Navajo Reservoir Operations Vol. II

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. II II I '. I . , , . ,II , II I I , II I II Backwaters are an important habitat for young native fishes, including Colorado pikeminnow. During studies of young stocked Colorado pikeminnow in the San Juan River, the fish were found in backwaters 60% of the time, but they were found in other low-velocity habitats (e.g. pools, pocket water) nearly 40% of the time. In the Green River, young Colorado pikeminnow are found in backwaters more often than fish in the San Juan River, and studies have shown that the San Juan River has relatively small amounts of backwaters compared with the Green and Colorado rivers. But the success of the stocked Colorado pikeminnow in the San Juan River has shown that this system has the habitats necessary for the survival and growth of these young fish. Studies assessing the flows needed to build and maintain cobble bars and backwaters similar to those used by Colorado pikeminnow were an important part of the 7-year research effort. These studies showed that relatively high flows were needed to build and clean these habitats. but that lower flows were needed to make them more abundant at the proper time of the year. During the 7-year research period, a number of responses to the reregulation of Navajo Dam were identified in the native fish community. Colorado pikeminnow young were found in very low numbers, or not at all, during low spring runoff years, and in larger numbers during higher flow years. The young of bluehead sucker and speckled dace, two other native species. were found in greater numbers during high flow years compared with low flow years. Flannelmouth sucker. another native species, tended to decline during the research period, but still remained the most abundant native species in the river. The change to a more-natural hydro graph during the research period resulted in more cobble and less sand habitats in the river, apparently favoring bluehead sucker and speckled dace rather than flannelmouth sucker. . Nonnative fishes in the San Juan River are potential predators and competitors with the native species and have been implicated in the decline of the native fishes throughout the Colorado River Basin. Populations of some nonnative fishes changed during the research period, but no major reduction in nonnative fish numbers were documented. Some authors have suggested that nonnative fishes may be reduced by high natural flows, but this was not the case in the-San Juan River during the 7 -year research period. Contaminants were also studied as a potential limiting factor for native fishes. but no pattern of contaminant concentrations and flow was found. Table S. I summarizes the biological and habitat responses that were found during the research period and the flows that were important in producing those responses. . FLOW RECOMMENDATlbN RiverWare, a generic hydrologic model, was used as the primary modeling tool for developing the flow recommendations. The model simulates the flow in the river at various gages at different points in time, including the past, present, and future. It does this by incorporating all past, present, and potentially future water development projects into the model. The 1929 to 1993 period of record was used in the model to simulate flows under the various development scenarios. Existing gaging stations were used to calibrate the model to ensure it was working properly for historic conditions. SJRIP Biology Committee May 1999 5-3 Executive Summary Aow Report