Over the past 20+ years, our research has shown that many valley corridors in the unglaciated
Mid-Atlantic region of the US preserve detailed records of the geological and ecological
history of these hydrodynamic systems spanning the last 10,000 years or more. This talk
concerns the rapid transition from stream-wetland complexes - stable for much of the warm
Holocene Epoch (the last 11,500 years) - to valley corridors choked with legacy sediments that
are less than 350 years old. Following European settlement, soils eroded from newly cleared
and tilled landscapes were trapped in millponds behind valley-blocking milldams. This was not
the only mechanism for legacy sediment formation, as erosion and deposition still occurred in
valley bottoms where mill dams were absent, but millpond sedimentation was the dominant
mechanism for this region. There were tens of thousands of milldams in the Mid-Atlantic
region, with possibly 16,000 in Pennsylvania alone, lining most 1st-3rd order valleys from
head to mouth. Water powered mills were used for nearly every industrial/mechanical
purpose, and the pervasiveness of millpond sedimentation led to the near eradication of
Holocene stream-wetland ecosystems and burial of vegetated floodplains.
Geologically speaking, this transformation was so rapid that we refer to it as the “Pompeii
Effect”. By early-20th Century, milldams were no longer needed or maintained, leading to
widespread dam failures. Once breached, surface waters incised through the millpond
sediments creating slotted channels with high, eroding banks. Single-thread meandering
stream channels with high banks did not exist in these valleys before legacy sediment
deposition and dam breaching, and are mere artifacts of 300-year-old anthropogenic
disturbances to these valley ecosystems. An understanding of this geological/anthropogenic
evolution offers the key to valley ecosystem restoration designs. Since restoration is meant to
restore an altered ecosystem to a state that approaches its natural ecological functions, we
must recognize that legacy sediments are the impairment and buried Holocene wetlands the
restoration target.
