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Table 2. Ecological responses to alterations in components of natural flow regime.' <br />Flow component Specific alteration Ecological response Reference(s) <br />Magnitude and Increased variation Wash-out and/or stranding Cushman 1985, Petts 1984 <br />frequency Loss of sensitive species Gehrke et al. 1995, Kingsolving <br /> and Bain 1993, Travnichek et <br /> al. 1995 <br /> Increased algal scour and wash-out of Petts 1984 <br /> organic matter <br /> Life cycle disruption Scheidegger and Bain 1995 <br /> Altered energy flow Valentin et al. 1995 <br /> Flow stabilization invasion or establishment of exotic species, <br /> leading to: <br /> Local extinction Kupferberg 1996, Meffe 1984 <br /> Threat to native commercial species Stanford et al. 1996 <br /> Altered communities Busch and Smith 1995, Moyle <br /> 1986, Ward and Stanford 1979 <br /> Reduced water and nutrients to floodplain <br /> plant species, causing: <br /> Seedling desiccation Duncan 1993 <br /> Ineffective seed dispersal Nilsson 1982 <br /> Loss of scoured habitat patches and second- Fenner et al. 1985, Rood et al. <br /> ary channels needed for plant establishment 1995, Scott et al. 1997, <br /> Shankman and Drake 1990 <br /> Encroachment of vegetation into channels Johnson 1994, Nilsson 1982 <br />Timing Loss of seasonal flow peaks Disrupt cues for fish: <br /> Spawning Fausch and Bestgen 1997, <br /> Montgomery et al. 1993, Nesler <br /> et al. 1988 <br /> Egg hatching Nxsje et al. 1995 <br /> Migration Williams 1996 <br /> Loss of fish access to wetlands or backwaters junk et al. 1989, Sparks 1995 <br /> Modification of aquatic food web structure Power 1992, Wootton et al. 1996 <br /> Reduction or elimination of riparian plant Fenner et al. 1985 <br /> recruitment <br />Invasion of exotic riparian species <br />Horton 1977 <br /> Reduced plant growth rates Reily and Johnson 1982 <br />Duration <br />Rate of change <br />Prolonged low flows Concentration of aquatic organisms Cushman 1985, Petts 1984 <br /> Reduction or elimination of plant cover Taylor 1982 <br /> Diminished plant species diversity Taylor 1982 <br /> Desertification of riparian species Busch and Smith 1995, Stromberg <br /> composition <br />Physiological stress leading to reduced plant et al. 1996 <br />Kondolf and Curry 1986, Perkins et <br /> growth rate, morphological change, al. 1984, Reily and Johnson 1982, <br /> or mortality Rood et al. 1995, Stromberg et al. <br /> 1992 <br />Prolonged baseflow "spikes" Downstream loss of floating eggs Robertson 1997 <br />Altered inundation duration Altered plant cover types Auble et al. 1994 <br />Prolonged inundation Change in vegetation functional type Bren 1992, Connor et al. 1981 <br /> Tree mortality Harms et al. 1980 <br /> Loss of riffle habitat for aquatic species Bogan 1993 <br />Rapid changes in river stage Wash-out and stranding of aquatic species Cushman 1985, Petts 1984 <br />Accelerated flood recession Failure of seedling establishment Rood et al. 1995 <br />'Only representative studies are listed here. Additional references are located on the Web at http://Iamar.colostate.edu/-poff/natflow.htmi. <br />ability of floodplain habitats (Wel- <br />comme 1979). The match of reproduc- <br />tive period and wetland access also <br />explains some of the yearly variation <br />in stream fish community composition <br />(Finger and Stewart 1987). <br />Many riparian plants also have <br />life cycles that are adapted to the <br />seasonal timing components of natu- <br />ral flow regimes through their "emer- <br />gence phenologies"-the seasonal <br />sequence of flowering, seed dispersal, <br />germination, and seedling growth. <br />The interaction of emergence phe- <br />nologies with temporally varying <br />environmental stress from flooding <br />or drought helps to maintain high <br />species diversity in, for example, <br />southern floodplain forests (Streng <br />et al. 1989). Productivity of riparian <br />forests is also influenced by flow <br />timing and can increase when short- <br />duration flooding occurs in the grow- <br />ing season (Mitsch and Rust 1984, <br />Molles et al. 1995). <br />The rate of change, or flashiness, <br />in flow conditions can influence spe- <br />776 BioScience Vol. 47 No. 11