Transient versus sustained biophysical responses to dam removal

Authors: Francis Magilligan*, Dartmouth College, Keith H. Nislow, USDA Forest Service, Northern Research Station, James T. Dietrich, University of Northern Iowa
Topics: Geomorphology, Water Resources and Hydrology, Environmental Science
Keywords: dam removal, watershed connectivity, channel recovery
Session Type: Paper
Day: 4/6/2019
Start / End Time: 9:55 AM / 11:35 AM
Room: Hoover, Marriott, Mezzanine Level
Presentation File: No File Uploaded


Taking advantage of the removal of the 5 m high Pelham Dam in 2012, we evaluated the immediate geomorphic and ecological responses (Magilligan et al., 2016). Following the same sampling and surveying protocols, we have been continuously monitoring the 1.2 km reach for changes in channel morphology, bed sedimentology, slope, fish demography, and sea lamprey redd construction. We augmented the initial geomorphic assessments by deploying ~300 RFID pit-tagged gravel clasts in 2016 in reaches above, within, and below the former impoundment to quantify the processes of sediment re-connectivity. The initial geomorphic adjustments have been generally sustained since the initial removal, especially for bed sediment size that fined initially and has been subsequently sustained. Sampling of RFID pit-tagged clasts indicates that bed load grain sizes correspond to median grain sizes below the former dam. Channel cross-sectional area varied downstream within the first year but has been subsequently unchanged. Pool filling has been a more transient response as initial bed aggradation has been followed by subsequent sediment evacuation. Bed fining has contributed to increased number of sea lamprey redds although their numbers vary yearly. In contrast to effects on species richness, abundances of all species (including both species of resident salmonids) in dam proximal sites were sharply lower in 2013 compared to pre-removal. However these sections currently have equivalent or greater abundance, suggesting that the reduction in abundance was a transient effect. Dam removals thus provide an important proxy and experimental design to evaluate stream channel recovery from a range of disturbances.

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