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I INTRODUCTION i The closing of Glen Canyon Dam in March 1963 produced a major change In temporal discharge patterns of the Colorado River in Glen and Grand canyons (Figure 1.1). During the predam period of measure, annual discharge volumes and maximum discharges varied greatly from year to year.. Maximum flows exceeding 50,000 cubic feet per second (cfs) were common. For 20 years following the dam's closure, during which time Lake Powell was filling, regulated maximum flows exceeded 50,000 cfs in one year and 30,000 cfs in only two additional years. Since early 1983, however, a combination of high runoff from snowmelt and the high stage of Lake Powell has resulted in maximum flows at Lee's Ferryl between 45,000 cfs and 92,600 cfs. In contrast, minimum flows, both pre- and post-dam, have been An the range of 1,000- 4,000 cfs during most recorded years. Prior to the closure of Glen Canyon Dam, there was little daily variation in discharge of the Colorado River in the study area. Rather, the variation was seasonal, largely due to snowmelt runoff (April-June) with occasional summer floods and periods of summer drought. Since the onset of regulated flows, the seasonal pulse in discharge from snowmelt has continued, although much abated in most years. Much of the variation, however, now occurs on a daily basis with dam releases dictated by needs for hydropower in the growing metropolitan areas of the southwestern United States. This variation, which is reminiscent of tides in marine systems, has no known corollary in the evolutionary history of the native flora and fauna of the Colorado River. During peaking power periods, minimal water is released from the reservoir at night when the demand for power is low, and discharge is increased dramatically during high demand daytime hours. Weekends and holidays are often characterized by extended periods of low flow. 1Throughout this report we will observe the spelling "Lee's Ferry" in reference to the sight formerly occupied by John D. Lee and used to ferry boats across the Colorado River. In this respect, we follow Rusho and Crampton (1981) and concur with their finding that the spelling "Lees Ferry" as commonly used is "...both in poor grammar and poor history, since no person named Lees was ever involved." -9-