Franklin & Marshall College Franklin & Marshall College

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Colonial Mill Ponds

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Colonial Mill Ponds of Lancaster County as a Major Source of Suspended Sediment to the Susquehanna River and Chesapeake Bay

Robert Walter and Dorothy Merritts
Department of Earth and Environment
Franklin and Marshall College
Lancaster, PA 17603

From the 18th through the early 20th Centuries, mills, mill dams and mill ponds were unusually abundant on Lancaster County streams. By 1840, over 500 water-powered mills were in operation in Lancaster County alone, which equates to one mill pond every 3 km of stream length. On some streams, such as on the West Branch of the Little Conestoga and the aptly named Mill Creek, the density was as high as one mill pond every kilometer. Such intense ponding, we surmise, essentially converted Lancaster County streams into linear slack-water ponds. Large volumes of eroded topsoil from post-settlement deforestation and tilling was trapped and stored in stream valley bottoms behind mill dams. Today, the mill ponds are completely filled with sediment and most dams are gone, some within the last few years as part of a conservation strategy to return streams to their natural state. We predict that much of the sediment load carried by the streams of Lancaster County today is not from modern soil runoff, as is often assumed, but rather is from reactivated, centuries-old sediment stored in valley bottoms behind historic mill dams.

The timing and ultimate source of sediment in Lancaster County streams has far-reaching implications beyond local interests in farmland conservation. The Conestoga watershed is an environmental hotspot, yielding by far more suspended sediment to the Susquehanna River per acre of land than any other part of the Susquehanna watershed. Sediment is the greatest stream pollutant in the United States, and it is especially devastating to the Chesapeake Bay as a contributor to low oxygen levels and habitat degradation.

We are working in partnership with faculty and students in the Departments of Earth and Environment, History, and Biology at Franklin and Marshall College, and with colleagues from the Lancaster County Historical Society, the U. S. Geological Survey, and Case Western Reserve University. We received a small grant from the Franklin and Marshall College Committee on Grants to support preliminary research, and we have submitted a proposal to the National Science Foundation for two-years of full research funding. It is anticipated that at least two undergraduate theses will be generated from this project.