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<br />", <br /> <br />..... <br /> <br />optimum flood magnitude and duration for building sand bars and forming backwaters will vary <br />from year to year and reach to reach depending also on availability of water in Lake Powell <br />reservoir and the supply of sand on the river bed. Designing flood magnitude and duration for <br />river conditions of a specific year instead of for the long-term average condition will necessarily <br />require some sophisticated long-term computational methods along with simpler sediment <br />budget calculations including monitoring of the status of sand stored within the river reaches. <br />Evaluation of other riverine resources will also necessarily be part of future flood designs. <br /> <br />OVERVIEW <br />The central premise of the experiment is that (1) floods are essential fluvial processes <br />necessary to maintain the Colorado River's pre-<lam geomorphic character, and (2) the river's <br />geomorphic character and associated processes influence aquatic and riparian ecology. <br />Geomorphic components include sandbar morphology, eddy sediment storage, and mainstem <br />sediment transport and storage, while ecosystem components that may respond to geomorphic <br />changes include benthic communities, indigenous and non-indigenous fishes, and riparian and <br />marsh vegetation. Sand bars deposited in zones of low velocity along channel margins are <br />features of special interest These deposits are used as campsites by river runners, form <br />backwater aquatic habitats that may be required by indigenous fish, and serve as substrate for <br />riparian vegetation established following dam closure. <br />Sand bars deposited in areas of flow separation along channel margins are common <br />geomorphic features of a wide variety of rivers, by no means unique to the Colorado. However, <br />height and steepness of walls and narrowness of the canyon are such that flow separation and <br />eddy-related sedimentation are unusually well-developed in Grand Canyon. An unusually high <br />proportion of sand bars form within these eddies, and the bars are essential recreational and <br />ecological resources. <br />The principal scientific questions to be addressed by the floOd experiment relate to the <br />optimum flood magnitude, duration, and frequency necessary for rejuvenating or rebuilding sand <br />bars and associated backwater-sand bar aquatic and terrestrial habitats in Grand Canyon. There <br />is a clear need to seek the optimum flood magnitude and frequency because such a staged evt>nt <br />entails substantial resource cost, primarily a loss of electric power generation due to loss of the <br />volume of water released in excess of power-plant capacity and a loss of a proportion of sand <br />stored in the main channel through transport into Lake Mead reservoir. <br /> <br />GeDeral WorkiUl! Hvootbesis <br />The null hypothesis to be tested by this dam release is that the magnitude and duration of this <br />flood will have no long-term effect on the Grand Canyon riverine ecosystem as a whole. The <br />alternative hypothesis is that the magnitude and duration of the flood will result in measurable <br />and predictable changes in the river geomorphology and thus the aquatic and riparian <br />ecosystem. <br /> <br />Awareness of Ecoloeical Risk: Development of an experiment to test these hypotheses <br />produces some risk for ecological resources involved Although not a reason to curtail the <br /> <br />4 <br />