GEOLOGICAL CONTROL OF BASEFLOW IN THE DEER CREEK WATERSHED, LOWER SUSQUEHANNA RIVER BASIN, PENNSYLVANIA AND MARYLAND

Characterizing the proportion of stream flow contributed by baseflow is essential for quantifying water availability, nutrient loading, and contaminant transport in watersheds. Bedrock composition (together with precipitation, land use, and topography) is known to be a primary control on baseflow. Baseflow separations, however, are generally derived from long-term streamflow gaging stations for which local geologic variation may not be a primary siting criterion. In this study, we report on the results of streamflow measurements from 12 sub-basins of the Deer Creek Watershed (DCW) in Pennsylvania and Maryland showing the impact of geological control on baseflow.

The DCW occupies approximately 171 square miles of agricultural, forested and subsidiary developed land in the lower Susquehanna River basin. Flow in Deer Creek is dominated by baseflow, with ~67% of total flow derived from groundwater, as calculated from USGS gaging station records. For this study, we completed sixty streamflow measurements (a minimum of three per sub-watershed) over a period of 6 months, under varying baseflow conditions. The unit baseflow per square mile was calculated for each subwatershed, following normalization to the USGS gage at Rocks, MD. Upper portions of the DCW are characterized by baseflow of 1.0 cfs/sq. mile. Lower portions of the watershed are characterized by values averaging 0.76 cfs/sq. mile. This variation can be attributed to the spatial distribution of the Wissahickon/Octararo Schist and the Baltimore Gneiss/Port Deposit Gneiss that underlie upper and lower portions of the watershed, respectively. The geologic contact between these areas approximately coincides with the location of US Route 1.

Pervious characterization of the DCW has generally relied upon long-term records at Rocks, MD in the upper DCW. Using population projections (25% increase over 25 years) and water budgets for average and drought years, it appears that lower portions of the watershed are those with the greatest potential for demand shortfall under future drought conditions – a result that would not be apparent from using the USGS gaging data from upper portions of the DCW.