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<br />" <br /> <br />, <br /> <br />. <br /> <br />. <br /> <br />A. Description of the Existing Environment <br /> <br />The existing environment of the Colorado River Basin has been described <br />in great detail in numerous environmental statements and studies prepared <br />by several Federal agencies in the past. A few of the general areas are <br />excerpted from these previous EIS and studies. 1/ <br /> <br />Geo logy <br /> <br />The upper or northern section of the Colorado River Basin in WYoming and <br />Colorado is a mountainous plateau 5,000 to 8,000 feet in elevation marked by <br />broad rolling valleys, deep canyons, and intersecting mountain ranges. <br />Many of peaks in these mountain chains rise to more than 13,000 feet <br />above sea level and many exceed 14,000 feet in elevation. Mountain lakes <br />exist in considerable numbers. The southern portion of the Upper Basin is <br />studded with rugged mountain peaks interspersed with broad, alluvial valleys <br />and roll ing plateaus. The main stream and its tributaries in Colorado <br />generally flow in deep mountain canyons. The Green River, primary tributary <br />of the Colorado River, flows in similar canyons in wyoming, Colorado, and Utah <br />after rising in. the Wind River Mountains. The San Juan River, a large <br />tributary, emerges from the mountains of southwestern Colorado, flows through <br />northwestern New Mexico, and then traverses the deep canyons of the San Juan <br />in Utah before joining the Colorado River in Glen Canyon. The Glen Canyon <br />section of the ma~n stream and tributaries lies almost entirely in deep <br />canyons. <br /> <br />Rocks of all ages from those of the Archean age (the oldest known geological <br />period) to t~e recent alluvial deposits, including igneous, sedimentary, and <br />metamorphic types, are found in the Colorado River Basin. The high Rocky <br />Mountains which dominate the topograpy of the upper regions are composed of <br />granites, schists, gneisses, lava, and sharply folded sedimentary rocks of <br />limestone, sandstone, and shale. Many periods of deposition, erosion, and <br />upheaval have played a part in the present structure of these mountains. <br /> <br />In contrast to the folded rocks of the mountains which fringe the basin, the <br />plateau country of southwestern Wyoming, eastern Utah, and northern Arizona <br />is composed principally of horizontal strata of sedimentary rocks. Slow but <br />constant elevation of the land area has allowed the Colorado River and its <br />tributaries to cut narrow, deep canyons into the flat-topped mesas. This <br /> <br />1/ <br />U.S. Bureau of land Management <br />(BlM) 1979. Proposed Domestic livestock Grazing Program for the Grand <br />Junction Resource Area. Final Environmental Statement. Apri~1979. <br /> <br />U.S. Bureau of Reclamation (USBR) 1971. Quality of Water Colorado River <br />Basin. Progress Report No.5. January, 1971. <br /> <br />U.S. Bureau of Reclamation (USBR) 1979. <br />Quality of Water Colorado River Basin. Progress Report No.9. <br />January, 19~ <br /> <br />o <br /> <br />5 <br />