circle of natural resources planners who were aware of the critical need for 

 improved information. 

 9. The President's water policy initiatives of June 1978 included water conserva- 

 tion and protection of the environment by "directing all Federal agencies to 

 incorporate water conservation requirements in all applicable programs. . .and 

 by requiring agencies to fund environmental mitigation plans at the same time 

 projects are being built." 



Methods Development 



In addressing an instream flow problem, the fishery manager is often confronted 

 with three sequential questions: 



1. How much water is needed to maintain the fishery? 



2. What happens if that much water (or a particular release schedule) cannot be 

 provided? 



3. How many fish are gained or lost with different levels of streamflow in the 

 river? 



A plethora of various methods has been devised to answer the first question. 

 Where conflicts over a supply of water are minimal, any of these methods may be 

 used with satisfactory results. However, as in the case of any resource in short supply, 

 conflicts regarding the allocation and use of water are the rule rather than the 

 exception. Therefore, the second question is frequently asked almost in the same 

 breath as the first one. Methods designed to determine a "minimum flow 

 requirement" have been found to be insufficient to address the question of 

 incremental effects of changes in discharge. 



Most all instream flow assessment methods in use during the 1970s fall into either 

 the rule-of-thumb (hydrologic based) or the physical habitat (hydraulic based) 

 categories. These have been reviewed in depth elsewhere. ^'^''^ The following summary 

 appears useful to denote the principal difference between the two categories (see 

 references 5 and 6 for similar discussions). 



The need for rule-of-thumb procedures came from the water planning and water 

 administration professions which are used to deal with good historical data bases of 

 streamflow and watershed (catchment basin) runoff records. Such methods based 

 upon specified percentages of average annual conditions gave rise to the "minimum" 

 flow concept for allocating water among offstream and instream uses. These methods 

 are useful when evaluating water availability on an annual basis for planning 

 purposes or granting of water rights under legislative processes. Most fishery 

 administrators do not approve of the use of rule-of-thumb derived "minimum"flows 

 for fishery maintenance when a stream is regulated (controlled by dams or 

 diversion). ^8 That is to say that the "operating rules" by which water is managed 

 must recognize the dynamic nature of the flow regime present in stream systems and 

 cannot be reduced to a single fixed flow value. 



This discontent with the rule-of-thumb approach seems to stem from the 

 hydrologist-engineers' perspective that the fishery does not require all of the 

 streamflow during any time other than infrequent drought conditions.* This 

 perspective has led to the unfortunate use by planners of such historic low flow values 

 as: the 7-day Qio (the lowest flow occurring for 7 consecutive days once in 10 years), 

 the 90% exceedence flow, 10% of mean annual discharge, and even the lowest flow of 

 record, as the selected minimum flow for instream protection. Such schemes fail to 

 recognize that the fishery is a dynamic resource which can tolerate extreme drought 

 conditions on infrequent occasions but cannot tolerate these low flows on a sustained 

 basis without extreme reductions in the production and yield of the fishery. 



Tessman' adequately summarized this concern when he stated "the best minimum 

 flow model is one that mimics nature. . . The year is a continuum of cyclic events to 

 which the natural community is adapted. Minimum flow expressed as total volume 



125 



