This report presents Lake Mead Water quality data obtained from 1964 to 1966. The effect of filling Lake Powell on the water quality of Lake Mead is evaluated. General limnological principles and the present limnology of Lake Mead are discussed. Lake Mead has a warm monomictic annual temperature cycle characterized by summer stratification, fall overturn leading into a continuous circulation throughout the winter; temperatures never fall below 39 deg F (4 deg C). During stratification, lower dissolved oxygen values were recorded in the thermocline than in the epilimnion and hypolimnion. Mineral content increases from the upper to the lower end of Lake Mead. The greatest increase is in calcium and sodium sulfates and chlorides, although there is an overall decrease in bicarbonate. The filling of Lake Powell intensified the deterioration of water quality in Lake Mead during 1965, as evidenced by increased temperature, conductivity, and total dissolved solids and decreased dissolved oxygen. Las Vegas Bay reach was found to be a major source for degradation of water quality in Lake Mead because of its large input of dissolved salts and algae nutrients. The monitoring station at Hoover Dam has been a useful indicator of water quality in the lower reach of Boulder Basin
