The Yucaipa area is a small alluvial-filled basin bordered on three sides by crystalline bedrock. Several faults that transect the alluvial deposits retard the flow of ground water and divide the area into seven separate ground-water subbasins. Each of the subbasins was evaluated as a potential recharge site, and Wilson Creek subbasin was selected as the most favorable. The study indicates that artificial recharge of 6,000 acre-feet per year should be possible through surface spreading in Wilson Creek subbasin.
The test drilling and infiltration test conducted at the Yucaipa damsite indicate that material underlying the site is highly permeable and readily transmits water. An unlined reservoir at this site would initially lose a considerable quantity of water through infiltration and would be difficult, if not impossible, to fill.