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-
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