The aquifers (Paleozoic) of central Texas, which include both park districts, provide varying amounts of water to the area. Locally, these aquifers provide minor amounts of water for domestic and livestock use.

The initial flow of the major streams within the seven-county area comes from seeps and springs, primarily from the Edwards-Trinity aquifer.  Groundwater in the area is generally available from the strata of Hickory Sandstone, Ellenburger Limestone, Hensell Sand and Edwards Limestone.