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increased demand for water in storage and changes ~n reserV01r operating policy. Plains fisheries near the Ft. Union formation--in the lower Yellow- stone, the Heart, the Knife and the Little Missouri--face both flow reduction and water quality degradation from coal development. Like blue ribbon sport fisheries, wild and scenic rivers tend to be head- water reaches that are remote from energy projects. Exceptions are the Little Missouri, the Dolores, and segments of the Green, the Yampa and the Upper Colorado Mainstem. III. INSTREAM FLOW ASSESSMENT AND PROTECTION , A. Methodologies Given a survey of critical reaches, one wants to know how energy and water development projects affect instream uses and how these uses can be protected. Energy development has three basic effects on the physical system. First, virtually any energy activity consumes water, thus reducing the quantity of flow. Second, storage facilities, which are usually necessary for low flow periods, alter the timing of flow. Third, surface and under- ground mining affect the quantity, timing, and quality of runoff from aquifer discharge. A fourth possible effect, discharge of residuals in waste water, is less pertinent because of the stringent effluent limitations mandated by PL 95-217, the Clean Water Act. The most obvious consequence of energy develop- ment then, is alteration of the timing and rate of flow (ft3/ s). 4 How do changes in flow influence other hydraulic parameters such as depth, velocity, and temperature? In turn, how do these physical changes affect fish popula- tion and distribution, riparian vegetation, and recreational activity? Finally, how can instream values be protected? The simplest and quickest protection measure is recormnendation of flow regimes on the basis of average flows (called the Montana or Tennant method). A 10 percent (of) average flow is said to sustain short-term survival of aquatic life; 20-40 percent average is considered a satisfactory range for fishery flows, while 40-60 percent and 60-100 percent are con- sidered excellent and optimum ranges res- pectively (Stalnaker and Arnette, 1976). Such a rule of thumb is useful because of its broad applicability. It is less valuable when the concern is protection of particular fishes and of recrea- tional activities and aesthetic values. (The Montana method assumes that flows sufficient for fisheries are sufficient for other activities.) More- over, where competition for water is great, as in the Rocky Mountain/Northern Great Plains Region, more technical and exact information is necessary to justify the reservation of flows and the foreclosure of water development. One of the most promising approaches to streamflow evaluation is the "Incremental Methodology" being developed by the Instream Flow Group (IFG) of the US Fish and wildlife Service. The methodology has four components: 40ther m~n~ng and in-situ conversion water quality impacts are considered as special situations that should be addressed case-by-case. 3