Counties had the largest <br />increases, of about 12 and 13 percen~ respec- <br />tively, Population totals are projected to increase <br />for some of the counties (Delta, Eagle, Gunni- <br />son, Montrose, and Summit) in the basin by more <br />than 20 percent in the year 2000 (Colorado Divi- <br />sion of Local Governments, 1994), Population <br />in the basin for the year 2000 would be expected <br />to exceed 300,000, <br /> <br />ENVIRONMENTAL SETTING 11 <br />