The Legacy of Historic Mill Ponds

        Whereas many ponds today receive little or no stream inflow, a much greater proportion of ponds in the region were historically “in-line” impoundments directly receiving inputs from larger streams. Some of these remain today, while the locations of others can be deduced from historical records and evidence obtained from careful fieldwork. Based on extensive work by Walter and Merritts (2008), approximately 130 dams probably existed along streams in the Brandywine watershed during the colonial and early American eras. Most were intended primarily to channel water into mill races, providing power for a wide variety of activities such as grinding flour, cutting lumber, shaping metal, and weaving cloth. The occurrence of mill ponds behind these dams coincided with a time of substantial deforestation in upland areas and consequent soil erosion, especially from agricultural land, much of it transported down local streams. Because the mill ponds were “in-line”, large quantities of sediment were deposited in the ponds as water velocities declined behind the dams. When the dams were eventually breached (as has happened to some 3/4 of the mill dams in the Brandywine watershed), stream bank erosion eroded large quantities of sediment formerly trapped in the mill ponds at a greatly accelerated pace (Pizzuto and O’Neal 2009).
        A map of a former mill pond located on Plum Run just upstream of the Strodes Mill Art Gallery at the Corner of Rte 52 and Birmingham Rd. in East Bradford Township is shown in Fig. 6a. During its existence sediments accumulated behind the mill dam. Once the dam was breached, erosion of these “legacy sediments” from the former mill pond became a major source of downstream sediment/nutrient transport, and severely incised the current stream channel well below its banks (Fig. 6b).


Fig. 6a-b. (a) Map of a triangular mill pond at the junction of the east and west branches of Plum Run. Note the presence of a former mill race leading from the pond to the mill on the west side of the stream. (b) Incised bank along the east branch, near the location of its former entry into the mill pond. Note the very steep, rapidly eroding left bank.