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<br />Annual releases from Glen Canyon <br />Dam depend primarily on reservoir <br />content and hydrological <br />conditions. Hydrological <br />conditions can vary from periods of <br />drought to periods of very high <br />precipitation. These conditions <br />determine various annual, <br />monthly, and daily release volumes <br />from Glen Canyon Dam. Each <br />alternative will reflect the <br />following release conditions for <br />purposes of description and <br />evaluation: <br /> <br />. Low water years - annual <br />water release of 8.23 million <br />acre-feet of water from Glen <br />Canyon Dam. <br /> <br />. Medium water years - annual <br />release of about 12 million acre- <br />feet. <br /> <br />. High water years - annual re- <br />lease of about 16 million acre- <br />feet. <br /> <br />MIMIC PRE-DAM <br />CONDmONS <br /> <br />1be objective of each of the two <br />alternatives in this group are to <br />respond to the public scoping <br />comments that requested <br />ecosystem conditions similar to <br />those that existed prior to the dam. <br /> <br />The Run-oJ-the River alternative <br />would, to the extent possible, <br />approximate pre-Glen Canyon <br />Dam high spring flows and low <br />fall/winter low flows by matching <br />releases to reservoir inflows. <br />Spring releases woUld not exceed <br />the combined capacity of the <br />powerplant and outlet works (total <br />of about 45,000 cubic feet per <br />second (cfs)) except when Lake <br />Powell is full enough for the <br />spillways to operate. Inflows in <br />excess of 45,000 cfs typically occur <br />in May and June. <br /> <br />8 <br /> <br />Pre-dam median daily flows <br />ranged from about 5,200 cfs in <br />January to about 50,000 ds in June. <br />Water would be released through <br />the powerplant, outlet works, and <br />spillways (only at full reservoir <br />level), in that order. The <br />powerplant would operate as a <br />run-of-the-river facility. <br /> <br />Pre-dam conditions cannot be' <br />precisely duplicated in Grand <br />Canyon due to clear, cold water <br />releases, the effects of other water <br />projects upstream, and the routing <br />of flows through the reservoir. In <br />order to help compensate for the <br />lack of sediment and the cold water <br />temperatures, this alternative <br />includes potential sediment <br />augmentation, pumping river <br />bottom sand, and multi-level intake <br />structures. Together, these features <br />have the potential to increase <br />turbidity, add sand for beach <br />maintenance and increase <br />temperature to more closely <br />simulate historic conditions. <br /> <br />Sediment Augmentation <br /> <br />Oear water flood releases of more <br />than 90,000 ds in 1983, removed 24 <br />beaches, decreased the size of 64 <br />beaches, increased the size of 86 <br />existing beaches, and created 50 <br />new beaches, many of which have <br />since eroded. Most of the lost <br />beaches were in the reaches of the <br />river upstream from Phantom <br />Ranch; and most new beaches were <br />formed in down-river reaches, <br />from channel-stored sediment. <br /> <br />A sediment slurry pipeline would <br />supplement the sediment supplied <br />by tributaries and accelerate the <br />rate in which sand is deposited in <br />river channel pools. The slurry <br />pipeline would be part of a <br />sediment augmentation program to <br />reduce beach erosion and provide <br /> <br />sand for the natural building of <br />beaches. This would involve <br />dredging sediment from delta <br />deposits in Lake Powell or Lake <br />Mead and transporting it through a <br />pipeline to the Colorado River near <br />Lees Ferry on a continual basis. The <br />annual volume of water required <br />for the slurry pipeline would be less <br />than one percent of the minimum <br />annual release from Glen Canyon <br />Dam (8.23 million acre-ft). It is <br />assumed that the river would then <br />carry the sediment downstream for <br />deposition in main channel pools. <br /> <br />The sediment would be mostly <br />sand but would contain some silt <br />and clay. 1be areas of Lake Powell <br />to be considered as possible sources <br />of sediment are the upstream delta <br />along the main stem (Cataract <br />Canyon), the San Juan River, and <br />the Dirty Devil River. Other <br />sources of sand may include the <br />Colorado River delta of Lake Mead. <br />Sediments trapped in Lake Powell <br />(or possibly Lake Mead) are being <br />considered because they are <br />renewable and would have <br />naturally entered the Grand <br />Canyon if the dam were not there. <br /> <br />Pumping River Bottom Sand <br /> <br />Sandy beaches in the Grand I <br />Canyon are created when high <br />flows carry sand in suspension <br />from the main channel into areas of <br />slower velocity (eddies). A beach is <br />formed or enlarged when the sand <br />in suspension drops out as the high <br />flows recede. Pumping sand <br />deposited in the Colorado River <br />channel onto specific beac,hes <br />identified by the National Park <br />Service would enhance beach <br />formation and maintenance. This <br />action could be taken only where <br />channel deposits are available. A <br />source of river channel sand (eddy <br />