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7 years or more without a flow that removes woody vegetation from 50 percent is a long- <br /> term duration impact. <br /> • <br /> Intensity: Intensity is defined in terms of total change in percentage of the bottomland <br /> cleared for the 31 year modeled periods. <br /> 1) Negligible intensity is herein defined as a net change in the total bottomland cleared of <br /> vegetation by ± 10 percent. <br /> 2) Minor intensity is herein defined as a net change in the total bottomland cleared of <br /> vegetation in the range ± 10 to 25 percent. <br /> 3) Moderate intensity is herein defined as a net change in the total bottomland cleared of <br /> vegetation in the range in the range ± 25 to 40 percent. <br /> 4) Major intensity is herein' defined as a net change a net change in the total bottomland <br /> cleared of vegetation by ± 40 percent or greater. <br /> Type: An operation which clears the canyon bottomland to a degree significantly improving <br /> upon the No- Action could be beneficial. <br /> Table 1 summarizes the community composition expressed as total percentage of vegetation <br /> removed from riparian bottomland. <br /> Table 1. <br /> Peftwetitage Preimpound No - Alternative Alternative Alternative Alternat <br /> vegetation ment Action A 8 C D <br /> removed <br /> Total 33 21 23 25 26 23 <br /> Scour 30 11 14 18 18 15 <br /> Inundation 3 10 9 7 8 8 <br /> Percent -63 27 39 39 36 <br /> change by <br /> shearing <br /> (total) <br /> 'Change in No- Action relative to the preimpoundment condition <br /> 2 Change in an action - alternative relative to the No- Action <br /> CONTEXT: Operations governed by the No- Action remove scour woody vegetation from 63 <br /> percent less of the bottomland than the preimpoundment flows. The context is regional. <br /> Each of the action - alternatives A through D improves on the context of control of woody <br /> vegetation by scouring. Each action - alternative is effective in a local context. <br /> DURATION: Flows of 10,000 cfs scour about 50 percent of the bottomland (refer to <br /> Figure 2). These magnitude flows occurred about every 3 years. This average flow coupled <br /> with the longer- return flows of 14,000 cfs may have effectively prevented establishment of <br />