The geohydrology of Pinnacles National Monument, California

Water supplies for Pinnacles National Monument are obtained from a collection gallery in alluvium and from a flowing well which obtains water from fractured breccia and tuff near a fault.

Rocks other than alluvium, in all but the northern fourth of the monument, are dense igneous or metamorphic types that, unless fractured, have little potential for development of ground water. However, in some areas near faults these rocks probably will yield small supplies of water. The alluvium along Chalone Creek contains water of good chemical quality in quantities sufficient to supply the monument's needs. The Temblor Formation, a fanglomerate in the northeastern part of the monument, seems to be capable of absorbing and transmitting water but has not been drilled. A test well to determine the potential of the Temblor is suggested.