GIS mapping of Pennsylvania stoneflies revealed that some species are aligned within historical major river basin connections rather than present day flows.  These connections are especially pronounced for several  species of Alloperla, Sweltsa (Chloroperlidae), and Allocapnia (Capniidae).  Three of Pennsylvania’s four major river basins, the Delaware, Susquehanna and Potomac, flow into the Atlantic Ocean, through either the Delaware or Chesapeake Bays.  The fourth river, the Ohio, flows into the Gulf of Mexico via the Mississippi River.  A small portion of northern Pennsylvania flows into the Great Lakes-St. Lawrence River.  Despite its present connection with the Susquehanna River and Chesapeake Bay, the Potomac basin shares many species with the Ohio basin, likely due to its longer historical connection with the Ohio during the Permian through Jurassic, the period of major stonefly speciation.  The upper West Branch Susquehanna River, which once flowed north or west into the present Great Lakes or Ohio basins, also shares species with the Potomac and Ohio basins.  Headwater erosion, stream piracy, and the advance of glaciers through northeastern and northwestern Pennsylvania have more recently affected existing flow patterns and species distributions.  The Allegheny Front also acts as a divide between major physiographic provinces and stonefly species.