of instream requirements during the course of a year is meaningless unless 

 streamflow is distributed properly during this period." 



For the purpose of water planning and interim determinations of the availability of 

 water for future development, the median or average monthly flow values are 

 accepted as more representative of the flow necessary to maintain a heahhy fishery 

 resource.* In such analyses, flows in the "optimum" or "acceptable" range are much 

 less controversial among fishery managers and ecologists. Following are examples of 

 rule-of-thumb approaches used in reconnaissance level evaluations of water resources:' 



• Median monthly flow values equal to 79-100% of the average flow for each 

 month of record. 



• Monthly minimum flows equal to the mean monthly flow (MMF) if 

 MMF<40% of mean annual flow (MAF). If MMF>40% MAF, monthly 

 minimum flows equal 40% MAF. If 40% MMF>40% MAF, monthly 

 minimum flows equal 40% MAF. 



• Single values of 60-100% of mean annual flow or 70-130% of the natural 

 characteristic low or base flow. 



Most applications of these methods to early-planning now recognize that flood 

 flows are also needed to cleanse the substrate and otherwise maintain the physical 

 integrity of the stream channel. Bankfull flows are generally now recognized as 

 necessary for maintaining channel cross-sectional integrity. However, the frequency 

 and duration of these flows are the basis of much argument. 



Physical Habitat Analysis 



Many researchers have documented the preference of stream fishes for particular 

 ranges of depths, velocities, substrate size, cover objects, "''"''2»'3,i'» and tempera- 

 ture."''* Nearly all site specific methods proposed to date are based upon 

 measurement of these important stream variables. 



All physical habitat-flow analyses can be further grouped in two categories: (1) 

 those based upon threshold conditions at critical or limiting macrohabitat features, 

 and (2) those based upon microhabitat features within specified (sometimes called 

 representative) stream reaches. 



Threshold methods. The methods require that species criteria for depth, velocity, 

 and substrate be specified. These criteria usually take the binary form with a specified 

 range. (See Stalnaker and Arnette,^ and Wesche and Rechard'» for summaries of 

 reported criteria.) The other necessary step is the measurement of depth, velocity, 

 and substrate along transects placed over the stream channel. When measured at 

 several different discharges, the "usable width" across the measured transect can be 

 computed. Variations on this approach are described elsewhere. ^'^ Another method 

 which has often been used is the measurement of wetted perimeter at several 

 discharges. A plot of wetted perimeter vs. discharge is then produced. Such visual 

 methods rely upon either a peak or obvious inflection point on the curves which is 

 stipulated as the discharge which maximizes the "usable habitat" (i.e., the upper 

 threshold) in the stream channel studied. 



Arbitrary calculations for establishing the "minimum" threshold conditions have 

 been suggested such as: ( 1) 75-90% of the maximum or optimum value; (2) the value 

 at which a tangent, drawn through the origin of the graph, touches the curve; and (3) 

 the discharge which produces the maximum contiguous width along a transect 

 having some specified depth value. The "minimum" threshold values have no 

 documented biological basis and are the subject of much controversy among 

 ecologists. These threshold methods do not take into consideration the timing of flow 

 in the stream channel and, therefore, should be restricted to regulated stream 

 applications when storage in large reservoirs makes possible releases downstream for 

 maximizing fishery habitat conditions. 



126 



