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INsTREAM FLOws TO ASSIST THE RECOVERY OF ENDANGERED FISHEs 19 6000 T--------------------------------------------------------------------------------------------- 5000 ------------------------------------------------------------ - 0 Green ® Jansen o Green • naming Gorge Yampa Upper window 4000 ------------------------------- -lowerYNndow N LL 3000 3 0 LL 2000 -_.------.---_--- 1000 ro - gpmmc?o m0 Jun 92 Jul 92 Aug 92 Sep 92 Oct 92 Fig. 14. Summer and fall baseflows on the Green River at Flaming Gorge Dam and at Jensen in relation to unregulated flows from the Yampa River. Bold, broken lines delineate 1,800 cfs (upper) and 1,350 cfs (lower) baseflow operational windows recommended for recovery of endangered fishes (U.S. Fish and Wildlife Service 1992), as derived from the stage-backwater relationship determined by Pucherelli et al. (1990) and Lyons and Pucherelli (1992) (data from U.S. Geological Survey). substantially in over 100 years because the Yampa remains unregulated. Unvegetated, bare sand- bars and backwaters evident in photographs taken in 1871 were amazingly unchanged in pho- tos of the same spots in 1983. Record high flows in 1983 did not change this interpretation (Potter 1984). Clearly, the scouring effect of spring floods does limit the distribution of riparian plants into the channel and backwaters on the Yampa River, whereas riparian vegetation composed primarily of nonnative species such as reed canary grass (Phalaris arundinacea), salt cedar (Tamarix spp.), and Russian olive (Elaeagnus angustifolia) is gradually choking the regulated segments of the Upper Colorado River Basin. Two interactive processes are involved in the long-term succession of regulated stream riparian vegetation. First, reduction of peak flows allows encroachment of riparian vegetation into the channel, backwaters, and floodplain wetlands, if the latter two are still hydrologically functional after regulation. The riparian zone of regulated rivers is small but frequently dewatered and re- hydrated. Second, nonnative plants are more com- petitive in the stabilized environment that exists in the narrow saturated zone next to the river channel and backwaters, and they tend to domi- nate the community. Native plants are adapted to deal with extreme variations in flow and soil satu- ration, conditions that do not occur in the dynamic fashion that characterizes unregulated hy- drographs in the Colorado River system. That is, in the predam environment, the riparian zone was large and only periodically or seasonally flooded. Hence, the natural plant succession that followed scouring flood events has been curtailed or lost along regulated streams, as reflected in the nar- row, undisturbed riparian corridor along the wet- ted perimeter of the river and its backwaters (Gregory et al. 1991). Maintenance of cottonwood (Populus deltoides, P. fremontii) gallery forests, which once charac- terized the floodplains of the pristine Upper Colo- rado River Basin, was dependent on seasonal flooding and drying in the riparian zone. Seeds produced by cottonwoods in spring were deposited