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<br />C~!) <br /> <br />The following facts about the lower basin clearly show it to have high potential <br /> <br /> <br />as a source of salt and that evaluations, to be reliable, must be thorough: <br /> <br />C;,) <br />o <br />(.;.") <br /> <br />1. The river and its tributaries flow over terrain consisting almost entirely of <br /> <br /> <br />rocks of marine origin or soil derived from that type of rock. <br /> <br />0-:> <br /> <br />2. Groundwater is more saline than that in the upper basin. <br /> <br />3. Irrigation water is diverted over terrain composed almost entirely of rocks of <br /> <br />marine origin or soils derived from that type of rock. <br /> <br />4. When transported to surface waters, sediment derived from rangeland carries <br />with it associated salts. <br /> <br />4i) <br /> <br />Results of the '89-'91 studies notwithstanding, extrapolating data from non- c <br /> <br /> <br />reservation land in the upper basin to reservation land in the lower basin would <br /> <br />not be prudent because of the geologic changes which occur in Reach 4 as well as <br /> <br />the lack of data available for Reach 5. A more thorough study was requested by <br /> <br />the Shiprock Soil and Water Conservation District in order to determine the <br /> <br /> <br />efficacy and feasibility of taking project action on the Navajo Nation Unit. <br /> <br />In this report USDA presents the results of an investigation to determine whether <br /> <br />irrigated cropland in the Navajo Nation Unit might contribute significantly to <br /> <br />Colorado River salinity problems. <br /> <br />Rangeland contributions of sediment and salt to the San. Juan River were not <br /> <br /> <br />evaluated. The marine origin of the rocks and subsequent soils of this area make <br /> <br /> <br />it highly probable that a significant amount of salt enters the San Juan River by <br /> <br /> <br />sediment transport and underground water flow. The high probability and <br /> <br /> <br />potential amount of salt make it imperative that more than just a cursory look be <br /> <br />taken to determine the significance ~f the salt loading taking place from <br /> <br /> <br />rangeland and the potential for watershed treatment. <br /> <br />7 <br />