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,.: .' along the upper Green River (personal obselVation). Other flows over the last several decades have not produced cottonwoods. Second, how much of an effect will encroachment of vegetation into the river channel have on reconfiguration of the channel, if peak flows are reinstated? Studies are needed to quantify this very apparent relationship between reduction of peak flow events and changes within the riparia of the Upper Colorado River Basin. .Loss of Food Web Function in the Varial Zone: The Problem of Basejlow Instability Hydropower operations have produced erratic baseflows on the Gunnison (e.g., Figure 11) and on the Green River (e.g., Figure 10) that are especially problematic because they destabilize food webs in the "varial zone" of the river. The varial zone is the shallow area of the shoreline (as opposed to the middle or thalweg of the channel) that is inundated and dewatered by the g flow events. Hence, the varial zone includes riparia as well as portions of the primary and secondary channels and backwaters not normally considered part of the riparian zone. In an unregulated river the varial zone may be quite large and dynamic in the context of natural geomorphic variability described by Figure 2, or in the context of the gallery forest discussed above. The varial zone in a regulated river often is smaller, owing to reduction in peak flows; but, more importantly, the varial zone of a regulated river usually is repeatedly watered and dewatered by dam operations for hydropower generation. As markets for hydropower vary, so does water output from the dam. The result on the Green and Gunnison Rivers is reflected in high spikes above baseflow (e.g., at points of initiation shown by arrows in Figure II) often lasting several days (e.g., note also sudden changes in flow in Figure 10). The extreme nature of these flow changes are more evident when hourly flows are plotted for the same periods (Figures 12 and 13). Regulated flows below hydropower dams also often reflect the consequences of the dam operator's need to control electrical load ("peaking" operations), as on the Green River in 1992 (i.e., diel cycles evident in Figures 12 and 13). Peaking and other short-term operations water and dewater the varial zone of a regulated river with much greater frequency than would occur under natural conditions. Stanford and Hauer (1992) demonstrated that diel changes on the Middle Fork of the Flathead River, an 34