AN INTEGRATED GEOMORPHIC/HYDROLOGIC STUDY ON THE EFFECTS OF LITHOLOGY AND BASELEVEL INFLUENCE ON DRAINAGE DENSITY AND GROUNDWATER/SURFACE WATER INTERACTION IN SLATE AND CARBONATE WATERSHEDS IN EASTERN PENNSYLVANI

We studied thirteen drainage basins in the Great Valley of Eastern Pennsylvania to examine the effects of lithology and local baselevel on drainage basin morphology and groundwater/surface water hydrology. Most of the basins we are studying begin in the Ordovician Martinsburg slate and shale and flow across a suite of Cambro-Ordovocian carbonates before discharging into the Delaware or Lehigh Rivers. However, several drainage basins are underlain entirely by slate/shale or carbonates. The Lehigh and Delaware rivers are incised approximately 50m below the carbonate surface and 150m below the slate/shale surface respectively, and establish regional hydrologic baselevel.

Drainage networks are well-developed and deeply incised in the shale/slate terrane and less prominent in the karst terrane. Drainage densities measured from high resolution hydrologic maps are higher by nearly a factor of two in the shale/slate terrane compared to the carbonate terrane (average drainage density in the shale/slate = 2.27 km/km² vs 1.29 km/km² in the carbonates).

One might expect the streams to be groundwater fed from springs in the carbonates and dominated by surface water runoff in the shale/slate, however, we find the exact opposite during low to “normal” flow conditions. Tributaries frequently originate at springs formed along fracture and cleavage porosity in the shale/slate or at the surface cover/bedrock interface.

We measured discharge under baseflow or near baseflow conditions in five streams across the slate/carbonate contact to determine whether the streams are losing or gaining from segment to segment. In the shale/slate terrain the streams are gaining, but when the streams transition into the carbonates, they become losing streams. Whereas all four of the streams that traverse both shale/slate and carbonates switch from gaining to losing across the boundary between the two lithotypes, the groundwater/surface water interactions appear to be complicated by the length of the traverse across the carbonates. The drainage basin that is entirely underlain by shale/slate also becomes a losing stream as it plunges from the high upland surface toward the Delaware River baselevel. This suggests that surface water/groundwater interactions are influenced by the low regional groundwater baselevel as well as lithology.