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Table 2. Ecological responses to alterations in components of :nat~ral flow regime.- Flow component Magnitude and frequency Specific alteration Increased variation Flow stabilization Timing Loss of seasonal flow peaks Duration Prol~nged low flows Prolonged base flow H spikes'l Altered inundation duration Pro'longed inUndation Rate of change Rapi~ changes in river stage Accelerated flood recession EcologJeal res~onse i Wash.qut and/or ~tr.a,nding Loss o~ sensitive species Increased alga~ scour and wash-out of organic matter: . Life: cyc1~ disrtfption Altered energy: flow Invasion or establishment of exotic species, leading to: !'.' . Local extinction Threat to nat,1ve commercial species .Altered cOfI1l11uni~~ Reduce~ wate~ and nutrien~ to floodplain plant species, ~ausing: . " Seedllng desl~cation ' Ineffective s~d dispersal " Loss of scou~ed habitat patches and second- ary channels reeded for p]an~ establishment · Encroacmneryt of vegetation -into channels Disrupt cues f~r fish; Spawning : ~gg hatching' Migration ; _ Loss of fish access to wetlands or baclC.wat"ers Modification Of aquatIc, food web structure Reduction or elimination of riparian plant recruitment ; I~vasion of exptic riparian species ReduCed p]an~ growth rates 1 < Concentratia~ of aquatic organisms . Reduction or ~limJn~tlon of plant cover Diminished pl~nt species diversity DesertificatioI) of riparian species compOSition i ' Physiological stress leading to reduced plant growth ratet morphological change, or mortality ~ c , 1 'Downstream ~oss of floating eggs Altered plant ~over types i , J 'Change in ve~etatlon functJonal type Tree -mortalitX . - Loss ofrtffi.e ~abitat far aquatic species " . W ash-out an~ stranding -of aquatic species Failw-e of see.;lling establishment } , Reference(s) Cushnian 1985, Petts 1984 Gehrk~ et aI. 1995i Kingsolving and Bain 1993, Travnichek et al. 1995 Petts 1984 , Scheidegger and Bain 1995 Valentfn et a1 19~5 Kupferberg 1996~ Meffe 1984 Stanford et at. 1996 Busch and Smith 1995i Moyle 1986t Ward and Stanford 1979 Duncan 1993 Nilsson 1982 Fenner et al. 1985, Rood et at. 1995t Scott et al, 1997, " Shankman and Drake 1990 Johnson 1994", Nilsson 1982 - Fausch and Bestgen 1997, Montgomery et ,a1. 1993t Nesler et 'a1. 1988 " NeesJe et al. 1995 ~., ,.--.- , WiUiams .1996. <. ~ Junk at al. 1989, Sparks 1995 Power 1992J Wootton at at 1996 Fenner et a1. 1985 Horton 1977 Reily and Johnson 1982 Cushm.an 1985, Petts 1984 TaylQT 1982 Taylor 1982 " '- --- - Busch and Smith 1995. .Strot:nberg -: et al. 1996, Kondolf and Curry 1986J Perkins et al. 1984; Reny and Johnson 1982, Rood et al. 1995. Stromberg et aI. 1992 RobertSon 1997 Auble et aI. 1994 Bren 1992, ConnoLet aI. 1981 Harms et a!. 1980 Bog&n 1993 Cushman 1985, Petts 1984 Rood et a1. 1995 i .Only representative studies are llsted here. Additional references arellocated on the Web at http://1amar.colostate.edul.'';poff/na.tflow.html, , .j ral flow regimes tMrough their n emer- _gence phenologt~s~' -the seasonal sequence of floweftng I se~d dispersal, germhiati9n, anq seedling growth. The interaction Of emergence phe.. nologies with t~mporal1y varying environmental sttess from flooding or drought helps; to maintain high species diversityj in, for exampleJ ability .of floodplain habitats (Wel- comme 1979) ~ The m,atch of reproduc- tive period and wetland access also explains some of the 'yearly variation in stream fish communi,ty composition (Finger and Stewart 1987). Many riparian plants also have life cycles th~t are adapted to the seasonal timing components ofnatu- 776 southern floodplain forests (Streng et at 1989) ~ Productivity of riparian forests is also influenced by flow timing and can Increase when short- ' duration flooding occurs in the grow- ing season (Mitsch and Rust' 1984, MoUes et at 1995). The rate of change, or flashinessJ in flow conditions can influence spe.. BioScience VoL 47 No. 11