IFG Incremental Method - ■'■'• .•*" '"' ' 



The acceptance of less than 50% of the optimum flow recom- 

 mendations indicates that the IFG method in its present state 

 of development is not a consistent method for deriving instream 

 flows for the trout rivers of Montana. Possible means for im- 

 proving the present model for use on Montana's trout rivers are 

 briefly discussed as follows. 



1. The present IFG model uses the mean velocity in the water 

 coliomn as one of the variables for computing the weighted 

 usable area. The mean velocities probably have little 

 relation to the velocities commonly chosen by the trout 

 within the column, particularly in the high gradient, 

 cobble and boulder-strewn rivers of Montana. 



The impact of this premise on the optimum flow recommen- ' - 

 dations generated by the IFG method was evaluated using 

 velocity data collected in the subreaches. These data 

 were used to modify the existing probability-of-use 

 curves for velocity in order to adjust for the model's' 

 use of the mean velocities rather than the bottom velo- 

 cities, generally believed the velocities to which the 

 trout are oriented. 



Velocity data for depths 5-2.5 ft in three of the sub- 

 reaches show that the mean velocities in the column 

 are highly correlated with the velocities at 0.8 of • 

 the depth (Appendix Table 32) . These relationships 

 were used to adjust the velocity curves for adult trout. 

 For example, the curve for adult rainbow trout on file 

 with the IFG assigns a probability-of-use of .95 for a ■ 

 velocity of 1.0 5 ft/sec. From Appendix Table 32 a 

 bottom velocity (0.8 of the depth) of 1.05 ft/sec in 

 the Madison #1 subreach corresponds to a mean velocity . 

 of 1.90 ft/sec. A probability of .95 is now assigned 

 to a velocity of 1.90 ft/sec, the mean velocity in the 

 col\amn. All data sets for the velocity curves for adult ' "' 

 rainbow and brown trout in each of the three subreaches 

 were adjusted in this manner. In order to make this 

 adjustment it was assumed that the velocity relation- 

 ships in Appendix Table 32 also apply to depths less 

 than 2 .5 ft. - • 



This single adjustment increased the flows at which the 

 optimum weighted usable areas occurred by 10 to 80% 

 (Table 33) . However, the optimum flows were not suffi- 

 ciently increased in the Madison #1 and Gallatin reaches 

 to compare favorably to those derived from the trout- 

 flow data (Table 20) . While a velocity modification of 

 the existing IFG model is apparently needed, it is not 

 the only problem area. 



96 



