Effect of Dam Removal on the Unionid Mussel Community in the Salmon River, New York, USA

Dam removal has become a useful tool for restoration of fragmented habitats but predicting how the aquatic community will respond remains difficult: published studies of dam removal have shown a complex and variable response by the aquatic community. Predictions made in 2002 regarding the aquatic community response were used to evaluate the removal of the Fort Covington dam (Salmon River, Franklin County, NY) in 2009. The unionid mussel population was expected to remain stable with the exception of stranding within the former reservoir but what was not expected was the extent of the mobilization of sediment, primarily sand, which buried several mussel communities in the lower river. The draining of the 1.3-hectare reservoir required only 25 hours and resulted in a lowering of the water level by 47 cm at the reservoir center (3.3 m at the dam). The rapid draining of the reservoir stranded, and subsequently killed, more than 3200 mussels of eight species along the shorelines and in two adjacent ponds. Loss of shallow-water river habitat was estimated to be 30%, with a 90% loss of habitat in the ponds. The removal of the dam increased water velocity in the former reservoir from 12 cm/s to 34 cm/s during low flow and scoured sand deposits from the upstream riffle, shorelines, and sand bars that had formed in the reservoir. The volume of scoured sand deposited within the river five months after dam removal was estimated to be 42,480 m³, which covered 1097 m of the river bottom to a depth of up to 1.5 m. Continued scouring has resulted in deposition in downstream portions of the river, leaving steep-sided shorelines within the former reservoir with only unstable coarse sand or clay as mussel habitat. The mussel population downstream of the dam showed some recovery by 2012 but not in the former reservoir.