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<br />8 <br /> <br />CLARKSON ET AL. <br /> <br />Iatter factor is particuIarIy important, because our ability to use controlled floods as an <br />element in the controI of non-native fishes and the creation and maintenance of nearshore <br />rearing habitats is predicated on the availability of sufficient sediment in the system. <br /> <br />At this time the precise quantity and best method of delivery of augmented sediment is <br />unknown, but we believe the eIaborate, costly, and environmentally questionabIe pipeIine <br />proposed previously would not be necessary. HydroIogical and sediment transport models <br />presently under development by physical scientists, in conjunction with the improving <br />lcnowledge of sediment storage, should be applied to resolve these issues. One important <br />characteristic of sediment introduced to the system is that it incIude silts and clays, as well <br />as sand. ApproximateIy 70% of the pre-dam suspended Ioad in spring floods was in the <br />sand-size range, and the remainder was silt-clay (Howard and Dolan 1981). Grain sizes <br />tran,sported during tributary floods are dominated by fine-grained clays and silts., , <br /> <br />Turbidity produced by fine-grained sediments in transport has been indicated as an important <br />cover element for native fishes (Kaeding and Zimmerman 1983, VaIdez et aI; 1992), and <br />diminished water transparency can limit the effectiveness of sight feeding non-native <br />predators.':: Silts and clays also are the primary sources of the important plant nutrient <br />phosphorus, which appears to be limiting to primary productivity in the Colorado River in <br />Grand Canyon (Evans and PauIson 1983). <br /> <br />'. <br /> <br />.~ '-:"1-" ",-;" ,... <br /> <br />Augmentation of sediment supplies to Grand Canyon, with the attendant advantages to native <br />fiShes, should allow even further flexibility in the use of hydrology to maintain the important <br />baIlince between native and non-native fishes. We advance the working hypothesis-and <br />advocate its testing-that sediment augmentation in conjunction with water temperature <br />mOdification would allow -additl"bnal periods 'Of' controlled' tIoodsordie1 fluctuations to be <br />used as elements of destabilization to negativeIy impact non-native fishes. Optimal use of <br />GIen Canyon Dam as a tool to maintain, enhance, and restore the native fish community in <br />Grand Canyon will be rea1i7ed when the combined elements of hydrology,. water <br />temperature, and sediment can be used to maximize the flexibility of dam operations for that <br />PUrpQ~:' This flexibility shouId also allow best use of the dam as a tool . to. promote. the <br />mixture. of muItiple uses for which it was originally constructed. . "" ,. _p' <br /> <br />~~ ,L :,-~: ~-' " <br /> <br />. ", ~ .;....,. <br />- <br /> <br />r.. . '~ <br /> <br />BACKGROUND INFORMATION ...' <br /> <br />{-"-'..'{;j! ; <br /> <br />:..' ~. . h '.' .~.,. <br />;,..' "'.- ~. <br /> <br />thi~; Pre-dam . Environment-Scaicity of detailed pre-dam habitat descriptions for the <br />.c016lado River in the Grand Canyon region hinders our discussion of historic habitats, <br />.aItIiough.considerab1e information exists re~arding the environmental processes that acted to <br />'form those habitatS (Leopold and Maddock 1~3, Leopold et aI. 1964, Leopold 1969, Dolan <br />