HISTORIC MILLPOND SEDIMENT STORAGE AND POST-DAM BREACH EROSION, CHIQUES CREEK WATERSHED, PENNSYLVANIA

The Chiques Creek watershed, a tributary to the Susquehanna River in the historically heavily milled Piedmont physiographic province of south-central Pennsylvania, is critical for evaluating the state’s Watershed Implementation Plan (WIP) to reduce sediment and nutrient loads to the Chesapeake Bay. The WIP documents how a jurisdiction will achieve and maintain water quality standards of the Chesapeake Bay Total Maximum Daily Load (TMDL) by 2025. One of the key pollutants in this watershed, which is listed as impaired under 303(d) of the Clean Water Act, is fine sediment carried as suspended load. A major, but underappreciated, source of this sediment is erosion of historic sediment stored upstream of obsolete low head milldams (typically 1-3 m high). This watershed has special importance because a large number of its milldams remain intact. However, Pennsylvania leads the nation in low head dam removal, with several in the Chiques watershed removed in the past ten years. More milldams are slated for removal in the next few years, even as the state tries to meet its sediment reduction goals for the TMDL. Our field and lidar differencing analyses of two recent dam removal sites shows rapid channel incision and substantial bank erosion.

Here, we (1) quantify the volume of historic sediment that is stored upstream of approximately two dozen 18^th-early 20^th century milldams in the Chiques watershed, and (2) compare this amount to known rates of post-dam breach erosion of millpond sediment. At least 21 milldams exist along ~54 km of the main stem, which is equivalent to one dam every 2.6 km. More than 2 million m³ of historic sediment remains in storage along the valley bottom, yielding an average of 30 m³ of sediment per m of stream length. Our field mapping demonstrates that this sediment is largely silt, clay and fine sand with sufficient cohesion to maintain near vertical banks. Post-dam breach bank erosion rates vary from 12 to 0.5 m/yr within the first 10 years after dam breaching, and diminish slowly thereafter, remaining about 0.05 m/yr for the next 8-10 decades. As a result of the large amount of fine sediment in storage in this watershed, and the relatively high erosion rates following dam breaching, it is likely that more than 30,000 m³ (>40,000 tons) of sediment is, and will be, eroded from stream banks in this watershed each year