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mountain communities in the headwaters of the study unit. (Bureau of Census, 1992). In addition, the head- waters of the region are subject to increasingly intense year-round recreational activity. Point-source pollu- tion from urbanization includes discharge from waste- water-treatment plants, solid-waste disposal, leaking underground storage tanks, industrial discharges, and storm runoff, which also is a nonpoint source. All these sources have a potential to affect the surface and ground water by adding nutrients, pesticides, various chemicals, hydrocarbons, trace elements, and salts depending on the specific point source. Because of the naturally low phosphorus concentrations that probably limit algal growth, reservoirs, such as Dillon, can be extremely sensitive to additional phosphorus loading, which leads to accelerated eutrophication (Colorado Department of Health, 1992). Advanced wastewater treatment for many municipal facilities and control of nonpoint sources of pollution from urban areas that discharge into Dillon Reservoir and the Fraser River, Eagle River, Roaring Fork, and several other tributary streams have been necessary to maintain water quality. Many streams in the study unit tend to have higher pH values than in other basins of the State; therefore, strict un-ionized ammonia standards (0.02 mglL) have been required of wastewater facilities to protect cold-water aquatic life (Colorado Department of Health, 1992). In the mountain areas, communities and resorts generally are located in narrow valleys containing highly permeable gravelly sediments, which have some nitrate contamination in localized, shallow ground water. In areas where septic tanks are used for waste disposal instead of community waste-treatment systems, nitrate contamination is especially likely. Aquifers in fractured rocks of the mountainous areas also are vulnerable to nitrate contamination from individual septic systems. Nitrate contamination probably will continue to be the most widespread ground-water problem in Colorado (Colorado Department of Health, 1992). There are a number of potential agricultural nonpoint-source-pollution issues in the lower region of the basin. Agriculture in the basin can produce increased levels of salinity, sediments, nutrients, pesti- cides, selenium, and other trace elements, which have an adverse effect on the surface water, ground water, and biological habitats. Salinity is an important water-quality concern in the agricultural areas of the basin. High dissolved- solids concentrations occur in irrigated areas near the lower Gunnison and lower Colorado Rivers and their tributaries (Colorado Water Quality Control Division, 1989). Irrigation-return flows, as seepage from canals and reservoirs and from field irrigation, are the largest human source for dissolved-solids concentrations in the study unit. The sedimentary geologic units, which include a large part of the central and lower parts of the basin, contain soluble minerals that contribute to the dissolved-solids concentrations. Surface water that is diverted for irrigation is applied to fields and a large amount is lost to evapotranspiration. This process concentrates the dissolved solids in the remaining water that eventually returns to the stream. Also, the reuse of water has a high potential to increase salinity in the Colorado River. Agricultural practices also can add to the salinity problem as a result of natural chem- ical processes. The chemical process involves trans- formingcalcium sulfate (gypsum) to sodium sulfate by cation exchange in solution. Since sodium sulfate has a higher solubility in solution than calcium sulfate, the sodium concentration will be increased in the surface water. The cation-exchange process can result from irrigation practices or storm runoff. Sediments are formed as a result of erosion and runoff processes. These processes are affected by soil type, land slope, climate, and tillage practices, which all affect the movement of contaminants to surface and ground waters. Sediment erosion by wind and water can be increased by cultivation practices and by live- stockthat trample stream banks, which occurs in many semiarid to arid climates like the Colorado Plateau. Soil permeability is a key factor affecting water quality in agricultural areas. In areas ofwell-drained soils, such as alluvial valleys in the basin, nitrate and pesti- cide concentrations can increase locally in alluvial aquifers beneath cropland that is fertilized, particularly where irrigated. Although phosphorus is at low levels throughout the basin, in areas of poorly drained soils, nitrogen and phosphorus from cropland fertilizers move to the surface water as surface runoff. Manure can contribute nitrogen to reservoirs and streams. Nitrate generally is at low levels in the basin, but is at higher levels in the Roan Creek, Uncompahgre River, and lower Colorado River reaches, because the reaches receive large quantities of drainage water from irriga- tion. Pesticide data are limited for surface and ground water. The areal distribution of data sites, the number of samples per site, and the temporal distribution of samples are inadequate for an assessment of pesticide distribution and occurrence. Selenium concentrations in the Gunnison and Uncompahgre Rivers exceed U.S. Environmental Protection Agency criteria for protection of aquatic life, and the concentrations can be of concern for fish and waterfowl (U.S. Environmental Protection Agency, 1987). The Colorado River downstream from the Gunnison River and the lower reaches of the Gunnison River provide habitat for endangered fish 30 Environmental Setting and Implications on Water Quality, Upper Colorado River Basin, Colorado and Utah