Geomorphic response to the removal of a dam in Western North Carolina

Keywords: dam removal, channel response, Southern Appalachia, bed texture, sediment transport
Abstract Type: Paper Abstract

Abstract

Dam removals are a growing trend in the United States. Current removal studies largely indicate that ecological benefits of river re-connectivity outweigh the economic benefits and the costs of maintaining aging infrastructure. However, there is a lack of comprehensive studies in certain regions of the United States, including Southern Appalachia. In May of 2021 the Ward’s Mill Dam, a 6 m high run-of-river dam located on the Watauga River in Western North Carolina, was removed. We used this removal as an
opportunity to study geomorphic impacts to such removals in this understudied region with the
objective of identifying changes in channel form, analyzing bed sediment characteristics,
and quantifying rates of volumetric evacuation and deposition. To capture geomorphic
change repeated pre- and post-removal cross-sectional surveys, longitudinal profiles, and in-situ
particle size sampling was conducted in upstream and downstream reaches. Field collections the
day after removal show significant deposition of gravel sized particles immediately downstream of
the dam (≤200m) and shifted the channel slope to -1.11% grade. Channel bed texture remains
significantly finer across the 2km downstream study reach 506 days following removal. Erosion
rates of the impoundment during the deconstruction period were on average 860
m 3 /day and decreased to 75.5m 3 /day by day 74. Below average mean daily flows during this phase
were able to evacuate ~72% of impounded sediment. Tropical Storm Fred introduced a 2-year flow that remained above baseline flow for 8 day, evacuating 33% of remaining reservoir sediment.