INITIAL CHANNEL ADJUSTMENTS AND SEDIMENT MOVEMENT FOLLOWING REMOVAL OF A LOW-HEAD DAM AND CONCURRENT OVERBANK FLOW EVENT - BUSHKILL CREEK, EASTON, PA

Although hundreds of small dams have been removed in the U.S. alone over the past 30 years, very few have included detailed pre- and post-removal measurements of stream channel dynamics and observations of re-establishment of channel equilibria with the post-removal streamflow regime. Our long-term study on Bushkill Creek seeks to document the response of the stream to removal of three low-head dams formerly used to power mills. This presentation focuses on the first dam removal that occurred in summer 2023. The former Dam 1 was approximately 2.5 m high and 23 m wide with a pool extending approximately 130 m. This dam was located 530 m upstream of the confluence with the Delaware River and had been in place for over 100 years. Removal began on July 7 but was delayed due to a storm event on July 9 that generated a stage rise of 0.95m and increased stream turbidity enough to prevent further demolition. Removal continued the following week, but a large overbank flood event (2.7m stage rise) interrupted work a week later. This second event caused widespread flood damage in our region, and generated significant sediment movement and channel incision upstream of the former dam pool. After the stage subsided sufficiently to allow stream access, we measured cross-sections above and below the former dam site, and continued measurements during the following weeks. Throughout dam removal we measured turbidity downstream of the dam and collected suspended sediment samples to relate turbidity readings (NTU) to suspended sediment (mg/L) and thus estimate sediment transport. The presentation will include time-lapse images and an analysis of our cross-section measurements and sediment transport results following dam removal and the concurrent overbank flood event of July 16. A companion talk by Germanoski et al., will share the results of pre-removal channel dynamics measured from 2017 to 2023.