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<br />flood, despite the reworking and deposition of <br />high-elevation flood deposits. <br />Operations of Glen Canyon Dam greatly <br />reduced the magnitude of floods, increased the <br />magnitude of base flows, and greatly reduced <br />the amount of fine sediment delivered to the <br />river corridor. River flows are dictated by a <br />number of statutes, regulations, and adminis- <br />trative decisions that are collectively known as <br />the Law of the River (MacDonnell et aI., <br />1995). The most important of these "laws" <br />concerns the required annual release of 1.02 x <br />106 ha'm (8.23 x 106 ac.ft) of water from Glen <br />Canyon Dam to fulfill obligations to Mexico <br />and the Lower Basin states of Arizona, Califor- <br />nia, and Nevada. A second tier of consider- <br />ations concerns maximization of hydroelectric <br />power production by reduction of the magni- <br />tude of dam releases to those that can pass <br />through the power-plant turbines; the capacity <br />of the turbines is approximately 900 m3/s when <br />the reservoir is full. Other considerations that <br />determine the magnitude of base flows include <br />river navigation and fisheries. The median <br />discharge of the Colorado River for the period <br />between March 14, 1963, and September 30, <br />2000, was 74% higher and the seasonal varia- <br />tion in flows was much less than during the <br />pre-dam period (Topping et aI., 2003). The <br />10% exceedence flow for the post-dam period <br />was about 708 m3/s, and the 90% exceedence <br />flow was about 125 m3/s (Fig. 3). <br />Flow regimes of the post-dam river have <br />changed since 1963, and management of Lake <br />Powell and of water releases from Glen Can- <br />yon Dam can be divided into 3 time periods <br />(Fig. 2B). Between 1963 and 1980, the pri- <br />mary objectives were to fill the reservoir and <br />generate hydropower, while still meeting <br />legally defined downstream needs. In 1963 <br />and 1964, the Bureau of Reclamation (Recla- <br />mation) sought to quickly increase storage in <br />the reservoir, and dam releases were very low. <br />In May 1965, dam releases occurred as a <br />sequence of discrete pulses wherein flows <br />increased or decreased by 500 to 1000 m3/s for <br />periods of 1 day to 1 week (Fig. 2C). <br /> <br />Engineer's notes in the Reclamation archives <br />refer to the high releases of 1965 as a "channel <br />cleaning" flow. Releases subsequently met <br />annual requirements to downstream users, <br />hydroelectric power production was maxi- <br />mized, and the reservoir gradually filled. <br />After the reservoir filled for the first time <br />in 1980, dam operations were guided by <br />considerations of dam safety in addition to the <br />traditional constraints of hydroelectric power <br />production and downstream water needs (Fig. <br />2D). This period included the 4 years between <br />1983 and 1986 when inflow was unusually <br />large, and dam releases exceeded the capacity <br />of the power plant. We refer to the peak flows <br />of each of these years as post-dam floods. The <br />highest of these occurred on June 29, 1983, <br />and was 2750 m3/s at the Lees Ferry gage. <br />Between 1984 and 1986, annual peak flows <br />were between 1340 and 1515 m3/s. Between <br />summer 1986 and summer 1990, dam releases <br />were similar to those of the reservoir-filling <br />period. <br />The era of environmental management <br />began in summer 1990 during an 18-mth <br />period when dam releases varied for 2-wk <br />periods to facilitate river-scale experiments <br />(Beus and Avery, 1992). The daily range in <br />dam releases was thereafter constrained under <br />administrative rules associated with the Grand <br />Canyon Protection Act (Fig. 2E). A 7-day <br />experimental release of 1274 m3/s began on <br />March 26, 1996, and is hereafter referred to as <br />the 1996 Controlled Flood. Implementation of <br />the Record-of-Decision for the Environmental <br />Impact Statement for Glen Canyon Dam <br />Operations (U.S. Department of the Interior, <br />1995) formalized restrictions on the daily <br />range of dam releases. The magnitude of base <br />flows increased throughout this period, and <br />flows less than the pre-dam median of227 m3/ <br />s occurred rarely. There were 3 short-duration <br />experimental releases of 878 m3/s flows in <br />November 1997, May 2000, and September <br />2000. <br />Sediment supply to the Colorado River in <br />the post-dam era was reduced more than was <br /> <br />6 System-wide Changes in the Distribution of Fine Sediment in the Colorado River Corridor ... <br /> <br /> <br />. <br /> <br />. <br /> <br />. <br />