Multiple Transect Method , ' 



The wetted perimeter curves for a composite of cross- ,^,- 

 sections within each river reach generally did not provide . . 

 single, well defined inflection points on which to derive 

 minimum flow recommendations. VThen present, inflection points 

 were not as readily discernible as those in the single tran- ; 

 sect method and in some cases more than one were present. 

 While the multiple transect method did provide acceptable ab- 

 solute minimxim flow recommendations for the four reaches v . ..^ 

 having discernible inflection points, it had no advantage 

 over the single transect method. It was costlier, more time 

 consuming, required greater effort to locate sampling sites, .-. 

 sometimes difficult to interpret, and occasionally unpro- 

 ductive. 



The study results indicate that in most cases the multi- .-» . 

 pie transect method can provide acceptable absolute minimum 

 flow recommendations. It is probably best to use multiple 

 transect data to support the recommendations derived from a -; 

 more consistent field method such as the single transect me- 

 thod previously discussed. In critical instream flow situa- 

 tions where supportive recommendations are desired, the addi- .-,,• 

 tional time, expense and nanpov;er involved in collecting 

 multiple transect data may be justified. 



The reliability of the wetted perimeter curves derived for 

 the multiple transect method was questioned due to the greater 

 error associated with the predictions of water surface eleva- 

 tions by the WSP model. The accuracy of the predicted water 

 surface elevations can be improved by supplying water surface ,• 

 elevations for a series of known flows rather than a single ^ , 

 flow as was done in the study. These additional data were 

 available but not used in calibrating the WSP model. In past 

 years the Montana Department of Fish, Wildlife and Parks has 

 generally collected only one set of water surface elevations 

 due to time and manpower limitations. Since this has been a 

 typical practice, an evaluation based on more than one set of 

 calibration data was considered inappropriate. At present, 

 the author believes it is best to avoid using the WSP model 

 to generate wetted perimeter curves for the high gradient, 

 boulder and cobble-strewn rivers until additional testing r^ • .c 

 clarifies the model's reliability. , r. , ., ^ . -a -■■- 



The acceptance of the multiple transect method as a valid 

 means of deriving minimum flow recommendations implies that .» 

 the wetted perimeter curve for a composite of cross-sections 

 encompassing various habitat types somehow relates to the physi- 

 cal needs of adult trout. As previously discussed for the 

 single transect method, a precise explanation for a wetted 

 perimeter and standing crop relationship is presently lacking. 

 Acceptance of this methodology will have to be based solely 

 on the apparent reliability of its recommendations. 



94 



