by Hannah Connuck, Franklin & Marshall College
Conferences, meetings, and presentations bring large groups of scientists together annually, but it was a rare event to gather more than 30 scientists, professionals, and students for a day of field work at the “Big Dig” in Lancaster, PA this April, 2024. Over the past several years, the Chesapeake Watershed Initiative (CWI) team has collaborated with LandStudies, Inc., the USDA Natural Resources Conservation Service (NRCS), and other stakeholders to excavate numerous sediment trenches with backhoes for observation and data collection.
One might ask: what about the work at this site enticed so many specialists, some of whom traveled thousands of miles to see the trenches? Some geologists were hoping to glimpse the deeply buried rubbly material which dates back 12,700 years ago according to radiocarbon dating. The pebbles mixed with finer sediment help tell the story of ice, permafrost, and glaciers of the region and tend to be at the bottom of the 1.5-2m trench. Ecologists and botanists were lured by the next layer: dark marshy soil that F&M students have painstakingly scoured for seeds and pollen to reconstruct the plant community that developed as temperatures warmed following the ice age. Restoration professionals were interested to see firsthand the ancient marshy soils buried beneath millpond mud that support the legacy sediment removal restoration strategy. Generally, digging long, deep, trenches with the explicit purpose of carefully investigating the sedimentary layers is a rare opportunity for practical reasons, like cost, permitting, and planning, so many jumped at the chance to join the CWI for this Big Dig.
Digging the trench while observers look on and sediment is set aside (top right) captured by a drone. Photo: Alice Fodor
Experts gather around to see a piece of the side of the trench, with dark marshy soil on the bottom (left) and legacy sediment on top (right). Photo: Hannah Connuck
The reason for digging these trenches is to understand the history of wetland valley bottoms in Lancaster and the broader Mid-Atlantic region prior to disturbance that began during Colonial settlement. Over the past 20 years, Professors Dorothy Merritts and Robert Walter have used their expertise in geologic changes over time to identify the different sedimentary layers that are commonly found in valley bottoms. Notably, the CWI has developed the idea that colonial-era settlement, dam construction, and dam removal/failure have reshaped the landscape. A battery of tests on the different sedimentary layers across many sites over many years has revealed that the damming of streams changed wide, flat, wet meadows into what we see today: distinct stream channels with high, steep banks made of “legacy sediment” that built up behind dams and remains even after they fail or are removed.
Trench dug in February 2023 near the Big Dig site showing the rubbly bottom later, dark marshy middle layer, and the thick legacy sediment on top. Photo: Dorothy Merritts
These findings have a number of practical implications related to history, conservation, and habitat conditions, both locally and stretching to the Chesapeake Bay and beyond. Local historians – like Donald Kautz, known for his blog and books on mills in Lancaster County – have worked with the CWI to identify dams along local streams and the historical context of their placement, size, and longevity. Local firms like LandStudies Inc. and other stakeholders, including local landowners have leveraged CWI findings and research to help guide sustainable valley bottom restoration projects. Removing legacy sediment significantly reduces the steady input of phosphorus, nitrogen, and sediment into local streams, improving the water quality, ability to support plant and animal populations, and downstream accumulation of sediment in places sensitive areas like the Chesapeake Bay. Additionally, excavation lowers the ground surface, reconnecting the stream to cool groundwater that helps support populations of fish, plants, and other organisms that thrive at cooler water temperatures. The tangible benefits for landowners that live along mill sediment-impacted streams include flood reduction, better recreational access, and cleaner water for drinking, fishing, and recreation.
Graphic developed by LandStudies, Inc. showing the unrestored floodplain with deep channels cut into thick legacy sediment (top) and a restored floodplain sitting directly in the marshy soil layer with wandering shallow streams running through a grassy wetland.
The location of the Big Dig, tucked away behind a retirement home along the Little Conestoga Creek, is ideal for trenching because it is close enough to a failed dam that the legacy sediment is easy to see, but out of the way enough that sediment removal would not disturb any modern infrastructure. Luckily for Lancastrians, this location is also perfect for restoration, and over the next few years, will have legacy sediment excavated, a wetland kickstarted with clever engineering and planting techniques, and public access facilitated by the Blue Green Connector!
For more information about the Chesapeake Watershed Initiaitve’s work, visit our website at www.ChesapeakeWatershedInitiative.org, or follow us on Facebook and Instagram at chesapeake.wi and LinkedIn at Chesapeake Watershed Initiative.