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<br />248
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<br />Flooding and Aquatic Ecosystems
<br />
<br />and expand their range. Changes in the hydrology of a watershed can be
<br />especially detrimental to wetland plants in riparian and floodplain habitats
<br />(Mitsch and Gosselink, 1986). The relation of flooding to riparian commu-
<br />nities is discussed in detail by Kozlowski (1984) and by Friedman and
<br />Auble in Chapter 8 of this volume.
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<br />Multiple and varied land-use practices in a watershed affect streamflows
<br />(i.e., magnitude of discharge, frequency of occurrence, duration of high
<br />flows, and rates of change), sediment transport, and water temperature
<br />through changes in the natural hydrograph to which native aquatic life
<br />adapted (Baxter and Glaude, 1980; Brooker, 1981; Ligon et aI., 1995;
<br />Welcomme, 1995; Poff et 'aI., 1997). Such practices include, but are not
<br />limited to, construction of dams to store water for later release, dewatering
<br />of streams for irrigated agriculture, controlled releases of water (i.e., alter-
<br />ation of natural streamflows) for irrigation and generation ofhydtoelectric
<br />power, channelization to control flooding or road construction, dredging
<br />to maintain shipping channels, and reduced water quality through dis-
<br />charge of various pollutants and contaminants into flowing waters. Hu-
<br />man activities (e.g., agriculture, grazing, logging, mining, and recreation)
<br />on upland areas affect streamflows through decreased retention of water
<br />in soils, increased runoff, increased sediment loads into streams, water
<br />depletion from irrigated agriculture, and decreased water quality through
<br />release of wastewater that contains pollutants or contaminants (Wydoski,
<br />1978,1980; Wohl, Chapter 4, this volume). Consequently, a need for man-
<br />agement of large rivers and their floodplains on an ecosystem basis (i.e.,
<br />watershed or drainage basin) has been recognized by ecologists and conser-
<br />vationists (Welcomme, 1979, 1995; Fontaine and Bartell, 1983; Avies and
<br />Walker, 1986; Naiman and Decamps, 1990; Frissell et aI., 1993; Wesche,
<br />1993; Petts, 1994; Allan, 1995; Cairns, 1995; Dunne and Leopold, 1995;
<br />Sparks, 1995; Stanford et al., 1996).
<br />Dams convert lotic river reaches into lentic environments and the re-
<br />sulting reservoirs do not provide habitats normally used by native riverine
<br />fishes. Manystream fishes evolve with life history strategies adapted to the
<br />natural hydrograph (Moyle and Herbold, 1987; Welcomme et al., 1989).
<br />Nonnative fish species are introduced to utilize the newly created lentic
<br />environments and are often considered more detrimental than beneficial
<br />to ecosystems (Courtenay, 1995; Magnuson, 1976; Meffe,1985; Moyle et aI.,
<br />1986). Sometimes small populations of native fishes will persist in short
<br />river reaches with suitable habitat (Stacey and Taper, 1992) but less toler-
<br />ant species will decrease in number and possibly become extinct (Miller
<br />et aI., 1989). Benthic communities exhibit patch dynamics due to frag-
<br />mentation of Iotic ecosystems (Pringle et aI., 1988) in which the structure
<br />and integrity of benthic communities are drastically changed (Ward, 1976;
<br />Paine, 1980; Bain et aI., 1988; Resh et a!., 1988; Hildrew, 1992; Shannon
<br />et aI., 1994) and natural food webs are dramatically disrupted.
<br />The discharge of pollutants and contaminants through domestic, agri-
<br />cultural, and industrial wastewater is a serious problem in many large
<br />river-floodplain ecosystems. For example, decreased water quality from
<br />wastewater and industrial discharge was identified as the most important
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