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In 236 of 305 cases (77%), the 7-day minimum flow was less than IO% of the average flow, which I consider in the range of severe degradation for most elements of the aquatic environment. Criteria for 3-day minimum flows would be worse yet, and recommending the meager, alltime, historic minimum flow would be unthinkable. That would be like prescribing a person's alltime worst health condition as a rec- ommended level for a portion of his future well-being. Avoid recommending vacillatillg flow regimens specifically for fishes, coincidental with monthly (m.a.f.) or daily (d.a.f.) average flows because there are too many unknown, degrading effects on the life cycles of many other organisms in the aquatic environment and other uses of water to justify frequent ebb and flow changes in most streams. Quarterly variations in flow regimens might approximate climatic hydrology and be appropriate. The Montana Method can easily be modified to suit the individual convictions of any biologist or the monthly or daily hydrological variations of any given stream. I usually recommend variation in flow regimens for 6-month periods (e.g., columns 3 and 4 of Table I) that mimic nature and coincide most naturally with the so-called climatic year used by the U.S.G.S. and the u.s. National Weather Service. This offers the following distinct advantages:12 a) Stages of ground water are more nearly uniform on October 1 in most inland sections of the United States than on any other date, leading in general to fairly reliable com- parisons of annual rainfall-runoff relations because of the relatively small errors due to annual differences in the ground-water table on that date; b) In the arid and semi-arid parts of. the country, September 30 usually marks the end of the irrigation season and in humid regions 14