Another question is why does the wetted ^perimeter curve 

 for riffles, areas generally uninhabited by '"adult trout, pro- 

 vide acceptable flow recoimnendations. If one assumes that 

 trout populations are food limited, then the wetted perimeter 

 curves for riffles, which are generally considered the pri- 

 mary invertebrate producing areas of a river, may provide an 

 index to the river's capacity to produce trout food organisms. 

 Below the inflection point, the area available for food pro- 

 duction greatly diminishes. The acceptance of this premise 

 is unlikely since living space rather than food supply is gen- 

 erally believed a more influential limiting factor on Montana's 

 trout rivers. 



The acceptance of the single transect method as a valid 

 means for deriving minimum flow recommendations implies that 

 the wetted perimeter curve for a riffle cross-section somehow 

 relates or provides an index to the physical needs of adult 

 trout. At present, the acceptance of this method will have to 

 be based solely on its consistency as a predictor of minimum 

 flows since a realistic explanation for its apparent effective- 

 ness is lacking. 



The question of the reliability of the wetted perimeter 

 predictions derived from the IFG-4 model will not be totally 

 resolved until the data are rerun using a model that directly 

 predicts rather than approximates the wetted perimeter. The 

 author believes that the IFG-4 predicted wetted perimeters, 

 even though approximations, are still superior to those gen- 

 erated by the WSP model due to the greater accuracy of the 

 predictions of water surface elevations. 



Additional testing of the single transect method using a 

 better wetted perimeter predictive model will be needed be- 

 fore the method is fully accepted for use in Montana's in- 

 stream flow program. Existing cross-sectional data collected . 

 in other drainages of the state will be analyzed using a 

 wetted perimeter program being developed for the Montana De- 

 partment of Fish, Wildlife and Parks to determine if the 

 recommendations derived by the single transect method are 

 reasonable. Acceptance of these recommendations will be 

 based solely on professional judgment since little long-term 

 biological data is available for deriving comparable recom- 

 mendations. An additional question to be answered is whether 

 the site of the inflection point for a single riffle cross- 

 section is similar for all riffles within a river reach. A 

 comparison of the wetted perimeter curves for a series of 

 riffle cross-sections is needed to resolve this question. 



93 



