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<br />Flooding and Aquatic Ecosystems
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<br />habitats are required for survival and recruitment of larval and juvenile
<br />fishes.
<br />Some braided channels become ephemeral as the high flows of a river
<br />subside. Deposition of sediments will occur along the shoreline and fill
<br />in the head of a secondary channel in a braided reach. Such secondary
<br />ephemeral channels become backwaters that are important as low veloc-
<br />ity habitats for various life stages of invertebrate and vertebrate animals.
<br />Temporary eddies at the mouths of tributaries to the main channel are
<br />regularly used by various adult fish species as a refuge from high water
<br />velocity in the main channel during high flows. Likewise, embayments
<br />are used as feeding and resting areas by juvenile and adult fish.
<br />Water velocities in floodplain habitats are generally low and are partic-
<br />ularly important in the life history strategies of fishes adapted for river-
<br />floodplain ecosystems. Although the main channel is used by such species
<br />during most of the year, aquatic habitats in floodplains provide an essential
<br />habitat for adults during floods as refuges, spawning areas, and feeding ar-
<br />eas (Baker and Killgore, 1994; Bayley, 1995; Welcomme, 1985; Welcomme
<br />et aI., 1989). Flooding produces temporary wetlands but other wetlands re-
<br />main permanent by water percolation through gravel and sand substrates.
<br />The diversity of wetlands (e.g., open water, swamps, emergent wetlands,
<br />forested wetlands, and lotic wetlands) (Table 9.2) provide a habitat mosaic
<br />that is used by many species of plants and animals. Floodplain depressions
<br />(ponds and lakes) that connect with the river during high streamflows are
<br />important as nursery and rearing areas for juvenile fish. If the connection
<br />between the river and floodplain habitat is lost as the river recedes after
<br />flooding, larvae and juvenile fishes may remain in depression ponds and
<br />lakes until the next flood when connectivity reoccurs between the river
<br />and floodplain. In some instances, total mortality of fish may occur in
<br />floodplain depressions that dry up. The function and integrity of flood-
<br />plains are lost through human activities that prevent connectivity of the
<br />lotic and Ientic systems (Armoros, 1991; Mellquist, 1992; Pelts et aI., 1992;
<br />Ward and Stanford, 1995; Welcomme, 1995; Stanford et aI., 1996; Poff
<br />et aI., 1997).
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<br />Human Perturbations of Large River Ecosystems
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<br />Humans settled river corridors and floodplains because rivers provided a
<br />means of transportation and floodplains were prime areas for agriculture
<br />and industry (Stanford et al., 1996; Poff et aI., 1997). Human perturbations
<br />of aquatic ecosystems through activities in upland areas, the main chan-
<br />nel of rivers, floodplains, and riparian zones adversely affected all types
<br />of aquatic ecosystems (Nilsson and Jansson, 1995), including wetlands,
<br />streams, rivers, lakes, estuaries, and coastal areas (Table 9.5).
<br />Today. nearly all large rivers in the northern third of the Earth have
<br />been dammed and regulated to provide water for multiple uses, including
<br />irrigated agriculture, flood control, hydroelectric power, industry, and do-
<br />mestic use (Table 9.5) (Gregory and Walling, 1987; Dynesius and Nilsson,
<br />1994). These rivers have been dramatically altered (Wohl, Chapter 4, this
<br />volume) through physical changes (Gore, 1994; Gore and Shields, 1995),
<br />chemical changes (Sweeting, 1994), and biological changes (Courtenay.
<br />1993,1995). Some large tropical river systems remain undammed such as
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