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fans. Most soils in the study unit are deep, loamy, well-drained soils that are alkaline (Heil and others, 1977). Soils in the mountainous regions are (1) Cryob- oralfs-rock outcrop soils, which are light-colored, well-drained soils that formed from weathering of a variety of crystalline and sedimentary rocks and (2) Cryumbrepts-Cryochrepts-rock outcrop soils, which are dark- to light-colored, well-drained soils in the alpine areas straddling the Continental Divide (table 2) (Heil and others, 1977). Soils in the highest mountainous areas generally are medium to very strongly acidic. In the nonmountainous areas, the most widely distributed soil association is the Cryaquolls- Argiborolls-Calciborolls-Cryboralfs-Cryoborolls- Torriorthenric-Haploborolls-rock outcrop soils, which generally are dark-colored, well-drained, moderately deep to deep soils. The Haplargids-Camborthids- Torriorthents association, which ranges in depth from shallow to deep, has light-colored, well-drained soil, and has formed from weathering of in-place or locally transported sandstones and shales. The Haplargids- Natrargids-Calciorthids-Torriorthents association is deep, light-colored, well-drained soil formed from the weathering of sedimentary rocks and from eolian material. The soil association present at the lower altitudes is the Torriorthents, which has depths ranging from shallow to deep and is alight-colored, well-drained soil that formed from locally weathered sedimentary rocks such as shales and sandstones. Soils in the nonmountainous areas generally are alkaline in nature. Ecoregions Ecological regions (ecoregions) have been established to characterize homogeneous areas on the basis of landscape features such as vegetation, soils, geology, physiography, and land use (Gallant and others, 1989). The study unit has been subdivided into five ecoregions: high-altitude tundra; low- to high- altitude forests; low- to middle-altitude Semidesert shrublands; Semidesert shrublands and wooded uplands; and salt deserts (fig. 6). The distribution of vegetation is determined primarily by mineral avail- ability from soils and geologic materials and by amount of precipitation (Chaney and others, 1987). High-altitude tundra is characterized by a cold, humid to arid climate with vegetation above treeline containing low-growth shrubs, cushion plants, and forts. Along the forest-tundra interface, there are sparse stands of Englemann spruce, subalpine fir, Limber pine, and Bristlecone pine. The middle- to high-altitude forests consist of Englemann spruce and subalpine fir; some areas are locally dominated by aspen. These forests vary from cool humid to warm dry climates depending on the altitude. In the low- to middle-altitude for- ests, vegetation consists of aspen, Douglas fir, Ponderosa pine, Gambel oak, and Pinon pine- juniperwoodlands. Low- to middle-altitude Semidesert shrublands have a semiarid climate with vegeta- rionconsisting of greasewood, four-winged salt- brush, shadscale, and sagebrush often interspersed with grasses. Semidesert shru- blands and wooded uplands are in a semiarid to arid climate. Sagebrush, rabbitbrush, and grasses grow in the Semidesert shrublands. In the wooded uplands, vegetation consists of juniper and Pinon pine, grasses, and sagebrush. Salt deserts are arid and have vegetation consisting of saltbrush, greasewood, rabbitbrush, horsebrush, and desert grasses. In the ecoregions classified as salt deserts and Semidesert shrublands, non- nativevegetation grows as a result of agricultural activities in the area. Irrigated crops include barley, corn, fruit, hay, oats, vegetables, and wheat (Chaney and others, 1987). Population The 1990 population in the largely rural Upper Colorado River Basin was about 234,000, less than 10 percent of the total population of the State of Colorado (fig. 7) (Bureau of Census, 1990). The largestpopulation center in the study unit is Grand Junction (30,209), which is located at the confluence of the Colorado and Gunnison Rivers. Some other principal cities in the area include Delta (3,952), Glenwood Springs (6,841), Gunnison (4,762), and Montrose (9,306) (Bureau of Census, 1992). The larger cities in the basin are located predominantly near agricul- tural lands or in mountain recreational communi- ties. Population of many counties in the basin increased about 5 percent from 1990 to 1992. Eagle and Summit Counties had the largest increases, of about 12 and 13 percent, respec- tively. Population totals are projected to increase for some of the counties (Delta, Eagle, Gunni- son,Montrose, and Summit) in the basin by more than 20 percent in the year 2000 (Colorado Divi- sion of Local Governments, 1994). Population in the basin for the year 2000 would be expected to exceed 300,000. ENVIRONMENTAL SETTING 11