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<br />002467 <br /> <br />Although the authors made no specific flow recommendations with respect to the remainder of the <br />base-flow period (November through March), the Service believes that flows of sufficient magnitude <br />also are needed during this period to ensure over-winter survival of the endangered fishes. <br />However, uncertainties exist as to the magnitude of winter flows needed. Therefore, the Service <br />extended base-flow recommendations through March, adding a 33% (31-cfs) buffer to the 93-cfs <br />flow target beginning November I (Appendix D). That is, flows at Maybell should not fall below <br />124 cfs during the winter with any greater frequency, magnitude or duration than they had under <br />historic conditions. <br /> <br />This buffer is consistent with historic hydrologic patterns, wherein average base flows after October <br />31 rose by 33% or more in halfofthe years of record (41/83) with respect to comparable average <br />base flows prior to November 1. The Service made no numerical flow recommendations with <br />respect to spring peak flows, except that reductions in peak flows be minimized to the greatest extent <br />practicable. Nevertheless, the Service recognizes that some reduction of peak flows is unavoidable. <br />Under this plan, base flows in the Yampa River will be augmented, as necessary, to satisfy flow <br />recommendations; however, the use of reservoir storage to augment base flows will be evaluated <br />for its potential impacts to peak flows. <br /> <br />The CROSS was used to estimate volumes of augmentation needed to satisfy Service base-flow <br />recommendations. The CRDSS for the Yampa River is a hydrologic model encompassing a 90-year <br />historical set of atmospheric and hydrologic conditions, which serve as a template on which to <br />compare alternative water supply and demand conditions. The period of record for the Yampa River <br />CRDSS is water-years 1909-1998 (i.e., October 1, 1908 - September 30, 1998). While no model <br />can predict the future, climatic and hydrologic patterns in the foreseeable future are likely to be <br />replicated with the same frequency as they occurred in the recent past. In this context, the CRDSS <br />applies a statistical probability of future atmospheric and hydrologic conditions that affect both the <br />supply of and demand for water based on actual observations of past conditions. <br /> <br />The CROSS estimated stream discharges in AF/month at Maybell under historic, current and 2045 <br />demand conditions. These monthly discharges subsequently were distributed on a daily basis in <br />proportion to gaged flows during the same time period and converted to average daily flows in cfs <br />by applying the following conversion factor: <br /> <br />AF/day + 1.98 = daily average cfs. <br /> <br />Management Plan for Endangered Fishes in the Yampa River Basin <br /> <br />33 <br />