TWO CONTRASTING FALL LINE WATERSHEDS IN NW DELAWARE AND SE PENNSYLVANIA

Streamflow in natural watersheds is typically considered a mixture of baseflow from groundwater discharge plus transient increases in discharge from precipitation events. In contrast, baseflow within urban watersheds is considered to be limited by decreased infiltration due to extensive impervious land cover, with a flashy response to runoff from precipitation events. We examined two contrasting fall line watersheds in NW Delaware and SE Pennsylvania (White Clay Creek, a designated Wild and Scenic Waterway, and Darby Creek, respectively). Hourly average data for discharge, water temperature, and specific conductance over a one-year period (January 2022 to January 2023) from the USGS streamflow monitoring stations #01478650 (White Clay Creek in Newark, DE) and #01475510 (Darby Creek near Darby, PA) were augmented with daily to bi-weekly grab sample measurements of stable isotope ratios of H and O in water. Catchment areas above these stations were 277 sq. km and 199 sq. km, respectively.

Analysis of land cover and runoff curve numbers revealed that the Darby Creek watershed is 21% more developed than the White Clay Creek watershed and has a higher runoff curve number average. White Clay Creek has higher base flow discharge and lower average specific conductance than Darby Creek; Darby Creek water shows strong increases in specific conductance during winter months, especially around precipitation events, indicating likely input of road salt, whereas such increases are minor or negligible in White Clay Creek owing to its protected status. The average lag time between peak rainfall and peak streamflow is longer for White Clay Creek, about 8 hours, than for Darby Creek which is about 3 hours. We found a weaker correlation between peak discharge and lag time for Darby Creek than for White Clay Creek. Isotopic variability of water in the streams is similar but smaller than that of local precipitation. There is more variability in Darby Creek’s δ18O values than for White Clay Creek. During and after precipitation events, the relaxation times of discharge peaks were shorter at Darby Creek.