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",l:l. o ~ w:.. during the 1935-48 and 1951-60 periods (Table II). This has a pronounced effect on TDS because sulfate alone comprises nearly one-half the TDS in the Colorado River. The changes in rates of gypsum dissolution may, therefore, be an im- portant factor in causing the decrease in TDS observed in the Lower Colorado River Basin during recent years. Accuracy of Ion Budgets Precipitation and dissolution estimates based on mass balance calculations have been questioned in a recent salinity study [25J. Messer et al. [25J contend that salin- ity decreases attributed to calcite precipitation [10J can often be accounted for by salt storage in the impoundments. Sufficient data are rarely available to accurately estimate salt storage, and it was excluded from our calculations. It 'is l\lllikely, ho;rever, that this introQuced errors in the ion budgots. Salt storage is a function of salt loads in the inflows and outflows (discharge and diversions). Salt concentra- tions, however, can only vary with changes in inflow salt concentrations or evaporation, if precipitation or dissolu- tion processes are not occurring in the impoundment. The expected outflow concentrations, estimated from 10 yr flow- weighted average inflow concentrations, adjusted for net water losses, differed significantly from measured concen- trations for several ions (Figure 2). Outflow concentrations of carbonate were consistently lower' and sulfate consistent- ly higher than expected. These differences simply cannot be , explained by salt storage since the estimates encompass roughly three flushings of the impoundments. The stoichio- metry was not exact in that calcium lost by calcite precipi- tation did not balance that derived from gypsum dissolution (Table II). This probably reflects the influence of sediment diagenesis processes (e.g. sulfate reduction) [22J, but the net affects of such processes on ion concentrations are un- known. There is, nonetheless, little doubt that calcite pre- cipitation is a major loss, and gypsum dissolution a major source, of salinity in the Colorado River. It was not possible to estimate halite dissolution in Lake Mead during the 1970-79 period. The observed decreases in sodium and chloride concentrations were nearly 5 mg/l lower for each ion than those expected due to net water losses (Figure 2). This did not occur in Lake Mead during the 1951-60 period or in Lake Powell during the 1970-79 period, indicating that it was not caused by retention of sodium chloride in the impoundment. Rather, this appears to be caused by sampling problems that developed in Grand Can- yon after flows were regulated by Glen Canyon Dam. The Little Colorado River enters the main stem Colorado 12