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<br />This strange and marvelous interaction of water and land which has etched
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<br />arid rangelands. The boundaries of the Upper Colorado River Basin consist
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<br />out the basin over geologic time represents a truly unique resource sys-
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<br />of uplifted earth masses heavily dissected by erosion, glaciation, and
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<br />tem among the river basins of the country.
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<br />weathering. In the interior of the basin are plateaus, mesas, and basins
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<br />While the natural contrasts of verdant slopes, trout streams, deep
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<br />all considerably affected by erosion. Erosion above Grand Canyon, as
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<br />canyons, and desert ranges remain, man has wrought great change on this
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<br />estimated by the sediment load, is about 6.5 inches per 1,000 years. This
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<br />most arid, but most spectacularly beautiful river basin. The wild uncon-
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<br />erosion is attributed mainly to the Cretace~ua mArine shales which were
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<br />trolled river that John Wesley Powell ran in 1869 is now highly developed
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<br />essentially continuous across the Colorado plateau when it was uplifted.
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<br />and totally regulated. The waters of the Colorado now serve 15 million
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<br />These erodible shale beds are still a major source of river sediment.
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<br />people and many uses in supplying water for cities, irrigated agriculture,
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<br />Water Resources. The upper basin divides naturally into three major
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<br />energy production, industry, and mining, and in supporting wildlife, rec-
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<br />drainage systems (figure 1): the Upper Main Stem (UM), the Green (UG),
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<br />reation, and areas of unparalleled aesthetic value to the nation.
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<br />and the San Juan (US). The entire Green River drainage area comprises
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<br />For all these varied activities, demands are made upon the river as
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<br />the Green subregion. The San Juan subregion is the drainage area of the
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<br />both a source of water and a carrier of residuals and by-products of man-
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<br />upper basin between the junction of the Green River with the Colorado
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<br />made as well as natural processes. Consequently, over time as energy de-
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<br />velopment and other pressures increase water use and pollution inputs,
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<br />conditions exist within the Upper Colorado River Basin. The input of
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<br />River and Lee Ferry, Arizona. A broad range of climate and streamflow
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<br />life will be dependent on maintaining the quantities and qualities of
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<br />from over 50 inches in the high-elevation headwaters to less than 6 inches
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<br />the future ability of the Colorado to sustain its unique fish and wild-
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<br />water to the hydrologic system, in the form of precipitation, varies
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<br />water required for them.
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<br />in desert areas. Most of the streamflow is provided by snowmelt from the
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<br />Physical Characteristics
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<br />mountainous areas, which produce high rates of runoff during the snowmelt
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<br />Physiography. The 1,440 mile long Colorado River Basin, containing
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<br />one-twelfth of the land area of the forty-eight states, is the most var-
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<br />period each year. Historic unit discharge rates decrease rapidly as the
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<br />tributary streams flow from their headwaters at high altitudes into the
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<br />ied physical setting of any American river. High mountainous elevations
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<br />less humid areas and finally into desert areas (see table 1). The river
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<br />(~14,OOO feet) are ultimately succeeded by high plateaus and low desert
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<br />produces the lowest outflow per unit area (60 acre-feet per square mile)
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<br />valleys. Geological structures and formations include deep, intricately
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<br />carved river canyons, high mountain s~opes, large saline shale structures,
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<br />and long verdant irrigated river valleys. Fir forests are succeeded by
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<br />of any river basin in the United States.
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<br />Large variations in annual discharge occur also from year to year
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<br />due to variations in precipitation, and uver periods of years due to long-
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