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<br />002638 <br /> <br />The primary goal of the Modde and Smith 1995 report was to maintain a relatively natural <br />hydro graph. High spring flows were identified as necessary to support biological processes, with <br />relatively stable base flows to support fish through the late summer, fall and winter based upon <br />natural variability (Table I). <br /> <br />Table 1. Monthly base-fiow targets (cfs) based on 80% exceedance of estimated undepleted <br />daily fiows 1 of the Yampa River at Maybell, Colorado (Modde and Smith 1995). <br /> <br />NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT <br />172 157 187 221 305 1150 4153 3326 175 125 45 88 <br /> <br />1 Hydrosphere 1995 <br /> <br />In theirreport entitled De/ermina/ion of Habitat Availability, Habitat Use, and Flow Needs of Four <br />Endangered Fish in the Yampa River Between August and October, Modde et al. (1999) took a new <br />approach to estimate instream flow needs of the endangered fishes in the Yampa River during the <br />base-flow period. After testing several approaches, the authors selected a curve-break analysis to <br />estimate base-flow targets for the Yampa River. This approach simulated habitat availability at <br />several different base-flow levels to identify available amounts of three different meso-habitats- <br />riffles, runs and pools- as a function of discharge. Riffles are considered to be most sensitive to <br />changes in stream flow. They also contribute significantly to the production of macro invertebrates <br />that serve as the basis of a food web for the endangered fishes. Therefore, habitat data from riffle <br />transects were used in this analysis. The curve break was determined by plotting the availability <br />of several important habitat parameters, such as depth, velocity and wetted perimeter (y-axis) <br />against stream flow (x-axis) for each transect; calculating a linear regression of these data; and <br />determining at what flow a residual (difference) between the curve and regression line was greatest. <br />Using this methodology, an average curve break of all riffle transects, 93 cubic feet per second (cfs), <br />was determined to be the target base flow for the Yampa River from August through October. The <br />study concluded that flows of 93 cfs or greater would be sufficient to maintain instream riffle <br />habitats critical for production of prey organisms for the endangered fishes during this period. <br />However, the study also concluded that flows of this magnitude need only be achieved at their <br />historical frequencies and durations. In other words, Yampa River flows had fallen below 93 cfs <br />in the past and may do so in the future, as long as they do not fall below 93 cfs more frequently or <br />for longer periods than had occurred in the past under otherwise similar hydrologic conditions <br />(Modde et a!. 1999). <br /> <br />Base-flow Recommendation <br /> <br />By adopting the Modde et al. (1999) August through October base-flow target of 93 cfs in an <br />historical context, the Service has, in effect, modified its 1995 recommendations (Modde and Smith <br />1995; Table I). Moreover, gage data indicate that Yampa River flows at Maybell occasionally <br />have fallen below 93 cfs in July, as well as from November through February. Therefore, for the <br />purpose of developing a base-flow augmentation strategy, the Service extended the base-flow <br />period to include July through February. However, the Service recognizes that winter flow <br />needs of the endangered fishes are not as clearly understood and, given these uncertainties, cannot <br />justify extending the 93-cfs flow target beyond October. Nor can the Service reaffirm its 1995 <br />winter flow recommendations based exclusively on statistical analyses of historical data, without <br /> <br />~ <br />r <br />r <br /> <br />Appendix D - Revised base-flow recommendations for the Yampa River <br /> <br />D-2 <br />