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<br /> <br />'.-:,l <br /> <br />In past time, the Colorado River has eroded a deep wide channel <br /> <br />through the Mancos Shale formation which has since been refilled with <br /> <br />alluvial materials to a depth of some 50 to 60 feet. The present <br /> <br />channel of the Colorado River is incised in the alluvial materials <br /> <br />which are much more permeable than the shale and serve as a general <br /> <br />aquifer f~r the floodplain area. This aquifer is recharged by deep <br /> <br />percolation losses from irrigation and canal and lateral seepage. <br /> <br />In some places the aquifer is under artesian pressure, and the salinity <br /> <br />of the water in the aquifer varies from 5,000 to 36,000 parts per million <br /> <br />of total dissolved salts. Salinity concentrations normally increase with <br /> <br />depth. <br /> <br />The topography in the irrigated area is gently sloping to rolling. The <br /> <br />major overall slope is to the river. Most of the irrigated lands are in <br /> <br />the 0 to 2 percent and 2 to 5 percent slope groups. Some breaks and <br /> <br />hummocky soils are also irrigated. <br /> <br />The upland watershed or diffuse salt source area is the drainage above <br /> <br />the Grand Valley irrigated area. The study area encompasses some 839,000 <br /> <br />acres. Annual precipitation varies from about 8 inches in the lower <br /> <br />valley to 20 inches on a small portion in the northwest corner of the <br /> <br />bas in. <br /> <br />15 <br /> <br />