HIGH EROSION RATES IN EARLY AMERICA ESTIMATED FROM WIDESPREAD SEDIMENT TRAPPING IN THOUSANDS OF 18TH-20TH CENTURY MILL DAM RESERVOIRS, APPALACHIAN PIEDMONT, USA

With modern sediment yields (up to 405 tons/km²/yr) comparable to those in tectonically active mountain belts, the low-relief Appalachian Piedmont region of the eastern U.S. yields anomalously large amounts of fine-grained sediment to the impaired Chesapeake Bay. Beginning in the late 17^th Century, generations of residents of the mid-Atlantic region unwittingly participated in what we refer to as “The Great Sediment Experiment”: Rapidly denuding a forested landscape with a thick residual soil; eroding large amounts of soil from the cleared land by straight-row tilling; storing much of that soil in stream corridors behind tens of thousands of mill dams; then increasing storm water runoff through suburbanization; and finally flushing sediment out at high rates as dams breached and channels became deeply entrenched into the thick legacy sediments. The Chesapeake Bay Watershed is now in the last stage of this centennial-scale geomorphic and human-impact experiment, which increased upland erosion rates ~50 times the long-term geologic rate. From the 17^th to 19^th Centuries, while upland erosion rates were abnormally high because of widespread land-clearing for agriculture, fine-grained sediment accumulated behind 1-5 m high dams that provided hydraulic power for more than 34,000 mills in the mid-Atlantic region. Historic maps and our cesium-137 and lead-210 dating indicate that slack water reservoirs in southern Pennsylvania and northern Maryland filled to capacity in less than 150 years. Entire streams were converted to swamps and ponds, raising regional stream base levels up to several meters and burying pre-settlement floodplains and wetlands. Our field work and radiocarbon dating in the Appalachian Piedmont indicate that pre-settlement streams had low, stable bank heights for up to ten thousand years prior to European settlement, and modern “floodplains” often are reservoir fill terraces. Today, entrenched streams with steep, eroding banks resemble arroyos in the arid American southwest. Over the past ~100 years, breaching or removal of defunct mill dams has caused widespread channel entrenchment, even in un-urbanized and forested areas. Millions of tons of remobilized legacy sediment contribute >50% of the current suspended sediment load in some watersheds.