ASSESSING LONG-TERM GEOMORPHIC ADJUSTMENT AND STREAM RESTORATION STRUCTURES IN NORTH-CENTRAL PENNSYLVANIA

A preliminary evaluation of stream restoration projects in north-central Pennsylvania is being conducted to determine their effectiveness to (1) limit channel incision, (2) reduce bank erosion and sediment inputs, or (3) improve aquatic habitat. The restoration projects were completed over the past twenty years using Natural Channel Design (NCD) methodology and are situated in the Piedmont province on relatively high gradient, bed load streams with moderate to low cohesive banks. To date over 400 structures have been located and assessed on 21 streams. One outcome of this project is to improve regulatory and watershed group guidelines for future restoration projects.

Field surveys reveal that approximately 75% of the restoration structures are damaged by channel erosion or gravel deposition (ranking >1). Thirty-five percent of the structures have sustained significant damage (ranking >2) or significant erosion or deposition (ranking >3). Most of the damage is occurring as flood-induced pulses of logging-legacy gravel work their way downstream, burying the structures and filling in the pool they were intended to create. One stream has been realigned and modified so many times that successive restoration stages or “NCD tells” are visible in the channel banks and valley bottoms. J-hooks and rock vanes sustained the most damage as compared to cross vane structures, with millions of dollars spent rebuilding and maintaining them. The results suggest that in this region conventional NCD methodology may not be effective over the long term because the fluvial systems remains in a protracted phase of disequilibrium following extensive logging and deforestation in 1800s.

Considering these factors, an adapted restoration approach should be used, whereby relic logging features such as splash dams and berms are removed to enable the stream to reconnect with its floodplain, increase habitat complexity, improve flow in abandoned side channels, and permit more uniform distribution of energy throughout the fluvial system. Rather than constrain the channel in a static position using hardened, in-stream NCD structures, restoration efforts should be directed at encouraging the stream to naturally create new meanders, bars, or pools and develop a new equilibrium condition on its own.