THE GREAT SEDIMENT EXPERIMENT: POST-SETTLEMENT LAND USE, EROSION RATES AND THE LEGACY OF SEDIMENT STORAGE IN THE MID-ATLANTIC REGION OF NORTH AMERICA

We use the Conestoga watershed, Lancaster County, PA, to demonstrate how Mid-Atlantic streams responded to land use change and mill dam construction during the 18th to 19th Centuries. Historic air photos indicate that rilling and gullying on agricultural slopes were widespread in the 1930s-40s, common in the 1950s, and rare since then, yet sediment yields in the Conestoga are among the highest measured in the Chesapeake Bay watershed (61 to 827 tons/km²/yr). By 1840, the largely denuded Conestoga drainage basin contained >160 mill dam reservoirs, which converted streams to ponds, trapped sediment, and raised regional base level 1.2 m. Three approaches were used to estimate the volume of sediment eroded from slopes and stored in valley bottoms: (1) the product of average mill dam height, pond length and valley width (47 x 10⁶ m³); (2) an average of 1.2 m of sedimentation in every stream valley (124 x 10⁶ m³); and (3) LIDAR to map mill pond and backwater fill surfaces (64 x 10⁶ m³). Based on ¹³⁷Cs and ²¹⁰Pb analyses, we observe that mill ponds were filled with sediment in 150 years and, assuming a trapping efficiency of 100%, we calculate a minimum sediment yield of 540 tons/km²/yr, an upland denudation rate of 438 m/My (~49 times higher than the long-term geologic rate), and a loss of ~7 cm of soil, comparable to other estimates of soil erosion in the Piedmont. If reservoir trapping efficiencies were 50%, these values become 1080 tons/km²/yr sediment yield, 876 m/My denudation rate, and 14 cm soil erosion. Beginning in the late-17th Century, generations of mid-Atlantic residents unwittingly participated in “The Great Sediment Experiment”: eroding large amounts of soil from cleared, tilled land; storing much of that soil in stream corridors behind mill dams; increasing stormwater runoff through suburbanization; and finally flushing sediment at high rates as dams breached. As mill dams breach, streams become entrenched and resemble arroyos in the American southwest. Suspended sediments yields from streams entrenched into legacy sediments in the stream corridor remain high despite modern conservation farming practices on upland slopes. Channel surveys and geochemical fingerprinting of in-stream sediments indicate that bank erosion could account for at least 50% of the suspended sediment load in the Conestoga River.