IFG Incremental Method 



The IFG incremental method enables the biologist to quantify the potential 

 impact of changes In stream flow on the habitat of a selected fish species and 

 life stage. The physical variables of depth, velocity and temperature as 

 associated with substrate are assumed to be the most important fish habitat 

 variables when quantifying habitat changes under differing flow conditions. 

 The habitat index calculated from these variables is termed the "weighted 

 usable area" (WUA) , The IFG model calculates a WUA value for each flow and 

 life stage of interest. This method is described in Bovee and Cochnauer 

 (1977), Bovee (1978) and Main (1978 and 1978a). 



Flow recommendations were derived from the calculated WUA-discharge 

 relationships for the life stages of interest using minimum variance matrices, 

 a technique originally developed by Denwood Butler, U. S. Fish and Wildlife 

 Service, Ft. Worth, Texas and modified by the IFG staff (Ken Bovee, personal 

 communication). This technique is used somewhat differently than recommended 

 by the IFG staff since our sole objective is to identify the single flow that 

 maximizes the total available habitat for adult brown and rainbow trout within 

 each study reach. The derivation of monthly flow recommendations that maximize 

 the total available habitat for all species and life stages of importance 

 during each month and are within the constraints of water availability for a 

 median (normal) and one-in-ten (drought) water year are not objectives of this 

 paper. 



The minimum variance matrices, as used in this study, are constructed by 

 arraying flows in ascending order across the top of each matrix. Arrayed 

 down the side of the matrix are the species and life stages of interest (the 

 adult stages of brown and rainbow trout). For each life stage and flow, the 

 percent reduction from the optimum WUA is listed on the matrix. After the 

 matrix is completed, the maximum variation from optimum in each flow column 

 is listed below the matrix. The flow with the smallest maximum variation is 

 the flow that maximizes the total available habitat for adult brown and rain- 

 bow trout. The following example illustrates this technique as applied to 

 reach //I of the Madison River: 



