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reducing year-to-year variability in the magnitude of both peak and low flows. If the facilities are effective at capturing and storing water during the snowmelt peak (11), peaks from localized thunderstorms . on uncontrolled, downstream tributaries during the receding limb (111) may approach the magnitude of the snowmelt peak. These peaks may occur at the wrong times for cottonwood establishment and may kill newly established seedlings by scour or burial. In addition, the "flatter" hydrograph may increase the importance of ice scour in the augmented winter flow (V) period as a source of seedling mortality. Hydropeaking operations in which flows are dramatically varied over short time intervals to generate electricity during peak demand periods represent a qual i tatively different type of flow alteration. Sites that alternate between inundated and moist on a daily or shorter basis are unlikely to satisfy the establishment requirements of cottonwood. Irrigation Development Water development for irrigation generally decreases the total annual discharge as well as altering the shape of the hydrograph. Ignoring the special case of 1nter-basin transfers! flow alterations depend on the size of the divers ons, the nature of return flows, and the storage capacity of the development. In a system with little storage capacity, substantial diversions may not occur until late in the rising limb (1) of the hydrograph, maintaining a strong snowmelt peak (11). There will likely be some attenuation of the peak, however, and a shortening and steepening of the receding limb (111). This presents seedlings with a more rapidly declining water table and may increase drought mortality. If late summer diversions are not offset by return flows, the summer flow (IV) may be lower, producing additional drought stress on seedlings. Winter flow will be largely unchanged with some augmentation of base flow by groundwater-mediated return flow. Increased storage allows more of the winter flow (V), rising limb (1), and snowmelt peak (11) to be captured for later growing season releases in the receding limb (111) and summer flow (IV) periods. Return flows from these releases may augment summer low flows and reduce associated drought stress. However, the resul t ing "flatter" hydrograph may reduce the potential area available for establ1shment in the ways described above for hydroelectric development. Hi9her groundwater tables from return flows may shift m01sture conditions at higher positions 1n the floodplain enough to allow different plants to become established in the understory of mature cottonwood stands. At very high levels of storage and diversion, river systems may become essentially dewatered. This may produce an initial increase in riparian forest as the active channel narrows. These 244