8218 - Laserfiche WebLink

Home Browse Search

'NATtJRAL FACTORS Thunderstorms in arid environments, such as initJie ~estern part of the UCOL study unit, cause large suspended-sediment loads iJ1's.treams. Natmal hot springs contribute about' SOO,OOO.,tons ~! ~~~~?l~,~~ SQl" s (l~ fJercent of the total . ,i!Pnnally to ';l\,fP~:s!feam8.'1 asm. Physiography and Climate The physiography is characterized by high mountains in the east and plateaus and valleys in the west. Two physiographic provinces divide the study unit: the Southern Rocky Mountains in the eastern part and the Colorado Plateau in the western part (fig. I). The east- ern and southern boundaries of the study unit follow the Continen- tal Divide. Because of large differences in altitude between the physiographic provinces, climate varies substantially between the eastern and western parts of the basin. Daily, monthly, and annual average temperatures are highest at the lower altitudes to the west. Climate changes from alpine conditions in the east to semiarid to arid conditions in the west. Precipitation generally ranges from 40 inches or more per year in the eastern part of the basin to less than 10 inches per year in the western part of the basin (fig. 2). 10 to 20 to 30 >30t040 - >400rmore .~ --\ . ~~ , I I/I>~ o I o 20 , 4f MILES 20 40 KILOMEiERS Figure 2. Average annual precipitation (in inches) in the Upper Colorado River Basin study unit (Colorado Climate Center, 1984). Geology and Soils The underlying bedrock in the study unit consists predominantly of crystalline and sedimentary rocks. Alluvium, consisting of stream, landslide, terrace, and glacial deposits, is present in valleys throughout the basin. In figure 3, the bedrock geology has been divided into igneous and metamorphic rocks; sedimentary rocks (further subdivisions of this unit were made if a predominant rock type of carbonate, sandstone, or shale was identified); intrusive rocks (often associated with mineral deposits); and volcanic rocks. Weathering of the different geologic units affects water-quality conditions in the study unit. The igneous and metamorphic rocks of the upper basin contribute fewer solutes to the streams than do the sedimentary rocks of the lower basin. At higher altitudes in the upper basin, exposed igneous, metamorphic, and volcanic rocks are fairly resistant to weathering, and thus salinity in the water is low in these upper stream reaches. Also, mineralized areas in the upper basin contribute trace elements to the surface and ground waters. When the streams come in contact with outcrops of sedimentary rocks in the middle and lower reaches of the basin, salinity in the water increases. In the more arid climate at lower altitudes in the western part of the basin, precipitation commonly is in the form of thunderstorms, and runoff from thunderstorms can deliver large loads of sediment and salts to the streams. In addition, evaporation in this arid climate enhances the accumulation of salts on the soil, which then can be delivered to streams during storm runoff. Selenium is present naturally in the shale bedrock of the middle and lower reaches, and it occurs in the surface and ground waters. The presence of many mineral springs in the study unit also has a major effect on water quality. The springs primarily are located in carbonate rock units in the area surrounding Glenwood Springs, Colo. (figs. I and 3). The mineral hot springs contribute about 15 percent of the total salinity annually to the streams in the basin (U.S. Department of the Interior, 1995). Soils also can affect water quality because soils can be a source of suspended sediment and soluble materials (salts). Suspended sediment in streams predominantly is a result of channel erosion and soil erosion from storm runoff. Generally, suspended-sediment concentrations in streams increase from the eastern edge of the Colorado Plateau physiographic province to the outflow of the basin at the Colorado-Utah State line. Soluble materials are added to the streams by storm runoff. II Igneous and Metamorphic Sedimentary Undivided Carbonate Sandstone Shale 20 I 40 MILES I . Intrusive . Volcanic o 20 40 KILOMETERS Figure 3. Generalized geology of the Upper Colorado River Basin study unit (modified from Schruben and others, 1974).