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WATER CHEMISTRY During the course of this study mean and coefficient of variation of daily flows at Lee's Ferry, 26 km below the dam, varied from 3,487 cfs to 51,570 c f s and 0% to 100.5%, respectively. Except during periods of flooding when spillway releases occur, discharge through the dam is drawn from the depths of Lake Powell, more than 60 m below surface at maximum stage. These waters are perennially cold, varying only from 6-120C, and seldom warm to more than 15.50C at Diamond Creek. Tributary inflows are generally warmer in summer and colder in winter than the mainstem, but their combined discharge is insufficient to affect the Colorado River. Major ion proportions in the Colorado River are: Ca > Na > Mg > K; and S04 > HC03 > C1. Tributaries vary from dilute dolomitic waters with high proportions gf Ca, Mg, and HCO3 to saline sodium chloride waters, but they produced only a slight increase of Na and Cl proportions in the mainstream during this study. Mean conductivity values increased only from 709 uS to 744 uS between the upper and lower reaches of the study area. Mean mainstream N03-NO2 concentrations varied between 315 ug/l and 350 ug/l with no apparent downstream trend. Tributaries were more variable with means of 13-759 ug/l. Ammonia levels were generally <5% those of N03-NO2 in the Colorado River and its tributaries. Soluble reactive and total phosphate mean concentrations exhibited a downstream increase in the mainstream. Mean values ranged from 11.0-171.5 ug/l for the former and 15.8-290.9 ug/l for the latter. Observed increases were attributable largely to inputs from the Paria and Little Colorado rivers. J Molar N/P ratios in the Colorado River suggest that this system, if nutrient limited in primary productivity, is limited j by phosphorus. Such limitation may not occur frequently, however, except perhaps in the reach above the Little Colorado River. Below the Little Colorado, dissolved phosphorus concentrations increase, but this is accompanied by considerable I' reduction in light penetration during flooding.