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fll\ ,1 ;) We have also reviewed the routing provided us of a Probable Maximum Flood through the proposed reservoir. This flood had an estimated peak inflow of 497,000 cfs and a 10 day volume of slightly over 2,000,000 af. Using both spillways during passage-of the Probable Mmdmum Flood volume of over 2,000,000 ai, the reservoir operation would result in a reservoir level above Elev. 4428.5 for only slightly over 7 days. Analogously, for a Once in 1000 Years Flood of about 1,000,000 af, reservoir levels would not be above Elev. 4428.5 for more than about 3 days. This condition is considered to be of basic importance because it determines Elev. 4428.5 as the reason- able ma.'l:imwn design elevation of reservoir water surface for steady seepage conditions, inasmuch as it is our judgment that the rare and very brief incursions of surface flood- ing above that elevation could not possibly saturate the foundation of the long, right abut- ment, non-cutoff reach of the dam, as presently designed with a slurry trench cutoff to Sta. 231, suffiCiently to create steady state seepage conditions exiting at the downstream toe, except possibly at those locations to the right of the slurry trench where the grolID.d surface is below about Elev. 4400. Such locations are limited and would require special study and determination whether seepage exit control is justified. 3. Foundation Conditions a. Left Abutment and Riverbed. It is our understanding that this reach of the dam is underlain at shallow depth by the Pierre Formation (siltstone). Overburden consists of weathered siltstone, aeolian sand, and, in the riverbed, recent sandy alluvium. The feasibility design calls for an excavated cutoff to siltstone under the core zone, and minimal surface stripping elsewhere. It is our impression that the latter requirement should be reviewed, insofar as it seems probable that the river allu- vium should be expected to be at a low relative density, and the weathered Pierre may be thick and appreciably less strong than the compacted embankment fill to be placed on it. Stripping should, in our judgment, be done sufficiently to expose foundation of eqUivalent or superior strength when compared to the embankment fill strength. We concur that the Pierre Formation is low in permeability and would provide a suitable base for the seepage cutoff. We do have some concern about the Pierre as a foundation for the Service Spillway and its Chute and Bucket, and for the River Outlet 10.