Background/Question/Methods Ponds have increased more than 18-fold in the Brandywine watershed, southeastern Pennsylvania since 1937. These ponds trap sediments from their catchments, reducing sediment loading to stream networks and influencing pond longevity. This study investigated how sediment characteristics vary within ponds as a function of depth and distance from shore, and how sediment accumulation rates vary among ponds as a consequence of differences in size, age, and drainage basin. Ten ponds representative in size of most ponds in the watershed were sampled during summer 2009. Five had influent headwater streams, and five had no surface inflows. Ten sediment cores were obtained in a regular grid pattern from each pond, allowing for measurements of sediment characteristics: dry bulk density (dBD), organic matter (% OM), and % sand. Based on GPS-indexed depth measurements of the current and original pond bottom (n = 32-45), changes in pond volume were determined using ArcView 3D Analyst, and were used to compute total sediment volume, mass, and organic matter storage.
Results/Conclusions
Within ponds, dBD, % OM, and % sand were all strongly intercorrelated (p < 0.001), and differed significantly among ponds (p < 0.01). The presence of an inflow did not influence the sediment characteristics. Based on simple linear regressions, decreased dBD and sand content, and increased % OM, accompanied increasing distance from shore, and (to a lesser extent) increasing depth. Among ponds, total sediment volume, mass, and OM storage were all intercorrelated (p < 0.001). Older ponds and ponds with larger watersheds showed increased sediment volume, mass, and OM (p < 0.001). Larger ponds with inflows showed increased volume, mass, and OM, and increased rates of annual sediment accumulation (SAR; m3/yr). Area-specific sediment accumulation rates (SARAS; m3/ha/yr) declined with increasing pond size, however, and were significantly higher (p = 0.029) among ponds with stream inflows. The SARAS values reported here are lower than in studies of agricultural impoundments, and greater than rates for larger lakes and reservoirs. Given our relatively high SARAS estimates and rapidly increasing pond densities, small ponds show an increasing potential to modify water quality within the Brandywine Creek watershed.
