9700 - Laserfiche WebLink

Home Browse Search

Green River Floodplain Inundation 7 June 2005 3 SITE-SPECIFIC FLOODPLAIN INFORMATION Investigations of the use of floodplain habitats by native and nonnative fish, conducted since publication of the Flaming Gorge flow recommendations, have led to a better understanding of those floodplain habitats that are most important as razorback sucker nursery habitats and how those habitats should be managed to improve survival of native fish. In addition, a number of important floodplain habitats have been altered to allow inundation to occur at lower peak flows. This information was summarized by Valdez and Nelson (2004). This new information prompted our re-evaluation of the specific connecting flow magnitudes recommended by Muth et aL (2000) and consideration of possible opportunities for improving conditions for native fishes while reducing the need for bypass and spill at Flaming Gorge Dam. Floodplain habitats in Reaches 2 and 3 of the Green River can be classified as depression floodplains or terrace floodplains (Figure 2). Depression floodplains are usually separated from the main channel by an elevated levee (natural or constructed) and typically retain water for a relatively long time after river flows recede. Terrace floodplains are sloping features that fill and drain with changes in river stage (Valdez and Nelson 2004). Both of these floodplain habitat types may become inundated during annual spring peak flows. As peak flows recede, depression floodplain habitats retain water at an elevation determined by the elevation of associated levee features (Figure 2). During the base-flow period, the amount of water in depression floodplains will usually decrease due to evaporation and percolation losses. The length of time that water is retained in depression floodplains is site- specific, and some depression floodplains can hold water through one or more years. For these habitats, subsequent spring peak flows of sufficient magnitude will reconnect the floodplain to the main channel before the water in the wetland has been depleted. In contrast, terrace floodplains drain as flows recede and do not retain surface water during the base-flow period (Figure 2). Observations of growth, survival, and movement of larval and juvenile razorback suckers suggest that depression floodplains are far more valuable as razorback sucker nursery areas than terrace floodplains (Valdez and Nelson 2004). However, a major limitation to the survival of razorback sucker larvae and juveniles in depression floodplains is the presence of nonnative fish that compete with and feed on native fish. It is thought that periodically allowing depression habitats to dry out helps control nonnative fish populations in the depression habitats and resets conditions within the nursery habitats. The chronology of events under this "reset" management strategy would be to: (1) inundate specific depression floodplains during a period when larval razorback suckers are present in the main channel; (2) retain water within the depression wetlands for one or more consecutive years; (3) provide reconnection to the main channel duringaa subsequent spring peak flow so that juvenile or subadult razorback suckers can return to the main channel; and (4) periodically allow the depression wetland to dry out to eliminate nonnative fishes.