CHANNEL RECOVERY FOLLOWING DAM REMOVAL: LESSONS FROM THREE HISTORIC DAM REMOVALS OF THE MIDDLE ATLANTIC PIEDMONT

To determine the rates and processes of channel recovery following dam removal, we studied 3 sites along Muddy Creek in southeastern Pennsylvania. Garthridge Dam, 12.2 m high and located the farthest downstream, was breached in 1933. Highrock Dam, 1.8 m high and located the farthest upstream, was breached in 1972. Castle Fin Dam, 3.1 m high, was removed in 1997. We surveyed the longitudinal profile and channel cross-sections upstream of the former dam site. We also sampled the bed material and mapped floodplain and channel deposits. Undammed reaches far upstream were used as controls. At all sites, laminated muddy reservoir deposits are still preserved as terraces up to 5 m high bordering the channel. These deposits are persistent and will likely remain in place for decades, if not centuries. In addition, these deposits are a preferred habitat for exotic plant species, which had exceptionally high cover values at all three sites. At Castle Fin, laminated muddy reservoir deposits underlie the channel bed, indicating that the channel has not incised to its predam elevation after 5 years. Some reservoir deposits remain at high Rock, while vertical incision has removed all the reservoir deposits at Garthridge. An exponential model fit to these data suggest a “half-life” for incision of about 11 years. Bed material is finer grained near the former dam site than at the control reaches at all the sites, and the water surface slope is higher near the former dam site than at the control reaches. These data suggest that, even after many years, channels above locations of removed dams are noticeably different from nearby control reaches, possibly indicating that complete recovery from dam removal is an inherently slow process.