Green River Riparian Vegetation below Flaming Gorge Dam: Is there Evidence of Recovery from Serial Discontinuity?

Heather L. Bowen

Riparian vegetation has changed considerably on the Green River between Flaming Gorge Dam and Split Mountain since the closure of the dam. The Serial Discontinuity Concept predicts that river ecosystems recover from regulation impacts with increased distance downstream from the dam (Stanford and Ward 2001). Impacts of the Flaming Gorge Dam on the Green River have resulted in a decline in native Fremont cottonwood (Populus fremontii) abundance along alluvial and canyon reaches attributed to modified flow regimes, decrease in suitable sediment, change in suitable soil moisture, increased intra- and inter-specific competition, and increased herbivory. Reduced flood magnitude, number and timing, and increased base flows have resulted in a floodplain morphology that is not conducive to native plant species recruitment. In conjunction with the altered flow regime, an increase in non-native plant species abundance, especially tamarisk (Tamarix spp.), has caused channel narrowing throughout the Green River. The Yampa River is the largest tributary providing a natural flow regime in this stretch of the Green River. The Yampa offers sediment and moisture that promotes seed rain and recruitment and reduces island stabilization and channel adjustment. The smaller tributaries may also contribute a small, but less significant source of sediment and flow to the Green River. The stretch of the Green from the dam to the confluence of the Yampa appears to be sediment starved. Though the Yampa confluence does not correct this deficit, it drastically lessens it bringing the sediment budget to a quasi-equilibrium. As a result, experimental studies of the Green River suggest that impacts of the Yampa tributary do influence the recovery of Serial Discontinuity on the Green River.