This map was created by the USGS to demonstrate the hydrogeomorphic regions within the Chesapeake Bay. The underlying rock type and physiographic province influence the range of ground-water discharge values.  These characteristics were used to divide the Chesapeake Bay watershed into eleven "hydrogeomorphic regions" or HGMRs. The characteristics of the rock type in each HGMR provide a surrogate for permeability and mineral composition of aquifers.