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sediments through the system. Tamarisk, an aggressive, exotic woody plant that stabilizes stream <br />and river banks, colonized sand and cobble bars throughout the 15-mile reach. In the absence of <br />high flushing flows, seedlings were able to develop deep, extensive root systems such that <br />dislodging them once high water again returned would be very difficult. Data also indicated that <br />during these three back-to-back low-water years, non-native minnows capable of preying on or <br />competing with larval endangered fishes greatly increased in numbers. <br />Based on the rapidity with which these ecological changes occurred, as well as the need for <br />relatively frequent spawning success of the endangered fish, FWS proposed a range of minimum <br />peak flows along with a frequency with which they should occur. In addition to the one day peaks, <br />mean monthly flows capable of producing such peaks as well as serving to maintain the natural <br />shape of the hydrograph were also proposed. <br />In the study presented in the current report, data were collected that shed additional light on some <br />of the topics discussed earlier. Stream bed monitoring indicated that spring runoff in 1993, with a <br />peak flow of 25,900 cfs, was capable of mobilizing coarse bed materials and thereby winnowing <br />accumulated fines from the channel substrate. This was supportive of the earlier recommendation <br />of a peak flow during wet years to exceed a minimum of 23,500 cfs. Transects that crossed <br />backwaters within the reach were monitored over a four-year period. Changes in fill and scour of <br />fine sediments from one year to the next indicated that a spring discharge with a peak of about <br />12,900 cfs was capable of flushing accumulated sediments from the bottom of the backwaters <br />thereby restoring their depth. This observation allowed the recommendation for a minimum peak <br />flow during low-water years to be reduced from 14,800 cfs to 12,900 cfs. <br />M other ongoing studies in the upper Colorado River are completed, additional information will <br />help provide a greater understanding of the relationships between flow and rare fish habitat. Flow <br />recommendations will periodically need to be updated and refined accordingly. For now, we are <br />satisfied that the recommendations provided here are an improvement over those provided earlier <br />and feel strongly that timely implementation of these recommendations will make a significant <br />contribution to the recovery of endangered Colorado River fish. <br />Xiii