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cies persistence and coexistence. In <br />many streams and rivers, particu- <br />larly in arid areas, flow can change <br />dramatically over a period of hours <br />due to heavy storms. Non-native <br />fishes generally lack the behavioral <br />adaptations to avoid being displaced <br />downstream by sudden floods <br />(Minckley and Deacon 1991). In a <br />dramatic example of how floods can <br />benefit native species, Meffe (1984) <br />documented that a native fish, the Gila <br />topminnow (Poeciliopsis occidentalis), <br />was locally extirpated by the intro- <br />duced predatory mosquitofish (Gam- <br />busia affinis) in locations where natu- <br />ral flash floods were regulated by <br />upstream dams, but the native species <br />persisted in naturally flashy streams. <br />Rapid flow increases in streams of <br />the central and southwestern United <br />States often serve as spawning cues <br />for native minnow species, whose <br />rapidly developing eggs are either <br />broadcast into the water column or <br />attached to submerged structures as <br />floodwaters recede (Fausch and Best- <br />gen 1997, Robertson in press). More <br />gradual, seasonal rates of change in <br />flow conditions also regulate the per- <br />sistence of many aquatic and riparian <br />species. Cottonwoods (Populus spp.), <br />for example, are disturbance species <br />that establish after winter-spring <br />flood flows, during a narrow "win- <br />dow of opportunity" when competi- <br />tion-free alluvial substrates and wet <br />soils are available for germination. <br />A certain rate of floodwater reces- <br />sion is critical to seedling germina- <br />tion because seedling roots must re- <br />main connected to a receding water <br />table as they grow downward (Rood <br />and Mahoney 1990). <br />Ecological responses to altered <br />flow regimes <br />Modification of the natural flow re- <br />gime dramatically affects both <br />aquatic and riparian species in <br />streams and rivers worldwide. Eco- <br />logical responses to altered flow re- <br />gimes in a specific stream or river <br />depend on how the components of <br />flow have changed relative to the <br />natural flow regime for that particu- <br />lar stream or river (Poff and Ward <br />1990) and how specific geomorphic <br />and ecological processes will respond <br />to this relative change. As a result of <br />variation in flow regime within and <br />among rivers (Figure 2), the same <br />human activity in different locations <br />may cause different degrees of change <br />relative to unaltered conditions and, <br />therefore, have different ecological <br />consequences. <br />Flow alteration commonly changes <br />the magnitude and frequency of high <br />and low flows, often reducing vari- <br />ability but sometimes enhancing the <br />range. For example, the extreme daily <br />variations below peaking power hy- <br />droelectric dams have no natural <br />analogue in freshwater systems and <br />represent, in an evolutionary sense, <br />an extremely harsh environment of <br />frequent, unpredictable flow distur- <br />bance. Many aquatic populations liv- <br />ing in these environments suffer high <br />mortality from physiological stress, <br />from wash-out during high flows, <br />and from stranding during rapid de- <br />watering (Cushman 1985, Petts <br />1984). Especially in shallow shore- <br />line habitats, frequent atmospheric <br />exposure for even brief periods can <br />result in massive mortality of bot- <br />tom-dwelling organisms and subse- <br />quent severe reductions in biological <br />productivity (Weisberg et al. 1990). <br />Moreover, the rearing and refuge <br />functions of shallow shoreline or <br />backwater areas, where many small <br />fish species and the young of large <br />species are found (Greenberg et al. <br />1996, Moore and Gregory 1988), <br />are severely impaired by frequent <br />flow fluctuations (Bain et al. 1988, <br />Stanford 1994). In these artificially <br />fluctuating environments, specialized <br />stream or river species are typically <br />replaced by generalist species that <br />tolerate frequent and large varia- <br />tions in flow. Furthermore, life cycles <br />of many species are often disrupted <br />and energy flow through the ecosys- <br />tem is greatly modified (Table 2). <br />Short-term flow modifications clearly <br />lead to a reduction in both the natu- <br />ral diversity and abundance of many <br />native fish and invertebrates. <br />At the opposite hydrologic ex- <br />treme, flow stabilization below cer- <br />tain types of dams, such as water <br />supply reservoirs, results in artifi- <br />cially constant environments that <br />lack natural extremes. Although pro- <br />duction of a few species may in- <br />crease greatly, it is usually at the <br />expense of other native species and <br />of systemwide species diversity <br />(Ward and Stanford 1979). Many <br />lake fish species have successfully <br />invaded (or been intentionally estab- <br />lished in) flow-stabilized river envi- <br />ronments (Moyle 1986, Moyle and <br />Light 1996). Often top predators, <br />these introduced fish can devastate <br />native river fish and threaten com- <br />mercially valuable stocks (Stanford <br />et al. 1996). In the southwestern <br />United States, virtually the entire <br />native river fish fauna is listed as <br />threatened under the Endangered <br />Species Act, largely as a consequence <br />of water withdrawal, flow stabiliza- <br />tion, and exotic species prolifera- <br />tion. The last remaining strongholds <br />of native river fishes are all in dy- <br />namic, free-flowing rivers, where <br />exotic fishes are periodically reduced <br />by natural flash floods (Minckley <br />and Deacon 1991, Minckley and <br />Meffe 1987). <br />Flow stabilization also reduces the <br />magnitude and frequency of overbank <br />flows, affecting riparian plant species <br />and communities. In rivers with con- <br />strained canyon reaches or multiple <br />shallow channels, loss of high flows <br />results in increased cover of plant <br />species that would otherwise be re- <br />moved by flood scour (Ligon et al. <br />1995, Williams and Wolman 1984). <br />Moreover, due to other related ef- <br />fects of flow regulation, including <br />increased water salinity, non-native <br />vegetation often dominates, such as <br />the salt cedar (Tamarix sp.) in the <br />semiarid western United States <br />(Busch and Smith 1995). In alluvial <br />valleys, the loss of overbank flows <br />can greatly modify riparian commu- <br />nities by causing plant desiccation, <br />reduced growth, competitive exclu- <br />sion, ineffective seed dispersal, or <br />failure of seedling establishment <br />(Table 2). <br />The elimination of flooding may <br />also affect animal species that de- <br />pend on terrestrial habitats. For ex- <br />ample, in the flow-stabilized Platte <br />River of the United States Great <br />Plains, the channel has narrowed <br />dramatically (up to 85%) over a <br />period of decades (Johnson 1994). <br />This narrowing has been facilitated <br />by vegetative colonization of sand- <br />bars that formerly provided nest- <br />ing habitat for the threatened pip- <br />ing plover (Cbaradius melodius) <br />and endangered least tern (Sterna <br />antillarum; Sidle et al. 1992). Sand- <br />December 1997 777