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, Nutrient and Energy Transfer within a Large River Ecosystem flooding has an immediate effect on the composition of fish communities (Bain, 1990) and juvenile fish are especially susceptible to downstream displacement (Harvey, 1987), these changes are transient and fish commu- nities rapidly return to preflood conditions (Bain, 1990). In some fishes, the lateral movement of fish on floodplains decreases exponentially with reductions in streamflow (Kwak, 1988) that may prevent recruitment, par- ticularly if water levels remain low for several years in succession (Starrett, 1951). Larval fish must initiate feeding during the critical period before they reach a point of no return (Houde, 1987; Li and Mathias, 1982; Miller et al., 1988) or point of irreversible starvation when high mortality oc- curs from starvation (May, 1974; Li and Mathias, 1982) as larvae con- vert from endogenous nutrition (Le., egg sac) to exogenous feeding (Le., zooplankton and early instars of benthic invertebrates). Adequate den- sities of zooplankton and benthic invertebrate prey of the right size are important to the growth and survival of larval fish. At low prey densi- ties, predation as well as intra- and interspecific competition results in high mortality (May, 1974; Welker et al., 1994). The growth rate of larval fishes is extremely important to their survival because smaller larvae that are in poor condition (Le., starved) remain vulnerable to predation for a longer period of time (Leggett, 1986) and are more susceptible to preda- tion because they have less locomotive ability (Rice et al., 1987). However, fish larvae can resume growth quickly after short periods of starvation if they encounter high densities of food organisms of the right size be- fore they reach their point of irreversible starvation (MigIavs and Jobling, 1989). The year-class strength of fisheries is often determined by environmen- tal conditions such as suitable water temperature as well as the quality and quantity of food organisms during the critical period for fish larvae (Hjort, 1914, 1926). The density, size, timing, and duration of food availability must match the timing of the swimup stage for fish larvae. During years with optimum environmental conditions, high survival of larval and ju- venile fish produces strong year classes. The timing, extent, and duration of flooding greatly affect fish species that use floodplains and these fac- tors may exert a moderate-to-strong control in year-class strength of some fishes (Lambou, 1963; Bayley, 1991; Baker and Killgore, 1994). Aquatic Habitats in Large River Ecosystems A wide array of habitats occur in a river-floodplain ecosystem (Table 9.1). Clear, concise definitions of aquatic habitats are summarized by Armantrout (1998). The main channels of most rivers have a tendency to meander (Leopold and Wolman, 1957), producing deep habitats on the outside bends and shallow habitats on the inside bends. Stream- flow and size of the materials forming the river bed will sort to form rimes from deposition or pools from scouring. Both types of habitats are required by various life stages of fishes and other aquatic organ- isms. Many adult fish species require rimes with a specific water ve- locity and substrate composition for spawning and benthic macroinver- tebrate production for food. Pools with lower water velocity and depth provide cover and serve as resting or winter habitats. Shallow produc- tive waters such as backwaters and embayments in rivers and floodplain "f :~ 245 1 I j