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<br />Table 1. Physfcal responses to altered flow regimes. <br /> <br />Sou.rce(s) of alteration <br />Dam <br /> <br />Hydrologic change(s) <br /> <br />Capture sediment moving <br />downstream <br /> <br />D~rnt diversion <br /> <br />Geomorphic response!!) Refetence(s) <br /> <br />Downstream channel erosion and Chien 1985, Petts 1984. 19851 <br />tributary headcutting Williams and Wolman 1984 <br /> <br />Bed annoring (coarsening) <br /> <br />Chien 1985 <br /> <br />Reduce magnitude and frequency Deposi~on of fines in gravel <br />of high flows <br /> <br />Sear 1995, Stevens et at. 1995 <br /> <br />Ch~nnel stabilization and Johnson 1994, WHliams and <br />narrowing, Wolman 1984 <br /> <br />Reduced formation of point bars, Chien 198;5, Copp-1989. <br />secondary channe~st oxbows. Fenne!" at at. 1985 <br />and cha~ges in channel- planform <br /> <br />UrbanIzation, tiling. dra1nage Increase magnitude and frequency Bank erosion and channel widening Hammer 1972 <br />of high (lows <br /> <br />, . Downward incision and floodplain Prestegaard 1988 <br />disconne ction <br /> <br />Levees and channeUzat.ion <br /> <br />Reduced :Infiltration into soil <br /> <br />Reduced baseflows <br /> <br />Reduce overbank flows <br /> <br />Groundwater pumping <br /> <br />Channel restrictIon .causing <br />downcutting <br /> <br />Floodplain deposition and <br />erosion prevented <br /> <br />Leopold 1968 <br /> <br />Daniels 1960. Prestegaard <br />et a1. 1994 <br /> <br />Sparks 1992 . <br /> <br />Reduced Ghannel migration and <br />for:mat~pn of se.condary ~hannels <br /> <br />Lowered water table levels <br /> <br />Streambank 'erosion and channel Kondoll and Cuny 1986 <br />downcutUng after ioss of vegetation <br />stabUity . <br /> <br />Shankman and Drake 1990 <br /> <br />such a disruptiont it may take centu- <br />ries for a ,new dynamic equilibrium <br />to be attained by channel and flood.. <br />, plain adjustments to the new flow <br />regime (Petts 1985); in some cases, a <br />new equilibrium is never attained, <br />and the channel r~mains in a state of <br />continuous reGovery from the most <br />recent flood event -(Wolman and <br />Gerson 1978). These channel and <br />floodplain adjustments are some- <br />times overlooked because they can <br />be confounded with long-term re.. <br />sponses of the channel to changing <br />climates (e.g., Knox 1972). Recogni- <br />tion of human-caused physical <br />changes and _, associated biological <br />consequences may require many <br />years,' and physical restoration of <br />the river ecosystem may call for dra- <br />matic action (see box on the Grand <br />Canyon flood, page 774). <br />Dams, which are the most obvi- <br />ous direct modifiers of river flow, <br />capture both low and high flows for <br />flood control. electrical power gen- <br />eration~ ~ irrigation and -municipal <br />water n'eeds, maintenance of recre- <br />, ational reservoir levels, and naviga.. <br /> <br />December 1997 <br /> <br />tion. More than 85% of the inland <br />waterways within the continental <br />, United States are' now artificl~lly <br />controlled (NRC 1992) 1- including <br />nearly 1 millIon km of rivers that are <br />affected by dams (Echeverria et al. <br />,1989). Dams capture all but the fin- <br />est sediments moving down a riverj <br />with mariy severe do-wnstre'am con.. <br />sequences. For exa.rnple~ sediment- <br />depleted water released _from dams <br />can erode finer sediments from the <br />receiving channel. The coarsening of <br />the streambed cant in turn, reduce <br />habitat availability for the many <br />aquatic species living in or using <br />interstitial spaces. In addition, chan- <br />nels may erode1 or downcut, trigger- <br />ing rejuvenation of tributaries, which <br />themselves begin eroding and mi.. <br />grating hea.dward (Chien 1985. Petts <br />1984). Fine sediments that are con- <br />tributed by tributaries downstream <br />of a dam may ,be deposited between <br />the coarse particles of the strea.mbed <br />(e.g.~ Sear 1995). In the absence of <br />high flushing flows, species with life <br />stages that are sensitive to sedimen- <br />tatioflt such as the eggs and larvae of <br /> <br />many invertebrates and fish~ can suf- <br />fer high mortality rates. <br />'_For many rivers, it 'is land-use <br />activitiest -including tiffib'er harvest. <br />livestock grazing, agriculture, and <br />urbanization, rather than damsJ that <br />are the primary causes of, altered <br />flow regimes. For example. logging <br />and the associated building of roads <br />have contributed greatly, to degrada- <br />tion of salmon streams in the Pacific <br />Northwest, mainly' through effects <br />on runoff and sediment delivery <br />(NRC 1996) ~ Converting forest or <br />prairie lands to agricultural lands <br />generally decreases soil infiltration <br />and results in increased 'overland <br />flow, channel incisionf floodplain iso.. <br />lation, and headward erosion of <br />stream channels (Prestegaard 19~8). <br />Many agricultural areas were drained <br />by the construction of ditches or tile- <br />and-drain systems~ 'with the .result <br />that many channels have become en- <br />- trenched (Brookes 1988), , <br />These land-use practices, com- <br />bined with extensive drai'ning of <br />wetlands or overgrazing, reduce re.. <br />tention of water in 'watersheds and, <br /> <br />773 <br />