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. ., filled with bed materials (Lyons and Pucherelli 1992). In the Gunnison Gorge of the Gunnison River downstream from the Aspinall Units, summer thunderstorms in 1991-92 caused debris flows in normally dry side flow channels. This episodic inflow of rocks and soil created large alluvial fans out into the river, which have persisted owing to insufficient peak flows to flush alluvium , . downstream (Elliott and Parker 1992). Channel Encroachment by Riparian Plants The inability of the regulated river to redistribute alluvium allows encroachment of vegetation into the river channel. Dense vegetation down to the low water mark (Le., minimum flow channel) is an ecological feature that now characterizes the river corridor of the regulated segments of the Gunnison (Stanford and Ward 1984), Colorado (Graf 1978, Stanford and Ward 1986b, Osmundson and Kaeding 1991) and Green Rivers (Fisher et al. 1983). However, Fisher et al. (1983) also provided very clear evidence that vegetation along the shoreline of the Yampa River has not substantially changed in over 100 years because the Yampa remains unregulated. Unvegetated, bare sandbars and backwaters evident in photographs taken in 1871 were amazingly unchanged in photos 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 YampaRiver, whereas riparian vegetation composed primarily of nonnative species such as reed canary grass (Phalaris arundinacea); salt cedar (Tamarix pentandra) 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 riparia. First, reduction of peak flows allows encroachment of riparian vegetation into the channel, backwaters and floodplain wetlands, if the latter two are stilI hydrologically functional after regulation. The riparian zone of regulated rivers is small but frequently dewatered and rehydrated. Second, nonnative plants are more competitive in the stabilized environment that exists in the narrow saturated zone next to the river channel and backwaters, and they tend to dominate the community. 32