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9 box elder), there would be an expected decrease in population of that species because the area of the zone would shrink. If, however, the species was restricted to lower elevations within the zone (e.g., tamarisk, giant whitetop, and scouring rush), it was assumed that no decrease would occur because the population could move to higher elevations within the zone as flows increased. It was also assumed that upper zone species could not survive on substrates that were inundated continuously or daily for more than one month each year. The flow velocity of the sustained spring peak of the seasonally adjusted operational scenarios was not considered sufficient to remove rooted upper zone plants. Reductions in maximum flows would result in some expansion of upper riparian zone vegetation. These reductions in flow would not affect established deep-rooted vegetation in the upper riparian zone because their root systems would be able to utilize soil moisture at greater depths. Drier soil conditions at lower elevations, however, could result in new areas favorable for expansion of populations. Species that occur near the current boundary between the lower and upper zone - including tamarisk, giant whitetop, box elder, golden aster, and artemisia - would be expected to benefit most from such a change. It was assumed that upper zone species would not colonize substrates that were inundated continuously or daily for more than one month each year. The duration of the sustained spring peak of the seasonally adjusted operational scenarios would not be sufficient to prevent some expansion of vegetation. To support the wetland species typical of the lower riparian zone, fluctuations would have to occur frequently enough to maintain the high soil-moisture levels required by this vegetation, but without the drowning that would occur with continuous inundation. Continu- ous inundation (i.e., nonfluctuating high flows) for more than one month during the growing season was considered sufficient to drown existing lower-zone vegetation if the water was more than 1 ft deep. Additionally, if a two-month or longer period of continuous exposure occurred during the growing season (i.e., without inundation due to fluctuating flows), it was assumed that this exposure would result in a dieback of any existing drought-intolerant species (e.g., cattail) and replacement by more drought-tolerant species (e.g., common reed). Peak flows in the spring were not considered to be of sufficient duration to drown vegetation in the lower riparian zone. Because the vegetation of this zone (especially herbaceous species) can respond rather quickly (within several years) to changes in the moisture regime, adaptation to operational scenarios was assumed to occur rapidly. Although the boundaries for upper and lower riparian zones were predicted for different operational scenarios, these boundaries are not absolute and would vary according to site- specific conditions. For the upper riparian zone, the lower boundary of the zone might be at higher or lower elevations at different locations. Wetland vegetation typical of the lower zone could occur outside of the fluctuation zone along the river, especially near tributary mouths, springs, or seeps where soil moisture is not dependent on river flow alone. Unprotected areas exposed to scouring flows might not support any vegetation, especially at the lowest elevations above minimum flows. For all operational scenarios, inundation would be constant below the 800-cfs level, preventing establishment of nonaquatic vegetation below this level.