Evaluation of Instream Flow Methods and Determination

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~ o ~ ~ ~ ~ ~ " ~ 25 ame out with greater WUA than rainbow, brown, or cutthroat. This ~ndicates to me that something is inherently wrong with the brook trout probability curves and that a major review of brook trout 'curves is required. However, Bovee (personal communication) reports 'that if temperature probability curves are used in the matrix cal- culations for brook trout the correlation between WUA and biomass is much better for brook trout. , The percent weighted usable area (dependent variable) for various species, life stages, and streams was plotted against median year water discharge pattern, one in five low water year, and one in five high water year discharge patterns (independent variable). These graphs are found in Appendix C. Comparison of Minimum Flow Recommendations Recommendations made with the four methods were similar in most instances (Table 10). The greatest discrepancies occurred between the Hontana Method and the other three methods. In five of the eighteen streams reaches the Montana Method gave quite different results from the other three methods. In four of the five instances (Cucharas, Huerfano, South Fork of the Rio Grande, and Sangre de Cristo) these differences can quite readily be ex- plained by the placement of the U.S.G.S. gage in relation to the study area location. On these streams the gage is several miles below the study area and below the confluence with other tributaries. If the influence of these tributaries could be subtracted out, the Montana Method would closely approximate the recommended flows obtained with the other three methodologies. The only instance where the Montana Method gave a higher recommendation than the other methods and no direct explanation was discernible was on the Lake Fork of the Gunnison River. No tributaries or diversions increased or decreased the flow of this river between the U.S.G.S. gaging station and the study reach. How- ever, it has been observed by DOW biologists over the past few years that on large streams with peak flows in excess of two orders of magnitude greater than the annual minimum flow, the Montana Method generally gives a much greater recommended minimum flow than the R-2 Cross Method. This is probably the case with the Lake Fork of the Gunnison River where peak flows in excess of 3,000 cfs are contrasted against winter minimums of about 30 cfs.