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Peak (II) The magnitude and timing of the peak flow are perhaps the most critical elements of the hydrograph for successful regeneration of cottonwood, which relies on the synchronous timing of seed dispersal and moisture to germinate and establish successfully. Peak flows maintain a geomorphically active and dynamic channel, creating moist barren seedbeds of freshly deposited alluvium, and rafting the small, light seeds onto these ideal germination sites. Peak flows must be of sufficient magnitude to insure germination and seedling establishment at elevations that will not be subject to subse~uent flooding or ice scour. They must also cOincide with, or immediately precede, seed dispersal. On most natural snowmelt-dominated systems, peak flows occur from mid-May to mid- or late June. Over-bank flooding associated in spring also serves to recharge the alluvial aquifer, (Johnson et ale 1976), saturate the rooting zone, and increase bank moisture storage prior to the onset of growth (Reily and Johnson 1982). This raises the local water table and increases the amount of moisture potentially available for growth later in the rear. Reily and Johnson (1982) attributed growth dec ines in riparian plants, in part, to a decrease in the frequency of over-bank floodin~ and the resulting water table decline. The magnltude of peak flows determines the extent to which the alluvial aquifer is rechar~ed and thus plays another critical role in the establlshment of new seedlings and long-term survival of existing vegetation. Receding Limb (III) The shape and slope of the receding limb of the hydrograph is as important as the timing and magnitude of the peak. As stream discharge decreases and, water levels gradually recede, moist, freshly deposited alluvium is exposed and the rafted seeds germinate quickly (Engstrom 1948, Ware and Pen found 1949). To survive, these seedlings require a continuous source of adequate moisture. Consequently, river stage must decline at a rate that allows seedllng roots to maintain continuous contact with saturated or sufficiently moist substrate. If river discharge and the water table drop too rapidly, root contact with the saturated substrate is interrupted and seedling survival decreases appreciably (Mahoney and Rood 1991, Segelquist et ale 1993, Stromberg et ale 1991). Summer Flow (IV) Hydraulic position of the newly established seedlin9s relative to the channel thalweg is critical to their long-term survival. If they germinate and establish too low in the channel prOfile they riSk scour and removal by subsequent floods associated with summer thunderstorms and succeeding year's peak flows. If germination and establishment occur too high on the 242