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mountains generally are thin and poorly developed and
<br />primarily are a product ofphysical weathering ofrocks.
<br />Soils in the Colorado Plateau are thicker and are
<br />formed on deposits of recent geologic age. Suspended
<br />sediment in the basin predominantly is a result of
<br />channel erosion and soil erosion from overland flow.
<br />Generally, suspended-sediment concentrations
<br />increase from the eastern edge of the Colorado Plateau
<br />physiographic province to the outflow of the basin at
<br />the Colorado-Utah State line. Suspended-sediment
<br />concentrations are smallest during base-flow condi-
<br />tions and largest during spring runoffwhen streamflow
<br />discharges are large and sediment is available. Soils in
<br />the agricultural areas of the basin contain soluble salts
<br />that could greatly affect the concentrations of sodium
<br />and calcium in the surface and ground water.
<br />Human Factors
<br />Human factors can adversely affect water quality
<br />as a result of point and nonpoint sources of chemical
<br />constituents. Because the economy ofthemountainous
<br />region depends on outdoor recreation and water-based
<br />activities, such as fishing, white-water boating, flat-
<br />waterboating, camping, and hiking, this area is a prior-
<br />ity to the State's water-quality program. For example,
<br />State water-quality standards for aquatic life have not
<br />been met for cadmium, copper, lead, and zinc in
<br />streams downstream from active and abandoned mines
<br />in the headwaters. In addition, many of the shallow,
<br />unconfined aquifers in Colorado have become contam-
<br />inated with nitrates and salinity resulting from agricul-
<br />turalactivities (Colorado Department ofHealth, 1992).
<br />Human factors also can improve water quality; for
<br />example, reservoirs can trap sediments and metals,
<br />resulting in downstream reaches having better water
<br />quality than the upstream reaches.
<br />Human factors can be described according to the
<br />water and land uses discussed earlier in this report.
<br />Interbasin water transfers, mining, urbanization, and
<br />agriculture are the principal activities that affect water
<br />quality in the basin. In the Upper Colorado River
<br />Basin, these activities occur approximately in a down-
<br />stream order. Interbasin water transfers are in the head-
<br />waters, mining is located in the mountainous areas,
<br />urbanization is in the Southern Rocky Mountains and
<br />Colorado Plateau, and agriculture predominantly is in
<br />the valleys of the Colorado Plateau.
<br />Because Interbasin water transfers generally are
<br />made near the stream headwaters, the amount of
<br />streamflow diverted can be a substantial part of the
<br />streamflow near these sources; however, the effect
<br />decreases farther downstream as the volume of flow
<br />increases. Diversions from the basin account for about
<br />12 percent of the mean annual streamflow at the
<br />Colorado River near the Colorado-Utah State line.
<br />The Alva B. Adams Tunnel, which diverts the largest
<br />amount of water in the entire study unit, may be the
<br />cause of increases in dissolved-solids concentrations
<br />in the Colorado River near Glenwood Springs
<br />(Liebermann and others, 1989). Although 9,000 tons
<br />of dissolved solids are removed annually from the
<br />Upper Colorado River Basin through the Alva B.
<br />Adams Tunnel, the principal effect of this diversion is
<br />the removal of relatively pure water from the Colorado
<br />River system.
<br />Mining practices have affected water quality in
<br />several parts of the basin. The headwaters of several
<br />tributaries to the Colorado River such as the Blue,
<br />Eagle, Roaring Fork, and Gunnison Rivers drain one of
<br />the primary metal-mining regions in Colorado. A large
<br />number of active and abandoned metal mines exist in
<br />this region, which is referred to as the Colorado
<br />Mineral Belt. Metal-mining activities usually are in
<br />areas of high precipitation, resulting in a greater risk of
<br />perennial or ephemeral mine drainage aswell asstorm-
<br />inducedmine drainage from mine dumps and tailings.
<br />Streams have been affected by point-source mine
<br />discharge and nonpoint-source runoff from mined
<br />areas (Wentz, 1974). Concentrations of cadmium,
<br />copper, iron, lead, manganese, mercury, molybdenum,
<br />and zinc exceed State water-quality standards for local-
<br />ized reaches of these streams (Colorado Water Quality
<br />Control Division, 1989). Some reaches in the basin,
<br />such as Red Mountain Creek near Ouray, are affected
<br />by acid mine drainage. Although local reaches of these
<br />streams have been affected, little is known about the
<br />transport of these metals downstream into the larger
<br />tributaries and the Colorado River.
<br />Coal mining in the area can affect water quality
<br />by increases in dissolved solids, particularly sulfate,
<br />and increases in trace-element concentrations. There
<br />are a number of active oil and gas fields and large
<br />deposits of oil shale, primarily in Garfield County.
<br />Although the effects from oil and gas drilling on the
<br />surface and ground water can be considerable locally,
<br />little is known as to the areal extent of the effects from
<br />this activity. Uranium mining was once active in the
<br />basin, and mining and milling wastes pose serious
<br />threats to ground water from radionuclide contamina-
<br />tion. High radium concentrations occur in shallow
<br />aquifers in Montrose County in association with
<br />uranium mining and milling operations (Colorado
<br />Department of Health, 1992).
<br />Urbanization has an important effect on the
<br />water quality of the study unit. Population is increas-
<br />ing at arate ofabout 10 percent annually in some of the
<br />IMPLICATIONS OF ENVIRONMENTAL SETTING ON WATER DUALITY 29
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