ENVIRONMENTAL EFFECTS OF SEDIMENT ACCUMULATION BEHIND HISTORIC MILL DAMS ON PLUM RUN, SOUTHEASTERN PA

Mills powered by water were common in the Piedmont during the 18th and 19th centuries. Walter et al. (2006, GSA Abstr. 38(2), p. 84) show that mill dams formed a nearly continuous sequence of mill ponds along streams in the Conestoga River system. They propose that the ponds trapped sediment (‘legacy sediment') during the colonial period which is being released from dams breached in the last century. They have documented chemical and sedimentologic characteristics of legacy sediments and geomorphic features of streams to show the importance of legacy sediments in modern day sediment and nutrient transport.

The Plum Run watershed is a 935-ha sub-basin of Brandywine Creek in the Piedmont province of southeastern PA. We have investigated a number of small mill dams and associated sediment accumulations. The largest feature is the 3-m high rock dam at Strode's Mill that formerly captured about 650 ha of drainage. Total-station surveys show that a wedge of fine-grained sediment above the dam forms an exceptionally flat surface consistent with accumulation in a pond. The modern stream channel is entrenched within steep banks that are subject to rapid mass-wasting.

We estimate historic sediment yield and modern erosion rates from historic information, taking into account uncertainties in the model inputs. We also use geomorphic models and repeated topographic sections across the channel to estimate recent bank erosion rate. We consider how entrenchment of the low-sinuosity channel influences flood hazard and stream erosion downstream from the dam. Because Plum Run has a gradient of 0.01, much higher than the average of 0.001 for the Conestoga, the Strode's Mill pond was proportionately smaller, probably extending no further than 300 m upstream, and holding less sediment. We model the effect of stream gradient on the historic trapping efficiency of legacy sediment by mill dams.