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<br />I- <br />I <br />. <br />t <br />. <br />. <br />t <br />. <br />. <br />t <br />I <br />t <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />37G7 <br /> <br />Tributary <br /> <br />Salt <br />Discharge (tons/yr) <br /> <br />San Juan Main <br />piedra River <br />Los pinos River <br />Animas River <br />La Plata River <br />Mancos River <br />McElmo Creek <br /> <br />77,000 <br />47,100 <br />29,600 <br />155,200 <br />13,500 <br />39,000 <br />115,000 <br />476,400 <br /> <br />The La plata River Basin is the only area for which <br />sufficient data were available to allow the USGS to accurately <br />determine the effect of the activities of man on salt loads. The <br />irrigation of 16,500 acres along the La Plata north of the New <br />Mexico line contributed 6,500 tons of salt per year to the La <br />Plata River total salt load at the state line. In other areas of <br />the San Juan basin for which sufficient data were not available <br />to make similar determinations, calculations indicate that the <br />amounts of dissolved solids added to the streams as a result of <br />irrigation ranges from about 0.4 to 2.6 tons per acre per year. <br />Land underlain by valley alluvium similar to that of irrigated <br />areas in the Los pinos River Basin had the lower rate, and lands <br />underlain by Mancos Shale similar to the irrigated lands along <br />the Mancos River had the higher rate. <br /> <br />EPA - The Mineral Quality Problem <br /> <br />The EPA considered the effect of irrigated agriculture on <br />salt loads at two areas along the San Juan mainstem in Colorado. <br />The irrigation of 12,000 acres above pagosa Springs yielded 1,500 <br />tons of salt during the study period while irrigated agriculture <br />between pagosa Springs, Colorado and Carracus, New Mexico added <br />36,500 tons of salt to the system. The difference in yields <br />above and below pagosa Springs was attributed to the insoluble <br /> <br />-35- <br />