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20 BIOLOGICAL REPORT 24 with debris on the floodplain surfaces as flows declined after the spring spate. Gradually drying soils of fine riverine alluvium provided ideal sub- stratum and water supply for germination and growth of seedlings. As a result of this unique coupling of the tree's life cycle with the annual hydrograph, trees of even age can be used to date the extent of past high flow events. Moreover, cottonwood leaves dropped in fall and blown into the river provide an important allochthonous source of nutrients for riverine food webs. Only remnant forests remain today along the rivers of the Upper Colorado River Basin owing to regula- tion of flow, which limits distribution of seeds and conditions required for germination. Agricultural activities such as grazing and tillage, and flood- plain revetments also prevent establishment of cottonwood seedlings. Replacement of riparian forests of naturally reproducing cottonwoods and associated native plants by nonnative plants in a narrow fringe along the river corridor is a classic symptom of the severing of dynamic spatial and temporal connections between the river channel and its floodplain (Stanford and Ward 1986a, 1992a,1993). Two questions require resolution with regard to riparian ecology and imposition of reregulated flows in the Upper Colorado River Basin. First, how much flooding and what frequency of flooding does the riparian zone require to maintain native riparian vegetation? Fisher et al. (1983) showed that the Yampa corridor remains largely un- changed, although salt cedar has invaded throughout the lower half of the river. The 1983- 84 high floods allowed cottonwoods to reseed along the upper Green River (personal observation). 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 chan- nel if peak flows are reinstated? Studies are needed to quantify this very apparent relation- ship between reduction of peak flow events and changes within the riparian vegetation of the Up- per Colorado River Basin. Loss of Food Web Function in the Varial Zone: The Problem of Baseflow Instability Hydropower operations have produced erratic baseflows on the Gunnison (e.g., Fig. 11) and on the Green River (e.g., Fig. 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 inun- dated and dewatered by the peak flow events. Hence, the varial zone includes riparian vegeta- tion as well as portions of the primary and secon- dary channels and backwaters not normally con- sidered part of the riparian zone. In an unregulated river the varial zone may be large and dynamic in the context of natural geomorphic variability described by Fig. 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 opera- tions 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 Fig. 11) often lasting several days (e.g., note also sudden changes in flow in Fig. 10). The extreme nature of these flow changes is more evident when hourly flows are plotted for the same periods (Figs. 12 and 13). Regulated flows below hydropower dams also often reflect the consequences of the dam operators need to control electrical load (peaking operations), as on the Green River in 1992 (i.e., diel cycles evident in Figs. 12 and 13). Peaking and other short-term operations water and dewa- ter the varial zone of a regulated river with much greater frequency than would occur under natural conditions. Stanford and Hauer (1992) demon- strated that diel changes on the Middle Fork of the Flathead River, an unregulated snow-melt river in Montana, were consistently less than 5% per day during the baseflow period. Repeated flushing of the varial zone prevents establishment of food webs and resting areas for small fish, which are required to support riverine fisheries. Weisberg et al. (1990) demonstrated that standing crops of zoobenthos increased 100-fold in 1 year in a regulated river after eliminating peak- ing operations at the dam and thereby reducing the devastating ecological effects of unnatural, short- term flushing of the varial zone. Repeated flushing also removes plant growth nutrients and alters the natural thermal insolation of shallow backwaters, which are especially important for bioproduction of low velocity food webs in general and for growth of squawfish and razorback sucker specifically. Despite the laudable reregulation effort by op- erators of Flaming Gorge Dam to stay within flow