Shallow depressions found in karst terrains may contain temporary (vernal) pools that are inundated seasonally in response to changes in meteorological conditions. The hydrogeology of 16 pools (0.06–0.4 ha) was studied in an Appalachian karst valley in central Pennsylvania, USA. The objective was to determine the effect of the geologic substrate on pool hydroperiod. Meteorological, geophysical, and hydrogeological data collected from November 1997–August 1999 and from January 2002–January 2004 suggested that hydroperiod was primarily controlled by meteorological conditions (total annual precipitation) and surficial aquifer geology. Multiple regression models were found to predict most of the spatial variability of pool hydroperiod with the following variables: thickness of the surficial sandy aquifer; sediment electrical resistivity; and annual precipitation. It might be expected that hydroperiod would be longer for clay pools than sandy pools because clay sediments can act as a seal to perch shallow ground-water and surface-water. Our data revealed the opposite to be true. Sandy residual sediments helped capture infiltration and direct this water along perched ground-water lenses or sheets to seasonal pools. This resulted in annual hydroperiods that were 115 days longer for sandy pools when compared to clay pools. The results suggest that the geologic substrate can be a major control on the duration of hydroperiod.