The single transect method has other advantages. The extra effort and 

 uncertainties involved in the selection of representative subreaches and the 

 placement of multiple cross-sections are eliminated as is the need for large 

 field crews and elaborate boat operations. Field data can generally be col- 

 lected by a crew of two since most riffles are wadable. 



The acceptance of the single transect method as a valid means of deriving 

 flow recommendations for adult trout implies that the wetted perimeter-dis- 

 charge curves for riffle cross-sections bear some similarity to the relation- 

 ship between trout standing crops and flow. Below the inflection point flow 

 on the wetted perimeter-discharge curve, the capacity of each of the study 

 rivers to sustain adult trout greatly diminishes. Why the wetted perimeter- 

 discharge curves for riffle cross-sections relate to the carrying capacity 

 is presently unclear. 



One possible explanation is that the adult trout populations are food 

 limited and the wetted perimeter-discharge curves for riffles, which are 

 generally considered the primary invertebrate producing areas of a river, 

 provide an index to a river's capacity to produce trout food organisms. 

 Below the inflection point flow, the area available for food production greatly 

 diminishes. The acceptance of this premise as the sole explanation for the 

 apparent effectiveness of the single transect method is unlikely since living 

 space rather than food supply is generally believed a more influential limit- 

 ing factor on Montana's trout rivers. At present, the original premise that 

 the wetted perimeter provides an index of the amount of adult trout habitat 

 and recommendations derived from riffle cross-sections provide for more than 

 the minimal habitat needs of the adults appears to be the most acceptable ex- 

 planation. 



Multiple Transect Meth od 



The wetted perimeter-discharge curves for a composite of cross-sections 

 provided acceptable flow recommendations for adult trout in only three of 

 the reaches. Inflection points on the wetted perimeter-discharge curves were 

 generally not as well defined as those for the single transect method and in 

 one reach more than one were present. 



It is probably best to use multiple transect data to support the recom- 

 mendations derived from a more reliable field method such as the single tran- 

 sect method previously discussed. In situations where supportive recommen- 

 dations are desired the additional time, expense and manpower required to 

 collect multiple transect data may be justified. 



IFG Incremental M e thod 



The WUA values generated by the IFG incremental method for the rivers 

 of the study area do not provide an accurate index of the actual amount of 

 habitat that is available for adult brown and rainbow trout at the selected 

 flows of interest. As a result, the IFG flow recommendations for the five 



