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26 BIOLOGICAL REPORT 24 the Upper Colorado River Basin. Despite dem- onstration of important effects of predation and competition for food resources, little infor- mation exists about the ecology of nonnatives in the Upper Colorado River Basin. High flows seem to reduce numbers of nonnative species, and diversion dams installed many years ago (e.g., Redlands on the Gunnison River) may have segregated nonnative populations and limited range expansion; however, much more information is needed. I suspect that consider- able unpublished data exist in files as a conse- quence of the sampling effort required to col- lect significant numbers of the endangered fishes. If so, the information should be exam- ined relative to what is known about the native species and published. If not, sampling proto- cols should be developed to describe trends in nonnative populations in all segments of the river. In addition, experiments are needed to clarify interactions between natives and non- natives. 8. River ecosystems are too complex to be de- scribed by deterministic models or constructs of individual attributes. Ecosystem compo- nents are N-dimensional and inherently vari- able (stochastic), and they interact in complex ways that cannot be predicted from logistic equations. Construction of an ecosystem model that describes all of the dynamic processes discussed above is likewise unreasonable as a predictive tool. Therefore, the prudent alterna- tive is to use all available ecological informa- tion to derive and implement a flow regime for the Upper Colorado River Basin ecosystem and to quantify variables (e.g., location of serial discontinuities, bioproduction of food webs, condition and quantity of low velocity habitats, availability of spawning habitats, spawning success, population dynamics of native and nonnative fishes) that describe whether the ecosystem is changing in a way that favors recovery of the fishes. Derivation of Flows Currently Recommended to Protect the Endangered Fishes Review of Instream Flow Methodology For well over 2 decades many different re- searchers have toiled to derive a general (easy to use), precise (gives the same answer in repeated tries), and real (accurately describes the many interactive processes that occur in nature) model to predict stream flows to protect fish and inverte- brates. Considering the myriad factors that influ- ence the distribution and abundance of endan- gered fishes in the Upper Colorado River Basin, and how intractable controlling factors become when many different river systems and biota are of interest, the search for such a model is formida- ble. Nonetheless, instream flow modeling has been fostered by the extreme value of water and the unwillingness of water development interests to "experiment" with flows on a river-by-river or even segment-to-segment basis. Much litigation has re- sulted over the need to maintain flows within river segments to protect biota and channel and flood- plain features at the expense of flow depletion for other human uses or at the expense of less flexibil- ity for hydropower operations. Flow Threshold Models A two-volume proceedings (Orsborn and All- man 1976) of a special symposium on rationale for and approaches to instream flow methodology sponsored by the American Fisheries Society and the American Society of Civil Engineers set the stage for this endeavor to couple management-ori- ented aquatic science with the physical mechanics of water flow in stream channels. From the outset a fundamental tenet of the evolution of instream flow methodology was that something simpler (less mathematical) and more intuitive (to field personnel working for management agencies) than full-blown ecosystem simulation was needed. Consequently, the methodology has tended to focus on economically important fishes and their habitat "preferences," as determined by flow. This should not be surprising because a primary objective of wildlife and fisheries man- agement for decades has been to protect and en- hance species-specific habitats to maximize carry- ing capacity and hence maximize harvest of surplus biota. The first widely used methods were entirely based on the fact that, below some flow threshold, physical habitat becomes limiting to fish and other stream biota during some part of their life cycle. The most commonly used method was the "Montana" method (Tennant 1975 and various modifications, see Wesche and Rechard 1980 for review), which attempts to relate perceived prob- lems, though rarely quantified (my observation,