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<br />(I;) <br />~ <br />o <br />'-0 <br /> <br />CONCLUSIONS AND RECOMMENDATIONS <br /> <br /> <br />The data, gathered for this report during the 199b to 1992 irrigation seasons, <br /> <br /> <br />indicate that salt loading of the San Juan River is occurring in the Navajo <br /> <br />Nation Unit as a result of several phenomena, i.e., irrigation of agricultural <br /> <br /> <br />land, groundwater and subsurface drainage, and erosion and transport of sediment. <br /> <br />;~~~t <br /> <br />. <br /> <br />, <br /> <br />Based on salinity data obtained by monitoring drainage water, irrigation <br /> <br /> <br />agriculture appears to be contributing to increased salinity of the San Juan <br /> <br /> <br />River through the mechanism of deep percolation. By applying irrigation water ~n <br /> <br /> <br />excess of that needed by the crop plus a little extra to move salt out of the <br /> <br />root zone, irrigators make it possible for the excess water to percolate through <br /> <br />salt-bearing aquifers in sufficient quantities to dissolve salt. The water then <br /> <br /> <br />returns to the river in a decidedly more saline condition than when it was <br /> <br /> <br />diverted. <br /> <br />Salt budgets indicate that over one-half of the salt loading is from other <br />sources, not related t~ irrigation. Evidence found during drain monitoring ~~ <br />:.;;~~/i <br />indicates that subsurface flow of either groundwater or deep-percolating surface'..' <br />water, or both, unrelated to irrigation agriculture, is making its way to the San <br />Juan River. <br /> <br />It also is likely that.salt loading is occurring as rangeland erodes and sediment <br />is transported to the river. Studies in other areas of the Colorado River basin <br />have shown that sediment resulting from erosion of salt-bearing soils carries its <br />inherent salt with it. Despite the low annual rainfall of this area there is <br />ample visual evidence that active erosion is taking place in the watershed and on <br />the flood plain during summer thunderstorms. No attempt was made to determine <br />the significance of salt loading as a result of the management being done on <br />rangeland, or the natural processes taking place on it. It soon became apparent <br />that such a study should be undertaken separately from the one being done on <br />irrigated lands. <br /> <br />-"":--j..:; <br />\f.:J,' <br /> <br />14 <br />