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PART V HISTORICAL AND PRESENT SALINITY CONDITIONS e produced a slight downward shift in the TDS data when the sum of the constituents was used. 7. Salinity Control Projects The implementation of salinity control units as described in Part VII has prevented some salts from reaching the river in the 1980-82 period. While the local impacts of these projects are still being evaluated, no significant effects are likely to have occurred as yet below Lake Mead because of the 5- to 7-year actual flow hydraulic detention time of the reservoirs. 8. Erosion Several researchers[9,10,1l,12] have shown that erosion of saline shales and dissolution of efflorescence increase dissolved solids concentrations during runoff events. These and previous studies have primarily focused on conditions caused by summer and fall thundershowers. Lower elevation snowmelt events associated with saline marine formations may contribute a greater portion of the salinity budgets. Analyses of the Green River near the Green River station indicate that electrical conductivity remains high or may increase with flow peaks associated with snowmelt runoff events in January through April. e During the Bureau of Reclamation studies of the McElmo Creek Salinity Control Unit (described in Part VII), it was found that approxi- mately 32 percent of the total salt load could be related to runoff events. Similarly, recent salinity control investigations by Reclama- tion show that 21 percent of the Price River salt budget and 14 percent of the San Rafael River salt budget are related to natural runoff. Bureau of Land Management (BLM) salinity/erosion studies in the Price River Basin show that at least one drainage yielded significantly reduced salt loads where experimental contour plowing and gully plugs were used for erosion control. The feasibility of erosion control as a mechanism to control salinity (including snowmelt runoff events) needs further investigation based on the BLM Price River studies. 9. Geochemistry Water quality in the Colorado River Basin varies greatly. Most surface runoff originates from precipitation and is very low in TDS. TDS concentration steadily increases in its downgradient course due to natural and man-induced activity. e Dissolution of efflorescence on the surface or minerals in subsurface formations is a major source of TDS. Runoff from snowmelt and thunderstorms, which cause alluvial, bank, and gully erosion, sus- pends solids from barren marine shales. The increased concentrations of calcium, magnesium, and sulfate in these waters are due to dissolu- tion of gypsum (calcium sulfate) and dolomite (calcium or magnesium carbonate). Much of the sodium is contributed by exchange of calcium for sodium on clays found in saline marine shales. 28