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<br />li02a89 <br /> <br />Effective Discharge and the Design of Channel Maintenance Flows <br />for Gravel-Bed Rivers <br /> <br />E. D. Andrews <br /> <br />U.S. GeologIcal Surv~, Boulder, Colorado <br /> <br />James M. Nankervis <br /> <br />u.s. Forest Service. Fore Collins, Colorado <br /> <br />Water resource developmcnt.<; which deplete me quanti[)' or reduce the range of srre.a.mflows <br />usu.a.l..ly have a number of uninlended ~ffcctS on the channel downstteam, inc:luding loss of channel <br />capacily, loss of aquatic and riparian habitat, and channel instabilities. A method for identifying <br />a flow regime sufficient to maintain desired stream characteristics. while permitting significant <br />development, would have great pr3ctical value. Over me past decadl:. imponant advances have <br />been made in our understanding of fluvial processes in gravel-bed streams. Using these advances <br />as a basis. one can oudine a method for determining chaMel maintenance flows for gravel-bed <br />streams typic.a.llo the westem United Sta~. A commoo characteristic nf gravel.bed streams is that <br />bed panicles are ttan,porred only aboul 5.10 percent of lIIe lime during the highest flows, and. <br />eyeD thee. at 3. very low rate. Although oCc.1sional motion of bed particles begins at a discharge <br />as small as 60 percenl of the banJcfull value. general motioo of the bed surface is exceedingly rare. <br />The proposed method relies on an appropriate bedload transport function and specific reach <br />information to compute the quantity of bed material in each size fraction transported by increments <br />of discbarge in the pre. and post-regulation regimes. Evaluatioo of possible flow regimes indicates <br />m:lt bankfull cbannel capacity can be maintained in its pre-regulation condition where 35 much as <br />60 percent of the RaNral flows are diverted. <br /> <br />1. INTRODUCTION <br /> <br />Alteration of natural Streamflows by regulation :!lid <br />diversion is common throug)1out the arid western United <br />States. Indeed, there are relatively few Streams with' <br />drainage areas greater than a hundred square kilometers <br />even in mountainous regions thal have not been affected <br />substantially. The most sigaifi=t change in the natural <br />strearn1low regime is a decrease in the magnitude of annual <br />peak flows. commoaly by a factnr of 2 to more than 10, <br />Hirsch er ai. [1990]. An associated, although frequently <br />unintended, consequence of streamflow storage and <br />diversion is a.tl appreciable c1uUlge in quantity and size of <br /> <br />Natural and AnthropOgenic Influences in Fluvial <br />Geomorphology <br />Gcophysic.a.l Monograph 89 <br />This paper is not subjccl to U. S. copyright. Published in 1995 <br />by the American Geophysical Union <br /> <br />ZOO 'd <br /> <br />0991 86~ OL6:131 <br /> <br />sediment supplied to the stream channel. These changes, <br />together with more diffuse, but often extensive, land use <br />changes have greatly altered the quantity, seasonal <br />distribution, aDd relative proportion of water and sediment <br />supplied to str= channels throughout the westerD United <br />States. <br />When streamflow and sediment supply are substantially <br />altered over a period of years to decades. changes in the <br />stream channel, floodplain, and riparian margin are <br />common. The adjustment may include changes in channel <br />size, morphology, pattern, rate of chiUlllel migration, bed- <br />material panicle-size distribution, floodplain morphology, <br />and the composition and density of riparian vegetation, <br />e.g., Peus [1979]. Galoy [1983], and Williams and <br />Wolman [1984]. Frequently, these adjustments affect <br />downstream aquatic and riparian resources. Increased <br />frequency of flooding due to reduced channel size and <br />floodplain storage. loss of ecologic.aJ habitat in the stream <br /> <br />151 <br /> <br />381^~3S lS3~Od Vosn <br /> <br />L~:II I03MIL6.Ll-'d3S <br />