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D., pers. comm.). A standard project flood corresponds to the 500 year discharge event, and is the flood occurring under maximum weather conditions reasonably characteristic of the region. Based on the FIRM floodzones, a 100 year flood would inundate an area up to approximately the 85 foot contour on the USGS, Clements 7.5-minute-series topographic map and a 500 year flood would inundate up to the 90 foot contour. Velocity and depth of flow within the flood plain during the 100 and 500 year flood event can be expected to be low due to the large cross sectional area over which streamflow would be dissipated. It is highly unlikely that a peak discharge event would provide sufficient velocities and depths at the proposed quarry site to alter channel alignment. The stability of the current channel alignment relies on both the containment of floodwaters by levees during peak streamflow events and the reduction of peak discharges by Camanche Reservoir. A review of aerial photos (Satz Joaquin County, Department of Public Works) indicate virtually no change in channel alignment in the project vicinity between 1937 and 1989. Several peak discharges well above the present planned maximum release of 5,000 cfs occurred during this time period. Discharges of 28,800 cfs and 27,300 cfs occurred in water years 1951 and 1956 respectively, before the completion of Camanche Reservoir. Vegetation encroachment on former channel point bars (upstream of the project vicinity) is also apparent in the aerial photos since 1963, and is indicative of reduced flood flows due to the reservoir. The probability of change in channel alignment is small based on this review of historic flows. The statistical chance of experiencing an 18,000 cfs streamflow in any single year is one in a hundred (100 year storm), and this discharge has already been exceeded without change in channel alignment. This analysis of channel stability is predicated on the assumption that there would be no levee failures due to factors other than the quarry operation. The quarry operation would remain at least 1,000 feet from the south bank levee, and would not affect the levee's structural integrity. If the levee were to fail in conjunction with an extreme flood event, there would be an increased possibility that the Mokelumne River could alter its present alignment by developing a channel that flows through the quarry. However as previously discussed, this would be unlikely since at least 1,000 feet of cross sectional area within the flood plain would provide dissipation of streamflow velocities 32