BASELINE CHANNEL DYNAMICS PRIOR TO REMOVAL OF LOW-HEAD DAMS FROM BUSHKILL CREEK, EASTON, PA

Since 2017 we have been doing detailed measurements of the lower portion of Bushkill Creek in Easton, Pennsylvania in anticipation of the removal of three low-head dams. Bushkill Creek is a tributary to the Delaware River and has a drainage area of 207 km². Numerous mill dams were constructed for water power along the lower reach of the stream from the 18^th century to early 20^th century. The planned dam removal will reconnect the lower portion (~3.5 km) of the Bushkill Creek to the Delaware River to facilitate fish migration for spawning and to give trout access to thermal refugia during periods of elevated temperatures in the summer. We also anticipate that fish movement upstream will transfer freshwater mussel larvae to re-establish mussel beds that are currently absent. Delays in permitting and funding the dam removal project have afforded us the unique opportunity to monitor pre-removal stream channel dynamics over a six-year period, which allows us to compare “natural” variations in channel morphology with the post-dam removal readjustment of the channel. We have been measuring physical, biological, and water quality attributes of the Bushkill Creek upstream and downstream of each of these dams intensely since 2017, with biological monitoring since 2010. At each dam we have surveyed upstream and downstream channel cross sections to evaluate channel movement. In addition, in 2019 we installed a stream gauge below the lowest dam to monitor stage, turbidity, temperature, and conductivity. Between the years 2018 and 2021 the stream experienced several large hydrological events capable of generating sufficient shear stress to mobilize the bed of the channel. Whereas some cross sections were remarkably stable with less than 0.1 meters of fluctuations in the bed elevation as an average across the profile, some sections experienced as much as 0.5 meters of average incision or filling. Sections upstream of dams were a bit more likely to experience significant change, perhaps as expected because bed material size was often finer in pools upstream of the dams. A companion talk by Brandes et al., will share the results of the initial changes that occurred since removal of the first two dams in summer 2023.