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Show CEA Report Environmental SEDIMENTS ^ Cfoo THICK TO DRINK, too thin to plow" is an old expres- - L sion referring to the amount of sediment in the Colorado River. In fact, the Colorado River was named by a Spanish explorer for the reddish color of its waters, which comes from the sediments contributed by the colorful sandstones and other rocks along its course. Sediments moved downstream to form marginal sandbars and terraces and a vast delta at the river's mouth at the Sea of Cortez. In Grand Canyon, these sediments provided substrate for riparian vegetation and habitat for wildlife. Additionally, sediments protected cultural resources along the river corridor and in the side canyons. Today, dams trap much of the vast quantity of sediment and nutrients that once flowed through the river. 29 Sediment accumulation above the dam alters river flow in tributaries through the process of aggradation. Sedimentation will eventually render the dam useless for power generation, flood control or recreation. • Prior to dam construction, the Colorado River carried 60 to 180 million tons of sediment each year through Grand Canyon. 2' 23 • Glen Canyon Dam traps 85% of the sediments which previously flowed through the Grand Canyon, thereby depriving the canyon of much of the sediments and nutrients necessary to maintain beaches and natural habitats. 2,8 Controlled floods, of flows amounting to half the pre- dam spring runoff, were released through Glen Canyon Dam in 1996 in an attempt to mitigate the sediment- related environmental problems in Grand Canyon. The experimental floods did not bring new sediment in, but redistributed that which was already within the channel. Although the experiment was somewhat successful, it was not a long- term solution. Most of the short term benefits were lost within a year. 2 The reservoir has already lost 932,000 AF of storage capacity due to sedimentation. 2 The reservoir will totally fill with sediment in 200- 800 years, depending on future sediment inflow rates. 2 The power plant intakes will be clogged sooner and will require expensive modifications to keep the dam operating. The process of aggradation deposits sediment upstream above the level of the reservoir. The backing up of sediments negatively impacts tributaries, such as the San Juan River and Cataract Canyon. Without the reservoir, built up sediments in the main channel upstream of the dam would be flushed out in 2 to 6 years. 2 Upon exposure to the elements, many of the sediments in the side canyons have been shown to migrate quickly downstream. Reservoir Powell Sediment Fill Curves 40 60 80 100 120 140 160 River Miles upstream from Glen Canyon Dam 30000000 S 25000000 | 20000000 :> 15000000 I 10000000 I 5000000 • 140,000af/ yr 200 400 600 Years 800 1000 Figure 2 Sediment accumulation in main channel beneath Powell reservoir in 1986 versus prior to dam construction ( BOR 1996 EIS). Figure 3 Predicted sediment volume with time based on two sediment inflow rates. At the faster rate ( typical of the years prior to 1941) the reservoir would be totally full in 200 years. At the slower rate ( typical of more recent years), the reservoir would be totally full in 800 years ( Myers, 1998). page 6 |
