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The magnitude and frequency of <br />'high and low flows regulate numer- <br />ous ecological processes. Frequent, <br />moderately high flows effectively <br />transport sediment through the chan- <br />nel (Leopold et al. 1964). This sedi- <br />ment movement, combined with the <br />force of moving water, exports or- <br />ganic resources, such as detritus and <br />attached algae, rejuvenating the bio- <br />logical community and allowing <br />many species with fast life cycles and <br />good colonizing ability to reestab- <br />lish (Fisher 1983). Consequently, the <br />composition and relative abundance <br />of species that are present in a stream <br />or river often reflect the frequency <br />and intensity of high flows (Meffe <br />and Minckley 1987, Schlosser 1985). <br />High flows provide further eco- <br />logical benefits by maintaining eco- <br />system productivity and diversity. <br />For example, high flows remove and <br />transport fine sediments that would <br />otherwise fill the interstitial spaces <br />in productive gravel habitats (Beschta <br />and Jackson 1979). Floods import <br />woody debris into the channel (Keller <br />and Swanson 1979), where it creates <br />new, high-quality habitat (Figure 4; <br />Moore and Gregory 1988, Wallace <br />and Benke 1984). By connecting the <br />channel to the floodplain, high <br />overbank flows also maintain <br />broader productivity and diversity. <br />Floodplain wetlands provide impor- <br />tant nursery grounds for fish and <br />export organic matter and organ- <br />isms back into the main channel (Junk <br />et al. 1989, Sparks 1995, Welcomme <br />1992). The scouring of floodplain <br />soils rejuvenates habitat for plant <br />species that germinate only on bar- <br />ren, wetted surfaces that are free of <br />competition (Scott et al. 1996) or <br />that require access to shallow water <br />tables (Stromberg et al. 1997). Flood- <br />resistant, disturbance-adapted ripar- <br />ian communities are maintained by <br />flooding along river corridors, even <br />in river sections that have steep banks <br />and lack floodplains (Hupp and <br />Osterkamp 1985). <br />Flows of low magnitude also pro- <br />vide ecological benefits. Periods of <br />low flow may present recruitment <br />opportunities for riparian plant spe- <br />cies in regions where floodplains are <br />frequently inundated (Wharton et <br />al. 1981). Streams that dry tempo- <br />rarily, generally in arid regions, have <br />aquatic (Williams and Hynes 1977) <br />River Stane Freauencv <br />WL <br /> <br />E <br />Annual <br />Figure 4. Geomorphic and ecological functions provided by different levels of flow. <br />Water tables that sustain riparian vegetation and that delineate in-channel baseflow <br />habitat are maintained by groundwater inflow and flood recharge (A). Floods of <br />varying size and timing are needed to maintain a diversity of riparian plant species <br />and aquatic habitat. Small floods occur frequently and transport fine sediments, <br />maintaining high benthic productivity and creating spawning habitat for fishes (B). <br />Intermediate-size floods inundate low-lying floodplains and deposit entrained sedi- <br />ment, allowing for the establishment of pioneer species (Q. These floods also import <br />accumulated organic material into the channel and help to maintain the characteristic <br />form of the active stream channel. Larger floods that recur on the order of decades <br />inundate the aggraded floodplain terraces, where later successional species establish <br />(D). Rare, large floods can uproot mature riparian trees and deposit them in the channel, <br />creating high-quality habitat for many aquatic species (E). <br />and riparian (Nilsen et al. 1984) spe- <br />cies with special behavioral or physi- <br />ological adaptations that suit them <br />to these harsh conditions. <br />The duration of a specific flow <br />condition often determines its eco- <br />logical significance. For example, dif- <br />ferences in tolerance to prolonged <br />flooding in riparian plants (Chapman <br />et al. 1982) and to prolonged low flow <br />in aquatic invertebrates (Williams and <br />Hynes 1977) and fishes (Closs and <br />Lake 1996) allow these species to <br />persist in locations from which they <br />might otherwise be displaced by <br />dominant, but less tolerant, species. <br />The timing, or predictability, of <br />flow events is critical ecologically <br />because the life cycles of many <br />aquatic and riparian species are timed <br />to either avoid or exploit flows of <br />variable magnitudes. For example, <br />the natural timing of high or low <br />streamflows provides environmen- <br />tal cues for initiating life cycle tran- <br />sitions in fish, such as spawning <br />(Montgomery et al. 1983, Nesler et <br />al. 1988), egg hatching (Nxsje et al. <br />1995), rearing (Seegrist and Gard <br />1978), movement onto the flood- <br />plain for feeding or reproduction <br />(Junk et al. 1989, Sparks 1995, <br />Welcomme 1992), or migration up- <br />stream or downstream (Trepanier et <br />al. 1996). Natural seasonal varia- <br />tion in flow conditions can prevent <br />the successful establishment of non- <br />native species with flow-dependent <br />spawning and egg incubation require- <br />ments, such as striped bass (Morone <br />saxatilis; Turner and Chadwick <br />1972) and brown trout (Salmo trutta; <br />Moyle and Light 1996, Strange et al. <br />1992). <br />Seasonal access to floodplain wet- <br />lands is essential for the survival of <br />certain river fishes, and such access <br />can directly link high wetland produc- <br />tivity with fish production in the stream <br />channel (Copp 1989, Welcomme <br />1979). Studies of the effects on stream <br />fishes of both extensive and limited <br />floodplain inundation (Finger and <br />Stewart 1987, Ross and Baker 1983) <br />indicate that some fishes are adapted <br />to exploiting floodplain habitats, and <br />these species decline in abundance <br />when floodplain use is restricted. <br />Models indicate that catch rates and <br />biomass of fish are influenced by <br />both maximum and minimum wet- <br />land area (Power et al. 1995, <br />Welcomme and Hagborg 1977), and <br />empirical work shows that the area <br />of floodplain water bodies during <br />nonflood periods influences the spe- <br />cies richness of those wetland habi- <br />tats (Halyk and Balon 1983). The <br />timing of floodplain inundation is <br />important for some fish because mi- <br />gratory and reproductive behaviors <br />must coincide with access to and avail- <br />December 1997 775