Riparian vegetation and channel change in response to river regulation: A comparative study of regulated and unregulated streams in the Green River Basin, USA

The effects of river damming on geomorphic processes and riparian vegetatio n were evaluated through Field studies along the regulated Green River and the free-flowing Yampa River in northwestern Colorado, USA. GIS analysis of historical photographs, hydrologic and sediment records, and measurement o f channel planform indicate that fluvial processes and riparian vegetation of the two meandering stream reaches examined were similar prior to regulat ion which began in 1962. Riparian plant species composition and canopy cove rage were measured during 1994 in 36, 0.01 ha plots along each the Green Ri ver in Browns Park and the Yampa River in Deerlodge Park. Detrended corresp ondence analysis (DCA) of the vegetation data indicates distinctive vegetat ion differences between Browns Park and Deerlodge Park. Canonical correspon dence analysis (CCA) indicates that plant community composition is controll ed largely by fluvial processes at Deerlodge Park, but that soil chemical r ather than flow related factors play a more important role in structuring p lant communities in Browns Park. Vegetation patterns reflect a dichotomy in moisture conditions across the f loodplain on the Green River in Browns Park: marshes with anaerobic soils s upporting wetland species (Salix exigua, Eleocharis palustris, Schoenoplect us pungens, and Juncus nodosus) and terraces having xeric soil conditions a nd supporting communities dominated by desert species (Seriphidium tridenta tum, Sarcobatus vermiculatus, and Sporobolus airoides). In contrast, vegeta tion along the Yampa River is characterized by a continuum of species distr ibuted along a gradual environmental gradient from the active channel (rude ral species such as Xanthium struminarium and early successional species su ch as S. exigua, Populus deltoides subsp. wislizsenii, and Tamarix ramossis sima) to high floodplain surfaces characterized by Populus forests and mead ow communities. GIS analyses indicate that the channel form at Browns Park has undergone a complex series of morphologic changes since regulation bega n, while the channel at Deerlodge Park has remained in a state of relative quasi-equilibrium with discharge and sediment regimes. The Green River has undergone three stages of channel change which have involved the transforma tion of the historically deep, meandering Green River to a shallow, braided channel over the 37 years since construction of Flaming Gorge Dam. The pro bable long-term effects of channel and hydrologic changes at Browns Park in clude the eventual replacement of Populus-dominated riparian forest by drou ght tolerant desert shrublands, and the enlargement of in-channel fluvial m arshes. Copyright (C) 2000 John Wiley & Sons, Ltd.