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~::.:.~ " (,) , .' The influences of limited bioproduction on distribution and abundance of fishes is not clear, but may contribute to observed decline of trout with distance from the darn, Nwnbers of indigenous minnows, including the endangered hump back chub, and suckers also are low throughout the river corridor from the dam to Lake Mead, while non-indigenous fishes are often surprisingly abundant in the turbid lower most reaches (Valdez 1993). Fish distribution and abundance may be supported to some extent by ephemeral inputs of terrestrial insects and detritus and the ability offish to move in and out of tributaries to forage. Many may be migratory from Lake Mead. Moreover, fishes may be able to utilize low- velocity habitats (e.g., backwaters) created where large eddy systems develop. Eddies tend to entrain drifting organic, matter that fishes can feed on efficiently and bioproduction of fish forage may be stimulated in backwaters by inputs of nutrient-rich ground water derived from interstitial flow through the eddy complex alluvium. However, enhanced bioproduction in backwaters, relative to the river channel, usually is negatively influenced by fluctuating flows which destabilize substratwn, nutrient supply, temperatures and water clarity (Stanford, 1993). The highly autotrophic dam tailwater (dam to Paria River) produces and exports POM which apparently serves as the prinwy energy source for heterotrophic ~ses that control bioproduction within turbid reaches downstream. Cycling of drifting particulate organic matter in the turbid reaches of the river is however likely flow dependent Higher flows scour more POM in the tailwaters but also transport it further downstream before it can be entrained and recycled. Lower flows scour less POM in the tailwaters, but may allow more efficient entrainment and recycling in downstream food webs. Understanding flow-mediated controls on bioproduction of riverine food webs clearly should be examined in a long term context along with geomorphic processes as determinants of flow regimes to protect and enhance canyon resources. Episodic high flows are needed to maintain geomorphic features and it is assumed that, owing to the coupling ofbioproduction to its physical setting (sensu Southwood, 1977), episodic flooding is important to food web structure and function (Stanford, 1993). Base flow operations OCClU'most of the time and have been assessed for certain food web variables (Blinn and Stevens 1994). Although, influence of flood flows on the riverine food web has not been assessed quantitatively, studies during GeES Phase I and Phase n indicate that as the dam discharge increased into the upper range of power plant capacity, sediment and organic matter became entrained (Leibfried and Blinn 1988). In many cases, organic matter drift ClI.gH1 by these elevated discharges included fragments of Cladophora, both living and dead, a result of exposure and desiccation during extended low discharge periods. This last phenomenon has been suspected to be a cause of a reduced food base for the Lees Ferry trout population, identified by the Arizona Department of Game and Fish Department as a primary reason for reduction in vigor and size of trout during research flows. The significance of quantity and composition of organic drift relative to dischar~ is determined through our knowledge of the importance of the role of each component in the drift as a "loss" from one part of the riverine ecosystem or as a "resource" for other downstream components of the ecosystem. Thus, information on organic matter drift is important both on a system-wide and reach basis, the former indicating how drift from one reach or tributary might serve downstrearn reaches, while the latter indicating how organic matter loss and/or change in one reach might be detrimental to other aquatic components of that reach. ;, , , 1 i " " :, . ? ~, ;.; ~ 1\1 ~ ~ ~~ " j~ " ~" ,. .' ,. ~~ !:- ~; " ~ ~:.~ .!" ~."" '" ~ :k.~ .~ '.,. ;', 19