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10 The assessment of impacts of operational scenarios addresses only those riparian areas above the confluence of the Green River with the Yampa River. Below the confluence, the Green River experiences a combination of the seasonal flow regime of the unregulated Yampa River, with high spring flows, and the daily fluctuations of the upper portion of the Green River (Fischer et al. 1983). As a consequence, flows below the confluence are strongly and less predictably affected by input from the Yampa River, making the effects of operational scenarios on vegetation more difficult to predict. 4.2 EXPECTED RESPONSES OF RIPARIAN VEGETATION TO OPERATIONAL SCENARIOS On the basis of the assumptions discussed in Section 4.1, areas above and below water and above the 800-cfs flow (Table 5) were used to determine the areas of various riparian habitats, as presented in Tables 6 and 7. The expected elevations of riparian zone boundaries for different operational scenarios are depicted in Figure 8. The expected boundaries for each operational scenario are discussed in Sections 4.2.1 through 4.2.4. 4.2.1 Year-Round High Fluctuation The year-round high fluctuation scenario would feature higher maximum daily flows (4,700 cfs) than occurred historically (4,200 cfs). The boundary between the upper and lower zones would shift upward, resulting in a slight change in the area occupied by each zone. The resulting changes in area represent a 5% decrease in the size of the upper riparian zone and a 13% increase in the size of the lower riparian zone. Species in the upper portion of the lower zone are most likely to increase, including Canada thistle, carex, common reed, coyote willow, juncus, and scouring rush. Box elder is likely to decrease because this species occurs across the entire upper zone and the total area of this zone would decrease. Species such as giant whitetop and tamarisk, which occur near the boundary of the two zones, would probably be unaffected by this scenario. 4.2.2 Seasonally Adjusted High Fluctuation Under the seasonally adjusted high fluctuation scenario, the lower boundary of the upper riparian zone is expected to shift upward to the elevation of the 4,700-cfs flow. Although lower maximum flows would occur from mid July through October, 4,700-cfs releases would occur through the entire first half of the growing season (to July 10). Any species of the upper zone that colonized the area below the 4,700-cfs elevation during the second half of the growing season would not survive the prolonged inundation during the following year. Under this scenario, the lower riparian zone is expected to occupy the area between the elevations of 800- and 4,700-cfs flows because of the periodic, but not constant, inundation that would occur between these stages. The area between the elevations of 3,000- and