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Dropping its sediment load in Lake Powell, the Colorado River below Glen Canyon Dam now runs cool and clear, excepting times of infrequent local thunder showers. Thousands of trout are now caught where only trash fish formerly survived. The 15 mile calm water reach between Lees Ferry and Glen Canyon Dam is especially attractive to large numbers of successful fishermen. :1 ~ , , :, =.1 Most spectacular, however, are the changes effected at Lake Mead, whose useful life is being greatly extended by upstream retention of sediment. Cold water released from the depths of Lake Powell, for power generation at Glen Canyon Power Plant, has lowered by 5 1/2 degrees (Fahrenheit) the temperatures of inflow to Lake Mead. Thus, the annual rate of Lake Mead surface evaporation is being reduced approximately 10 percent (8 inches in depth) with a substantial water savings of 75,000 to 100,000 acre feet annually. .' " I ~_I ", Present uses of this reach of the Colorado River and environmental changes incident to the operation of Glen Canyon Dam are presented herein. River travel through Grand Canyon is only by National Park Service permit, who supplied the data pertaining thereto. The Arizona State Fish and Game Department furnished the sport fishery data. Glen Canyon Dam and Power Plant is under the jurisdiction of the Bureau of Reclamation, who supplied the river operating data. Evaporation at Lake Mead was computed by the Geological Survey and is published in its Annual Water Supply Papers. The author also drew upon his personal knowledge of the river system, gained over a 3l-year period of employment with the Bureau of Reclamation and six years as Chief Engineer to the Upper Colorado River Commis sion. The author's latest of several river trips was in June 1972. . ~ , :. ',. , ,'.' ,- v THE RIVER'S NEW REGIMEN i~1 t. ,', Glen Canyon Dam was constructed for the primary purpose of regulat- ing the Colorado River in order that there could be further uses of the river in the Upper Colorado River Basin and still meet Compact water obligations in the Lower Basin, the point of division between the two basins being one mile below Lees Ferry. In the past, the annual runoff at Lees Ferry has greatly fluctuated, varying from over 20 million acre feet in the high runoff year of 1921 to as small as 4 million acre feet in the low runoff year of 1934. High runoff is now being stored in Lake Powell for later release to supplement low runoff. The result is presently a more or less uniform discharge at Lees Ferry of about 8.5 million acre feet annually with neither extreme high or extreme low seasonal flows, a more ideal regimen for the river travelers. ,.' ~.:. ~i IS ;.~ ~-i:~ . ~:~ Under regulated flow conditions, generation of hydroelectric power becomes attractive. Therefore, a large revenue-producing power plant was installed at Glen Canyon Dam and its energy output integrated into a system 2