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BIOLOGICAL REPORT 90(5) 31 Minimum Flow is a Myth by Clair B. Stalnaker U.S. Fish and Wildlife Service National Ecology Research Center 4512 McMurray Avenue Fort Collins, Colorado 80525 During the warm water stream symposium in 1980, when discussing low flow as a limiting factor in warmwater streams, I warned fisheries manage- ment personnel that the concept of a single minimum or base flow for fishery habitat maintenance that has evolved in the western region of the United States could very well become a real threat to low gradient, eastern, warmwater stream fisheries (Stalnaker 1981). The minimum flow concept rose from western water law as a mechanism to either reserve an amount of water from future appropriations or as a means of granting an instream water right for fishery purposes. This led to the myth that a consis- tent methodology could be used to establish a single minimum discharge value for any given stream. Ex- perience has shown that as water becomes fully ap- propriated to upstream use or storage, the minimum flow, if not frequently violated in time, tends to become the average flow condition. Too often the minimum becomes the objective rather than the means to achieve some riverine fishery or recrea- tion management goal. Such persistent low flows are not necessarily desirable from the water manage- ment perspective, being inflexible in the face of com- petitive uses or during unusual water supply condi- tions (e.g., drought), and certainly do not meet all the desired environmental needs. This difficulty with minimum flows arises in part because all the in- stream uses for which flows may be needed are not identified. Most often overlooked are necessary periodic high flows that move bedload, flush sedi. ments, rejuvenate the floodplain, and generally maintain the structural characteristics of a stream channel, which should be maintained in dynamic equilibrium with its watershed (Stalnaker 1979). A common misconception among water manage- ment personnel and consumers is that inclusion of all the identifiable instream uses of water in an in- stream flow requirement will dictate an additive treatment of their respective needs. This, it is fur- ~ ther assumed, will result in total allocation of the stream flow to instream uses. Contrary to this view, a considerable degree of compatibility exists among many in stream uses and downstream delivery requirements for offstream or consumptive uses. However, in order to deal with these compatible uses, the instream flow advocate and the water resource manager must be aware of both the timing and the magnitude of all the demands being placed on the stream system. Such a common under- standing, which should lead to the identification of in stream flow requirements, will protect all compli- mentary uses as well as meet downstream delivery requirements. It is evident from reviewing the literature and from the discussions during this workshop that many methods for evaluating instream flow needs have evolved since the 1960's. I prefer to categorize such methods as "standard setting" or "incremental." Standard setting methodologies, on one hand, refer to those measurements and interpretive techniques designed to generate a flow value (or values) in- tended to maintain the fishery or recreational use at some acceptable level (usually dictated by policy). Incremental methodologies, on the other hand, are organized and repeatable processes by which (1) a fishery habitat-stream flow relation and the hydrol- ogy of the stream are transformed into a baseline habitat time series, (2) proposed water manage- ment alternatives are simulated and compared with the baseline, and (3) project operating rules are negotiated. Triheyand Stalnaker (1985) suggested that a hier- archical approach to hydro licensing and relicens- ing be followed that in essence takes advantage of both the standard setting and incremental ap- proaches. A three-tiered hierarchy was suggested including reconnaissance, feasibility, and operational or design studies for evaluating hydro projects. It is important to recognize that such licensing is