Serial Discontinuity Concept Applied to the Hydrology of the Green River

Maxwell Agnew

The natural flow regime of the Green River, from Flaming Gorge Dam to the mouth of the Colorado River, has been altered since dam closure in November 1962. The pre-dammed river system had extremely high late-spring to early-summer peak floods originating from snowmelt and relatively low baseflows during summer, autumn and winter (Muth et al 2000). Different eras of dam operation have had different effects on the natural flow regime. From closure to 1992, dam managers released water based on electricity demand, resulting in a highly unnatural flow regime. Profound changes to the natural hydrograph included the loss of high late-spring/early-summer peak floods and a change to higher than natural base flows. Many fish species in the Green River system are endemic, having evolved lifecycles dependent upon the characteristics of the natural flow regime. Several native fish in the river system became endangered, mainly as a result of Flaming Gorge Dam operations. From 1992 to 2005, Flaming Gorge Dam was operated under the Biological Opinion, which provided flow and temperature recommendations necessary for endangered fish recovery, while still meeting power generation and irrigation demands. The overall goal of the Biological Opinion was to create a flow regime resembling, as close as possible, the natural flow regime. Truly natural flow can never be achieved because of hydraulic limitations of Flaming Gorge Dam, diversions for irrigation, and the added hydrologic effects of other dams in the Upper Colorado watershed (Muth et al 2000). The serial discontinuity concept (SDC) was applied to the hydrology of the Green River from Flaming Gorge Dam to the Green River gaging station. Beginning in 1992, more natural flows have been observed at the USGS stream-flow gage near Jensen, UT and the gage near the town of Green River, UT. Since more natural flows have been observed, the hydrologic discontinuity distance has decreased. The success of the Biological Opinion flow recommendations will be judged according to the long term population trends of the endangered fish.