A contour map showing minimum depth to ground water from 1973 through 1979 was constructed for the upper Santa Ana River valley region. The map was prepared as an initial step in an ongoing liquefaction-potential study, but is not a liquefaction-hazard map. The contour map indicates where ground water shallower than 50 ft below land surface occurred at least once during the 1973-1979 period, and also indicates the probable future distribution of ground water shallower than 50 ft below land surface for periods when climatic conditions and water-management policies similar to those in the 1970's recur. This contour map does not show how the water table actually looked at any particular instant during the reporting period, nor does it show average or typical ground-water conditions during the reporting period. Instead, this map shows what the regional ground-water table would look like if the shallowest water level measured in each well during the 1973-1979 period is used as the basis for constructing a map of minimum depth to ground water.

This map identifies twenty areas within the upper Santa Ana River valley where water levels in wells were shallower than 50 ft below land surface at least once during the period 1973-1979. In the greater Santa Ana River area, between the San Jacinto ground-water barrier and Prado flood-control dam, ground water was shallower than 50 ft below land surface intermittently throughout the 1973-1979 period. In this area, shallow water generally reflects shallow depths to impermeable bedrock and the ease and frequency with which ground water is replenished by natural and artificial recharge. Most of the other areas of shallow ground water identified on this map experienced pervasive shallow water levels only after mid-1977. Prior to mid-1977, ground water in these areas generally was deeper than 50 ft below land surface.

During later parts of the 1973-1979 period, water tables rose mainly because of two factors: (1) wetter-than-normal winters in 1977-1978 and 19781979 contributed increased volumes of surface runoff and natural recharge in the upper Santa Ana River valley region; and (2) commencing in 1972, ground water in the Valley region has been replenished by artificial recharge of imported water derived from the California State Water Project. The accelerated natural and artificial recharge of ground water basins in 1977, 1978, and 1979 raised ground water tables throughout the Valley region.

Water-level records more recent than September 1979 indicate that for most of the twenty areas of shallow ground water, water levels have remained shallower than 50 ft below land surface through December 1981. In some areas, water levels have risen even more. For example, in the San Bernardino area, rising water locally has invaded basements, undermined roadways, and affected foundation construction. Where post-1979 water levels have continued to rise, the areas underlain by shallow ground water have expanded and now are larger than the areas shown on the contour map of this report.