9700 - Laserfiche WebLink

Home Browse Search

Green River Floodplain Inundation 13 June 2005 4 IMPLICATIONS OF FLOODPLAIN CHARACTERISTICS ON PEAK-FLOW RECOMMENDATIONS AND IMPLEMENTATION 4.1 Timing and Duration of Connection to the Main Channel Drifting razorback sucker larvae are typically detected in the main channel Green River about 12 to 20 days after spawning occurs, and usually in Mayor June. In general, most drifting larvae are observed in the main channel over a period of approximately two weeks (Muth et al. 2000). Because drifting larvae are more likely to die if they are not entrained into suitable nursery habitats, these habitats must be connected to the river when larvae are drifting. Therefore, connecting flows must occur concurrently with drifting larvae. The Muth et al. (2000) flow recommendations identified a need for connecting flow durations of two weeks or more in one of four average years, and in all moderately wet, and wet years. The basis for the recommended duration of two weeks or more above 527 m3/s (18,600 cfs) was the observed two- week availability of drifting razorback sucker larvae. The specific magnitude chosen (527 m3/s or 18,600 cfs) was based on the relationship between flow and floodplain inundation with levees in place. However, recent modification to levees allow the same amount of priority depression floodplain to be inundated at flows of 396 m3/s (14,000 cfs) as would be inundated by flows of 527 m3/s (18,600 cfs) (Figure 4). Thus, it is possible that the same benefits for endangered fish could be achieved by maintaining longer duration floodplain connections with flows of 396 m3 Is (14,000 cfs) as with flows of 527 m3/s (18,600 cfs). The Muth et al. (2000) recommendations also specify that peak flows should be timed to coincide with peak and immediate post-peak flows of the Yampa River. This timing is consistent with the appearance of drifting razorback sucker larvae, and facilitates achieving the needed magnitude and duration of connecting flows. There is a relationship between the duration and magnitude of flows that can be supplied by Flaming Gorge Dam. For a given volume of water, a higher connecting flow will provide a shorter duration of connection than will a lower connecting flow. By reducing the magnitude of flows needed to maintain floodplain connections (e.g., by removing or modifying levees, as done recently by the Recovery Program), the duration of connection can potentially be increased. Peak flows that are much higher than those required to maintain connections between depression floodplains and the main channel probably provide no increase in available nursery habitat once flows recede. Increasing the period of time over which razorback sucker larvae can be entrained into the floodplain may provide a substantial benefit by increasing the probability of entrainment of larvae into nursery habitats. Thus for a given volume of water, it may be more beneficial to native fishes to provide a lower magnitude, longer duration connecting flow rather than a higher magnitude, shorter duration connecting flow. However, flows needed to adequately entrain larvae into floodplain habitats or that would allow successful out-migration of reared juveniles may be somewhat higher than flows that simply connect or inundate the depression floodplains.