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8 4 ASSESSMENT OF POTENTIAL EFFECTS ON RIPARIAN VEGETATION The assessment of potential effects of hydropower operational scenarios on riparian vegetation focused on changes in the boundaries of upper and lower riparian zones. The assessment was based on (1) information on species distributions collected during field studies and (2) the relationship between riparian area and flow determined from aerial videography. 4.1 ASSESSMENT APPROACH The daily and seasonal patterns and durations of inundation and exposure charac- teristic of each hydropower operational scenario were evaluated to determine impacts on the riparian zone. Flows that occurred during the growing season (mid April through mid October) were considered most important because plants can tolerate a wider range of conditions when they are in a dormant state (Walters et al. 1980). The impacts considered included changes in the size of lower and upper riparian zones and changes in the relative abundance of species within these zones. All projected impacts were compared with the existing characteristics of the riparian zone below Flaming Gorge Dam, as determined from field studies and aerial videography. These characteristics are the result of pre-dam river conditions and the effects of flow regulation. Changes in operational scenarios would not affect plants such as Fremont cottonwood, box elder, and rabbitbrush in the higher elevations of the upper riparian zone (above the elevation of pre-dam annual floods, i.e., about 7,000 cfs or 7 ft above the elevation of 800-cfs flows). These plants became established prior to construction of the dam and persist because they have well established root systems that can reach deeper soil with higher moisture levels. Recruitment at these higher elevations is limited, however, because of the reduction or elimination of the occurrence of large flooding events needed for germination; eventually this vegetation will be replaced by nonriparian xerophytic vegetation. Pre-dam vegetation below Flaming Gorge Dam is analogous to the vegetation of the old high- water zone of the Colorado River below Glen Canyon Dam, as described by Carothers and Brown (1991). Changes in the relative abundance of riparian species in the remainder of the riparian zone were inferred from changes in the hydrologic regime and observed relationships between species abundance and position along the elevation gradient. It was assumed that if a new hydropower operational scenario were established, plant populations would move up or down the gradient into newly favorable areas through normal processes of propagation. The ability of a population to move was determined on the basis of its current position along the gradient and the direction of flow changes as described below. Higher operational flows would force the populations of some upper zone species to move up the elevation gradient. If the species currently occupied the entire upper zone (e.g.,