Environmental setting, water budget, and stream assessment for the Broad Run watershed, Chester County, Pennsylvania

Abstract

The Broad Run watershed lies almost entirely in West Bradford Township, Chester County, Pa., and drains 7.08 square miles to the West Branch Brandywine Creek. Because of the potential effect of encroaching development and other stresses on the Broad Run watershed, West Bradford Township, the Chester County Water Resources Authority, and the Chester County Health Department entered into a cooperative study with the U.S. Geological Survey to complete an annual water budget and stream assessment of overall conditions. The annual water budget quantified the basic parameters of the hydrologic cycle for the climatic conditions present from April 1, 2003, to March 31, 2004. These water-budget data identified immediate needs and (or) deficits that were present within the hydrologic cycle during that period, if present; however, an annual water budget encompassing a single year does not identify long-term trends. The stream assessment was conducted in two parts and assessed the overall condition of the watershed, an overall assessment of the fluvial-geomorphic conditions within the watershed and an overall assessment of the stream-quality conditions. The data collected will document present (2004) conditions and identify potential vulnerabilities to future disturbances. For the annual period from April 1, 2003, to March 31, 2004, determination of an annual water budget indicated that of the 67.8 inches of precipitation that fell on the Broad Run watershed, 38.8 inches drained by way of streamflow to the West Branch Brandywine Creek. Of this 38.8 inches of streamflow, local-minimum hydrograph separation techniques determined that 7.30 inches originated from direct runoff and 31.5 inches originated from base flow. The remaining precipitation went into ground-water storage (1.71 inches) and was lost to evapotranspiration (27.3 inches). Ground-water recharge for this period-35.2 inches-was based on these values and an estimated ground-water evapotranspiration rate of 2 inches. Assessment of fluvial-geomorphic conditions included large-scale mapping of stream classes within the Broad Run watershed and in-depth study of three representative stream reaches also within the Broad Run watershed. Based on the total distance of all stream reaches classified within the Broad Run watershed, 61 percent were classified as C-class, 14 percent as E-class, 13 percent as B-class, 5 percent as F-class, 4 percent as undifferentiated B- and F-class, 2 percent as G-class, and less than 1 percent as A-class. The map of stream classes indicates that the Broad Run watershed currently has no large-scale areas of stream impairment and that, generally, the stream is not entrenched and the main branch of the Broad Run has an available, functioning flood plain. Smaller tributary streams, however, showed signs of localized entrenchment due to site-specific influences such as natural stream-channel evolution, localized channelization, localized contraction due to road and driveway crossings, and (or) increased localized runoff. For example, one small reach along a tributary channel was observed to become entrenched due to runoff originating from a new housing development. Entrenched stream reaches are merely located by large-scale mapping and require individual assessment to determine potential causes of entrenchment and (or) future restorative actions. Three in-depth geomorphic study sites showed that the Broad Run can currently be considered graded or in a state of dynamic equilibrium. The sites did, however, identify certain vulnerabilities to future changes within the watershed. These vulnerabilities included disruption of the present sediment supply, including both increases and (or) reductions in the current sediment loads within the Broad Run; increases in both magnitude and duration of storm-water runoff; encroachment of development onto present flood-plain areas, and (or) alterations to riparian zones. Assessment of stream-quality conditions includ