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~ Invertebrates play an influential role in continuing the assimilation of potential ~ productivity into aquatic food webs. Aquatic invertebrates feed on periphyton, • phytoplankton, and detritus, incorporating nutrients and biomass derived from the floodplain into the riverine food web. Grazer, shredder, and filter feeding invertebrates ~ are in tum eaten by predatory invertebrates and fish, thus assimilating floodplain ~ productivity into the top levels of the food web (Fig. 1.1). ~ ~ Fish ~ ~ ,Invertebrates ~ ~ Detritus ~ ~ ~ i ~ Floodplain ~~ ~ Inundation ~ ~ ~ Nutrients ~ Phytoplankton/ r eri h on • Figure 1.1 Food web of the middle Green River floodplain. ~ Zooplankton and aquatic invertebrate larvae are an important food resource to ~ juvenile fish. Therefore, the high density and diversity of invertebrate assemblages • within floodplain habitats represents an augmented food resource to juvenile endangered fish. Because of enhanced productivity of food organisms, floodplain ~ wetlands facilitate rapid fish growth and survival within the areas (Osmundson 1986) ~ and enhance fish survivorship and growth within the river system. Native young-of-the- ~ year (YOY) fish that leave the river and utilize floodplain wetlands should be larger than • if they remained in the river. When these larger native fish enter the river they should be less susceptible to predation by most nonnative fish in the main channel. Thus, it is ~ hypothesized that floodplain restoration will provide larval/juvenile endangered fish ~ access to high quality nursery habitat with enhanced food resources and favorable • physical conditions that will enhance survivorship. These potential benefits could be offset, however, by increased production of nonnative fish that also utilize floodplain ~ habitats. Successful establishment of nonnative fishes has already resulted in their ~ dominance in both the main channel (Day et al. 1999; Trammel and Chart 1999) and • floodplain habitats (Modde 1997) of the Green River. Many nonnative species now present in the Colorado River system are potentially very important as limiting factors to ~ native fish survivorship and recruitment. Channel catfish (Ictalurus punctatus), northern ~ pike (Esox Lucius), walleye (Stizostedion vitreum), green sunfish (Lepomis cyanellus), ~ and smallmouth bass (Micropterus dolomieur) all pose significant threats to native 1.4