MULTI-DECADAL POST-DAM BREACH CHANNEL EVOLUTION ALONG BIG BEAVER CREEK, SOUTHEASTERN PENNSYLVANIA

Alluvial streams were described as “authors of their own geometries” by Luna Leopold (1994), but modern streams in the mid-Atlantic piedmont were inexorably "edited" by anthropogenic processes associated with widespread land clearing and mill damming during the mid-1700s to early 20^th century. Here, we develop a model of post-dam breach channel evolution that accounts for both historic sediment in storage upstream of milldams and time since dam-breach along the unglaciated Big Beaver Creek (BBC) valley bottom in SE Pennsylvania.

At least 9 breached milldams with heights of 1-2 m exist along ~37 km of BBC, equivalent to one dam per 4 km. About 2 million m³ of historic sediment remains in storage as fill terraces along the valley bottom, totaling nearly 60 m³ of sediment per m of stream length. Our field mapping demonstrates that this sediment is largely silt, clay and fine sand with sufficient cohesive strength to maintain a near vertical bank. Based on regional lidar dem differencing with data from 2008 and 2014 (a 6.7-yr period), post-dam breach bank erosion rates vary from 12 to 0.5 m/yr within the first 10 years after dam breaching and diminish slowly thereafter, remaining ~0.05 m/yr for the next 8 decades. All but one milldam along BBC breached >50 years ago, so the majority of banks are eroding slowly at this time.

After post-dam breach channels incise and begin to migrate laterally, the outside of meander bends is eroded and Pleistocene periglacial gravel scoured from beneath the fine-grained historic sediment. The coarse gravels are transported short distances during large flow events and build point bars opposite the eroding banks. However, we find that bank erosion and channel corridor widening during the past >5 decades resulted in diminishing flow depths and hence bed shear stresses, bed load transport, and point bar growth. From 2008 to 2014, notable gravel bar formation only occurred at the site of most recent dam breaching (2006). At all other reaches, little bank erosion or bar growth occurred. We posit that extensive channel corridor widening via bank erosion since dam breaching has resulted in near-hibernation conditions for these anthropogenically altered streams. This understanding of decadal-scale fluvial processes can provide insights for effective stream restoration strategies.