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'.( . r? . ;.. ~ ... -'or; 001556 . . . . 'fraction.(Fig;4) were typical of all size classes and indicated that: transport capacity was 1-2 orders of magnitude greater than supply. The. next step was to evaluate how much the discharge could be reduced under post-project conditions without occurrence of aggradation. It was assumed that the reservoir would trap all particles in the gravel size fraction. AdditionalLy, based on Andrews' conclusions and lack of signifi- cant tributary sediment sources below the reservoir (within the forest boundary), it was assumed that there would be an insignificant supply of the gravel size fraction below the reservoir. Using the calibrated trans- port relationships, the discharge required to move the remaining sediment supply was, then evaluated for both sites A and B. Results indicated that 85 cfs would be required below Site A to move all the pre-project supply of sediment . finer than gravel, while 10 cfs would be required below Site B. To move all pre-project sediment supply (including gravels) would require 30 cfs below Site B, a value that is considered a very conservative main- tenance flow requirement.:: :Channel. stability under these flow conditions was not considered. to be impacted since the existing channel:. Jsupply. .- limited) is stable,.probabli due to armoring by coarse particles. VEGETATION ENCROACHMENT The above results suggested that a relatively small discharge would adequately prevent sediment deposition in the channel downstream of the project. Therefore, the flows required to minimize vegetation encroachment became the controlling factor in. defining required bypass flows. It is generally accepted that established vegetation cannot survive long periods of submergence or mean velocities higher than 5-6 fps. Under pre-project bankfull conditions (190 cfs), the mean velocity was 3.7 fps; therefore, it was concluded that a 16-day bypass time period (calculated from strict application of the Chapter 30 procedure) provided an adequate length of submergence to minimize vegetation encroachment. Under the post-project 85 cfs flow necessary to prevent aggradation below Site A, the velocity is only 2.5 fps. Therefore, this flow would also be required for 16 days to minimize vegetation encroachment below the waterline. The 85 cfs flow covers 98 percent of. the pre-proj ect bankfull wetted perimeter; conse- . '.'._quently, it can be.. concluded that a post-project flow of 85 cfs below Site. A is as effective as a channel maintenance flow as 190 cfs was for the pre-project condition. For Site B the 10 cfs flow required .to prevent aggradation leaves a large portion of the channel bed exposed which could be encroached on by vegetation. Under the 30 cfs flow required to transport all the pre-project sediment supply, 74 percent of the pre-project wetted perimeter is covered. To provide a more significant submergence factor would require about 50 cfs, for which 89 percent of the pre-project wetted perimeter is covered. Based on these results, a 50 cfs bypass for 16 days was considered neces- sary to control a majority of the vegetation encroachment below Site B.