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<br />discussed above is likewise unreasonable as a predictive tool. Therefore, the prudent alternative is <br />to use all available ecological infonnation to derive and implement a flow regime for the Upper <br />Colorado River Basin ecosystem and quantify variables (e.g., location of serial discontinuities; <br />bioproduction of food webs; condition and quantity of low velocity habitats; availability of <br />spawning habitats, spawning success, population dynamics of native and nonnative fishes) that <br />describe whether the ecosystem is changing in a way that favors recovery of the fishes. This <br />approach is described in detail below in Section VI. <br /> <br />N. Derivation of Flows Currently Recommended to Protect <br />the Endangered Fishes: Review and Synthesis <br />Review of In stream How Methodology <br />For well over two decades many different researchers have toiled to derive a general (easy to <br />use), precise (gives the same answer in repeated tries) and real (accurately describes the many <br />interactive processes that occur in nature) model to predict stream flows to protect fish and <br />invertebrates. Considering the myriad of factors that influence the distribution and abundance of <br />endangered fishes in the Upper Colorado River Basin described above, and further considering <br />how intractable controlling factors become when many different river systems and biota are of <br />interest, the search for such a model is fonnidable indeed. <br />Nonetheless, instream flow modeling has been fostered by the extreme value of water and <br />the unwillingness of water development interests to "experiment" with flows on a river-by-river or <br />even segment-to-segment basis. Much litigation has resulted over the need to maintain flows within <br />river segments to protect biota plus channel and floodplain features at the expense of flow depletion <br />(abstraction) for other human uses or at the expense of less flexibility for hydropower operations. <br /> <br />Flow Threshold Models <br />A two-volume proceedings (Orsborn and Allman 1976) of a special symposium on <br />rationale and approaches to instream flow methodology sponsored by the American Fisheries <br /> <br />45 <br /> <br />'. <br /> <br />'. <br /> <br />. <br />