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Last modified
7/14/2009 5:01:46 PM
Creation date
5/20/2009 11:04:50 AM
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UCREFRP
UCREFRP Catalog Number
8015
Author
Poff, N. L., J. D. Allan, M. B. Bain, J. Karr, R., K. L. Prestegaard, B. D. Richter, R. E. Sparks and J. C. Stromberg
Title
The Natural Flow Regime, A paradigm for river conservation and restoration
USFW Year
1997
USFW - Doc Type
BioScience
Copyright Material
YES
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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
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