IFG Incremental Method. The IFG recommendations are expected to exceed 

 the upper limits of the flow ranges derived from the trout-flow data since 

 tlu-y are intended to maximize the total available habitat for adult brown and 

 rainbow trout and, thereby, sustain near optimal adult standing crops. Three 

 of the 1 FC recommendations (Madison //I, Madison //3 and Gallatin //2 reaches) are 

 in fact less than the lower limits of these flow ranges. These results led to 

 an examination of the IFG model and its applicability to river environments. 

 Ways for improving the IFG model for use on Montana's trout rivers were sug- 

 gested by this examination. They are briefly discussed as follows: 



The mean velocities in the water column, one of the variables used by the 

 IFG model for computing the WUA, have little relation to the bottom velocities 

 commonly chosen by the adult trout inhabiting the relatively high gradient, 

 cobble and boulder strewn rivers of the study area. VJhen the IFG model is 

 adjusted to use bottom velocities rather than mean velocities for computing the 

 WUA, the WUA-discharge curves for the study reaches are markedly altered. Figure 

 3 illustrates the magnitude of the changes that can occur to the WUA-discharge 

 curves for adult brown and rainbow trout when bottom velocities are substituted 

 for mean velocities. The use of the bottom velocities is a more realistic ap- 

 proach since adult trout are generally considered bottom oriented. 



WUA-discharge curves were derived using bottom velocities for three of 

 the study reaches. When these "new" curves are incorporated into the minimum 

 variance matrices previously discussed, the flows that maximize the total 

 available habitat for adult brown and rainbow trout in the Madison #1, Gallatin 

 //2 and Big Hole //I reaches are now 31.2, 8.5 and 25.5 m /s, respectively. 

 These flows are 7.1, 2.1 and 11.3 m /s, respectively, greater than those pre- 

 viously derived using mean velocities. It is evident that the use of the mean 

 velocities by the present IFG model as one of the variables for computing the 

 WUA is unacceptable for Montana's trout rivers. 



Based on the comparison of these "new" IFG recommendations for the 

 Madison #1, Gallatin //2 and Big Hole //I reaches with those of the trout-flow 

 data, it was concluded that other modifications in addition to the incorpor- 

 ation of bottom velocities into the model are needed in order to improve the 

 reliability of the IFG recommendations. 



Gover, a variable influenced by flow and shown in many cases to be highly 

 correlated with trout standing crops, is one factor that has the potential 

 for altering the WUA-discharge curves and, thereby, improve the IFG recommen- 

 dations. The IFG staff is presently incorporating cover into their model. 



I'robnbi 1 i ty-of-use curves developed for river trout populations are 

 another modification that may be needed. The probabil ity-of-use curves de- 

 veloped by the IFG staff and used in this study were primarily derived from 

 data collected on streams and creeks. These curves may not be applicable to 

 the larger waterways. 



Data that would determine the magnitude of the changes these two modi- 

 fications (addition of cover and probabil Ity-of-use curves for river trout 

 populations) would have on the computed WUA and the resulting recommendations 



