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channel profile, seedlinq roots lose contact with the water table, desiccate, and die. Consequently, summer flows are critical to the lonq-term survival of newly established seedlinqs, and provide necessary moisture when evapotranspiratlon is hiqhest. Winter Flow (V) Ice scour associated with low flows durinq late fall and winter may cause seedlinq mortality and prevent recruitment to larqer size classes. Aqain, the position at which qermination and establishment occur often dictates seedling survival. Those individuals located too close to the channel are easily uprooted and scoured by ice, even during normal winter flows. Aside from the rising limb, all aspects of the hydroqraph play a vital role in the qermination, establishment, and long-term survival of many riparian species. In some snowmelt-dominated s~stems, cottonwood recruitment is apparently not llmited by seed qermination, but rather by the inability of the newly established seedlings 'to survive subsequent high flows, drouqht stress due to periods of low summer flow, or direct scouring by ice durinq the winter and early spring. FLOW ALTERATIONS Riparian veqetation dynamics are determined by a complex interplay of hydrology, fluvial qeomorphology, .and bioloqy. Furthermore, many western rivers have experienced a complex set of flow alterations over time. Thus, it is not surprisinq that a variety of responses to flow alteration have been observed. Consideration of how an altered hydrograph will satisfy the ecological requirements of various riparian plants is an important step in the analysis of potential impacts of a particular flow alteration on riparian veqetation. To illustrate this approach, we briefly discuss some qeneralized flow alterations of snowmelt rivers in terms of the ecoloqical requirements of plains cottonwood. Hydroelectric Development A run-of-the-river hydroelectric facility has little impact on the downstream hydrograph. As storage capacity increases, however, hydropower operations tend to attenuate the peak (II) and augment lower flows (IV and V), thus producing a "flatter" hydrograph. Because the timlng of the snowmelt peak (11) is often unchanqed, moist alluvial surfaces are still available during the receding limb (111) for germination. However, the "flatter" hydroqraph may substantially reduce the potential area for establishment by (a) reducing the difference between the peak flow and the summer or low flow, (b) reducing the rate of channel meandering through attenuation of the more powerful peak flows, and (c) 243