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0,14.2 stability of this reach may be threatened by under seepage related to the known presence of a deep, highly pervious aquifer centered on an old but now deeply buried channel of the South Platte River. Based on the information provided to us, following is our con- cept of the foundation conditions in the non-cutoff reach of the dam, with particular reference to the deep aquifer. The long, no-cutoff reach is surficially covered with loose aeolian sands, hav- ing thicknesses varying from 0 to 15 ft. The sands, where present, overlie alluvial fan deposits of heterogeneously stratified layers of sands, clayey sands and silty sands having a total thickness of 100 to 150 ft. and having a permeability of generally between 50 and 10,000 ft/yr. with an effective "average" on the order of 2000 fl/yr. We note that, probably to be on the conservative side, the USBR seepage analyses utilized sub- stantially higher permeability values. The deep aquifer, as we visualize it, passes under virtually all of the non-cut- off reach of the dam, is composed of pervious gravels, is from 5 to 70 ft. thick, and has an estimated permeability on the order of 100,000 to 300,000 it/yr. The existing basIc ground water table in the non-cutoff reach is on the order of 50 to 70 ft. below ground sllrface. Many, very localized, perched water tables prob- ably exist, where limited layers of clayey sand intercept downward seepage from pre- cipitation and irrigation. 4. Ground Water and Seepag:e Analysis. It is apparent that the potential seepage conditions around and under the right abutment reach of the dam have been in- tensively studied both by USBR engineers, and on their behalf by USGS specialists. The emphasis on this aspect of the project has related both to potential water loss from the reservoir and to potential safety and stability problems at the dam. It seems appar- ent that the present, primary concern is related to the characteristics of the deep aquifer that passes under the right abutment reach of the dam, as noted earlier. The questions have been basically: (a) How much additional seepage would the aquifer con- vey under project conditions? and (b) Will uplift pressures primarily associated with the known aquifer performance increase under reservoir conditions to a sufficient e.'(- tent to adversely pressurize the presently non-saturated alluvium that would be the 12.