Long-term Monitoring of Ecological and Geomorphic Adjustments to Dam Removal in an Upland Mesic Catchment

Abstract:

Although more than 1,100 dams have been removed nationally, only 10% have any post-removal assessments with < 3% having any combined geomorphic and ecological analyses. Taking advantage of the Fall 2012 removal of the 6-m high, 200-yr old Pelham Dam in central MA, we sampled geomorphic parameters in Amethyst Brook (23 km²) pre-removal and in each subsequent post-removal year through 2015. We combined these geomorphic assessments with quantitative electrofishing surveys of stream fish richness and abundance above and below the dam and with visual surveys of native anadromous Sea lamprey (Petromyzon marinus) nest sites. Post-removal assessments were complicated by 2 events: upstream knickpoint migration exhumed an older wooden crib dam 120 m upstream of the former dam, and the occurrence of a 20-yr RI flood 6 months after removal. Process-based erosion dominated in the initial post-removal months with significant bed erosion and knickpoint migration occurring through the former reservoir and upstream to the exhumed crib dam that now acts as a grade control. Similar to other removals, the bed aggraded (20 cm) and fined (~50%) downstream in the initial year, with subsequent coarsening (~ 10-20%) in Years 2 and 3, but with D₅₀ still significantly finer than the pre-removal armored bed. The initial fining and subsequent coarsening, unlike previous studies, does not reflect erosion of former reservoir fill but represents the re-connected upstream sediment supply. Ecologically, our monitoring has further underscored the importance of restoring sediment supply and removing barriers to movement to the diversity and abundance of the native fish assemblage. The observed fining has had major implications for Sea lamprey that require fine gravel for spawning, allowing them to spawn in previously unoccupied below-dam sections. Dam removal has also allowed three additional native species to rapidly expand their upstream distribution up to, but not beyond the exhumed dam. Downstream abundances of some species in the first year were markedly reduced in dam-adjacent sections in response to the major geomorphic changes associated with removal and a subsequent flood. Since then, abundances have generally stablilized or increased, demonstrating the resilience of these species, and the importance of a long-term approach.