Historic Reservoir Sediment Storage and Erosion Rates from Lidar and DEM Differencing, Chiques Creek Watershed, Pennsylvania

Dam removal is accelerating nationwide, but Pennsylvania, mostly within the Chesapeake Bay watershed, leads the nation in number of lowhead dam removals, even as state and federal agencies develop Best Management Practices and Watershed Implementation Plans to reduce sediment loads to the Bay. Fine sediment carried as suspended load is one of the key pollutants in the Chesapeake Bay, listed as impaired under 303(d) of the Clean Water Act. Understanding the source of this sediment is crucial to reducing its load. After milldams breach, stream channels cut down through the former sediment-filled reservoir and transport erodible, fine sediment downstream. The rates at which reservoir sediment erodes and volume of historic sediment remaining in storage were generally unknown until our work over the past few years.

For the Chiques Creek tributary (326 km^2 watershed area) to the Susquehanna River in the heavily milled Piedmont physiographic province of unglaciated south-central Pennsylvania, we located 48 milldams from historic maps and field work, with an average spacing of one milldam per 2.5-3.2 km of stream length and density of 0.11 milldams/km^2. Thirty-six of these obsolete milldams breached from disrepair since the early 20^th century. Three other dams were removed between 2013 and 2018, and 9 are still in place. More than 2 million m³ of historic sediment remains in storage along the valley bottom throughout the watershed, yielding an average of 30 m³ of sediment per m of stream length. Our field mapping demonstrates that this sediment is largely silt and fine sand with sufficient cohesive strength to maintain a vertical bank up to several m high where milldams are breached and stream channels incised to the level of buried periglacial rubble.

Based on lidar dem differencing for 2008 to 2014, more than 30,000 m³ (>40,000 tons) of sediment was eroded from stream banks in the Chiques watershed each year from 2008 to 2014. Our investigations at a recent dam removal (July 2018) site in this watershed are documenting the rate of erosion of reservoir sediment during the first months after dam removal and its impact on suspended sediment load.