HISTORICAL INFLUENCES ON 21ST CENTURY PROBLEMS IN A SUBURBAN WATERSHED, CHESTER COUNTY, PA

The Plum Run watershed is a 935-ha sub-basin of Brandywine Creek in Chester County, PA which is listed as having impaired water quality (PA DEP). Prior to the 20th Century Plum Run was dammed for water power and land was cleared for urban development (West Chester) and agriculture. More recently West Chester has become an area for rapid suburban development. As part of a multidisciplinary study to identify degraded portions of the stream, to establish a baseline for continued study, and to assess remediation we have measured channel morphology at 14 stations throughout the watershed. Cross-sections, profiles, and maps around each station have been incorporated into a GIS database along with land use and biotic and geochemical water quality data. The graphics are scaled to enhance visual comparison of channel characteristics among stations.

We model water height at 10 stations with sub-basins < 350 ha. The flow arriving at a station is estimated from the rational method based on a specified rate of precipitation and each sub-basin's area and land use. The height of water necessary to accommodate the flow is calculated recursively from Manning's equation based on our measured cross-sections and profiles. As an extension, we have incorporated our models into an Excel workbook. Excel has the advantage that it is easily accessible to students who can apply it to interpret watersheds in a classroom or undergraduate research setting. Our results suggest that present-day channel-forming flows are creating inflections in the cross-sections and that channels at some stations have been incised to an extent that virtually eliminates overbank flow. Incision and erosion of bank sediment seem to be major sources of sediment moving through the stream. Down-cutting in one channel and a tributary is associated with relatively recent (~1980's) straightening to accommodate an athletic field. Significant effects also persist from a broken 19th to early 20th century mill dam which exposes reservoir sediments to erosion, creating a 3-m deep channel.