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<br />W <br />\-" <br />t,';> <br />,-,J <br /> <br />Between 1965 and 1983 decreasing dissolved-solids concentrations <br />were detected at 20 stations on the Colorado River and tributaries <br />(Kircher, 1984). Increasing concentrations were detected in three <br />tributary streams (Dolores R. near Cisco, Utah, Little Snake R. near <br />Lily, Co.,. Virgin R. at Littlefield AZ). Three stations showed no <br />change. The decreased concentration can be due to several factors! for <br />example this period was one of major reservoir filling, improved <br />irrigation practices and, attempts to control point-source salinity. In <br />addition, beginning in about 1940 and continuing to about 1980 there was <br />a substantial reduction of grazing pressure on the Public Lands, which <br />was accompanied by a marked improvement of range conditions (U.S. Bureau <br />of Land Management, 1985). In order to understand the salinity problem <br />in the Upper Colorado River Basin short and long term salinity <br />variations must be understood, and this will involve a geomorphic <br />overview of the basins with special attention being paid to channel <br />changes with time. <br /> <br />Geology <br /> <br />The Colorado River Basin drains all or parts of five physiographic <br />provinces. Middle and Southern Rocky Mountains, Wyoming Basin, Colorado <br />Plateau, and Basin and Range Provinces. <br />Highly erodible shales are exposed throughout the Upper Colorado <br />River 8asin lowlands. Because of its thickness, usually over 600 m and <br />at times over 1000 m, the outcrop area of Mancos Shale is the largest <br />among the sedimentary rocks (Figure l-6). Mancos Shale has been studied <br />more than any other formation in connection with salinity because of its <br />large exposures, its erodibiE ty, and its high SMC. The name Mancos was <br />first applied in 1899 by Cross (Fisher, Erdmann, and Reeside, 1961) to <br />exposures of shale near the town of Mancos in southwestern Colorado. <br />The Mancos Shale is a shallow-water marine formation. It is thinly <br />bedded and dark gray, when fresh. The shale contains numerous veinlets <br />of gypsum .and calcite, and when weathered it is a lighter gray. The <br />surface is a friable, semi-powdery mass when dry that becomes sticky and <br />impervious when wet. It includes a few thin layers of bentonite, <br />calcareous sandstone, and shaly limestone. In places it includes <br /> <br />11 <br /> <br />1,. <br /> <br />.".,.,-'",-<",-,,, <br /> <br /> <br />-~ <br />-1 <br /> <br />';- <br />,j <br />j <br /> <br /> <br />i,;", :k,,_i <br /> <br />