Transient versus sustained biophysical responses to dam removal

Dams fragment the longitudinal flux of sediment and obstruct fish passage. To help remediate these impacts, dam removal has become an important tool in the river restoration toolkit. Scientifically, these removals also provide an important experimental design, fixed in time and space, to quantify geomorphic and ecological responses to removal of a disturbance. Using the removal of a 6 m high dam in Massachusetts (USA), we quantify the imme-diate and subsequent channel and ecological recovery to test whether the immediate ecological/geomorphic re-sponses were sustained or transient. Initial geomorphic adjustments have been generally sustained since removal, especially for bed sediment size, where initial fining and reduction in bed caliber size has been main-tained over the course of the study. Sampling of transported passive integrated transponder (PIT) tagged clasts indicates that the median of bedload grain sizes now corresponds to surface median grain sizes below the former dam. Channel cross-sectional area varied irregularly downstream initially but is now spatially uniform. Pool fill-ing, however, has been a transient response as initial aggradation shifted to sediment evacuation. Ecologically, upstream establishment of resident fish species previously restricted to below-dam locations occurred in the first year after removal. Upstream expansion increased upstream species richness and has been maintained throughout the study period, in spite of the variability in presence and abundance of rare and migratory species such as the anadromous Sea lamprey (Petromyzon marinus). In contrast, a reduction in abundance of resident species in dam proximal sites in 2012 appears to be a transient response to abrupt and intense bed movement associated with removal of the dam, as numbers recovered and, in some cases, exceeded pre-dam removal levels. Overall, we found that immediate (within the first 1-2 yr), sustained effects on geomorphology and ecology dominated the response to dam removal in this system, a finding that may be generally applicable to small dam removals in upland catchments. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

Dam removal is an important tool for river restoration and provides an experimental design to quantify environmental responses. The study of a 6 m high dam removal in Massachusetts showed sustained effects on geomorphology and ecology, with immediate improvements in bed sediment size and upstream fish species richness. The response to dam removal in this system is generally applicable to small dam removals in upland catchments.