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<br />C') <br />C:J <br />..... <br />~ <br /> <br />declined 27% on average from pre-dam to post-dam periods (Figure 3.14) during late summer and fall in <br />Reaches 1 through 4, which are below the suspected Colorado pikeminnow spawning areas. The low <br />peak flows during the post-dam period were not sufficient to create and clean backwaters, and relatively <br />high low flows in late summer did not maximize backwaters during base-flow periods. Starting in 1992, <br />the reoperation of Navajo Dam to mimic a natural hydrograph improved backwater availability by returning <br />it to near or above pre-dam conditions. Projected backwater availability under various future conditions <br />with the flow recommendations in place indicates that backwater habitat would remain higher than pre-dam <br />conditions for most scenarios (Bliesner 1 999b). The flow recommendations included 5,000 cfs for 21 <br />days, in part to maintain backwater habitat quality by flushing out fine sediments, and a base flow of 500 <br />cfs to maximize the extent of backwater habitat. <br /> <br />,""I <br /> <br />Flows needed for formation and maintenance of cobble bars are also important parts of the flow <br />recommendations. The recommendation of 8,000 cfs for 10 days was made, in part, for cobble bar <br />construction and maintenance. Cobble bar quality was also likely reduced during the post-dam period <br />when peak flows were reduced. In addition, the 10,000 cfs recommendation was made to assist in creating <br />and preserving islands, an important factor in creating habitat-rich areas. Figure 3.15 shows that as island <br />count increases, habitat count, a measure of habitat richness, also increases. <br /> <br />If Navajo Dam is operated as prescribed in the flow recommendations, key habitats for the endangered <br />fish species will be maximized in both quantity and quality, and they will be provided at the proper time of <br />the year for use by the fish, based on information gathered during the 7-year research period. New <br />information may improve the flow recommendations through adaptive management. However, larval <br />habitat availability, a major potential habitat limitation for both endangered fish species, was not explored. <br />In addition, YOY razorback sucker habitat availability was not researched. These potential habitat <br />limitations will be studied when the adult populations of both species increase sufficiently to produce millions <br />of larvae. <br /> <br />Habitat limitation is one of the primary factors restricting the population density of fish in any habitat, and <br />habitat will likely limit the two endangered fishes in the San Juan River at some point. The river's recovery <br />potential will not be known until the adult populations of these two species are increased and natural <br />recruitment occurs. Based on the habitat quantity and quality studies conducted during the 7 -year research <br />period, as well as the retention and growth of the stocked endangered species, the San Juan River does <br />provides habitat for both of these species. Because of this, the San Juan River has the potential to make <br />a substantial contribution toward recovery of both Colorado pikeminnow and razorback sucker. <br /> <br />September 2000 <br /> <br />3-35 <br /> <br />Program Evaluation Report <br />