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~ Q ~ ~ average of 397 x 103 tfyr to 180 x 103 tfyr over the same period [16J. The greatest decrease occurred in the Upper Basin where rates dropped from 240 x 103 tfyr to 28 x 103 tfyr. The changes in calcite precipitation were therefore closely related to changes in productivity. Calcite precipitation in Lake Powell averaged 23 mgfl during the 1970-79 period (Table II). Reynolds [17J demon- ,strated that polyphenols in the Colorado River inflow to Lake Powell significantly inhibited calcite precipitation in the upper end of the impoundment. The polyphenols are de- , rived from forested regions of the Upper Colorado RiverBa- sin, and concentrations vary directly with seasonal flow patterns [12J. During spring, 'when the river forms an over- flow in Lake Powell, polyphenol concentrations are suffi- cient to inhibit calcite precipitation in the upper one- third of the impoundment. Calcite precipitation is limited primarily to summer months and only occurs in the lower end of I,ake Powell where dilution reduces polyphenol concentra- tiona [12, 17J. Rates of calcite precipitation in Lake Powell during the 1970-79 period were roughly one-half as high as those estimated for Lake l~ead during the 1935-48 and 1951-60 periods (Table II). This could reflect differences in factors influencing solubility (temperature) or possibly indicate that polyphenol inhibition was not as high in Lake , Mead when it received runoff directly from the Upper Colo- rado River Basin. Ratios of autochthonous organic carbon sedimentation to autochthonous calcite precipitation indi- cate'that inhibition was, and still is occurring in the Upper Basin of Lake Mead [16J. These ratios did not change , appreciably after Lake Powell was formed indicating that , polyphenols are still being supplied to Lake Mead either via export from Lake Powell or possibly from inputs in the Grand Canyon. Thus, even though rates of calcite precipitation were relatively high in Lake Mead during early impoundment, , it is likely that they would have been even higher were it not for the inhibition that appears to be caused by poly- phenols. The inhibition of calcite precipitation that occurs in both impoundments, and decreases that occurred in Lake Mead after Lake Powell was formed, reduce the combined effective- ness of the impoundments for calcite removal. However, the two impoundments still removed an average of 38 mgfl of calcium carbonate during the 1970-79 period (Table II) which is extremely significant from the standpoint of salinity control. Lake Powell increased sulfate concentrations by 16 mgfl, but rates of gypsum dissolution have decreased con- siderably in Lake Mead. The combined impoundment system now contributes 53 mgfl calcium sulfate to the river, but this is still considerably lower than what occurred in Lake Mead ;--; c.:: 11