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Executive Swnmary In October 1992, the Congress enacted the Grand Canyon Protection Act of 1992 (title XVIII of P,L. 102-575), which required the Secretary of the Interior to complete the envirorunental impact statement by October 30, 1994. The act also required that GAO audit the costs and benefits of the various operating alternatives identified in the final environmental impact statement. In preparing the statement, Reclamation studied the potential impact of various flow alternatives on selected resources, Reclamation reported the results of these studies in the final environmental impact statement on March 21, 1995, As discussed with the responsible congressional committees, for the purpose of this audit, GAO examined (1) whether Reclamation's impact determinations were reasonable and (2) what, if any, concerns still exist about the Glen Canyon Dam's final envirorunental impact statement. The act also requires that on the basis of the findings, conclusions, and recommendations made in the envirorunental impact statement and the GAO audit report, the Secretary is to adopt criteria and operating plans for the dam. Background Before the construction of the Glen Canyon Dam, the Colorado River's sediment-laden flows fluctuated dramatically during different seasons of the year, Annual daily flows of greater than 80,000 cubic feet per second were common during the spring runoff. In contrast, flows of less than 3,000 cubic feet per second were typical throughout the late summer, fall, and winter. Water temperatures ranged from near freezing in the winter to more than 80 degrees Fahrenheit in the summer. The construction of the Glen Canyon Dam altered the natural dynamics of the Colorado River corridor through the Glen and Grand canyons, The dam replaced the dramatic seasonal flow variations with significant daily fluctuations, greatly reduced the amount of sediment in the water, and resulted in nearly constant water release temperatures of about 46 degrees Fahrenheit. As early as 1982, the Secretary of the Interior initiated the Glen Canyon Envirorunental Studies of the effects of the dam. These studies were led by Reclamation and conducted by a number of different agencies, In 1989, the Secretary designated Reclamation as the lead agency in preparing an envirorunental impact statement. Other agencies and individuals participated in these efforts, including federal and state resource agencies, Indian tribes, private consultants, universities, and river guides, To protect the downstream resources until the completion of the impact statement and the adoption of a new operating plan for the dam, in November 1991 Reclamation implemented interim operating criteria, The interim Page 3 GAOIRCED-97-12 Glen Canyon Dam's Environmenta.llmpa.ct Statement