The Loma Prieta (California) earthquake (October 17, 1989; M 7.1) caused significant changes in the hydrology of the San Lorenzo and Pescadero drainage basins, northwest of the epicenter. Streamflow increased at most gauging stations within 15 min after the earthquake. Ionic concentrations and the calcite saturation index of the stream water also increased. Streamflow and solute concentrations decayed significantly over a period of several months following the earthquake. Ground-water levels in the highland parts of the basins were locally lowered by as much as 21 m within weeks to months after the earthquake. The spatial and temporal character of the hydrologic response suggests that the earthquake increased rock permeability and temporarily enhanced ground-water flow rates in the region.