A risk assessment study of water quality, biota, and legacy sediment prior to small dam removal in a tributary to the Delaware River.

The proposed removal of three run-of-river dams (all ≤5-m height) in eastern Pennsylvania along lower Bushkill Creek, a tributary of the Delaware River, has provided a valuable opportunity for multidisciplinary research involving the collection of more than 5 years of pre-removal monitoring data, analysis of heavy metals in legacy sediment cores, and associated toxicity assays to determine the singular and interactive effects of lead, copper, and cadmium on survival and behavior of a common macroinvertebrate found in Bushkill Creek. Monitoring data were collected from sites approximately 35 m upstream and downstream of dams and reference sites located approximately 5 km upstream of all dams. Results indicate that oxygen levels, macroinvertebrate diversity, and proportion of sensitive taxa were significantly lower upstream and downstream of dams in comparison with upstream reference reaches. The strong correlation between water quality and macroinvertebrates in this system implies that removal of the lower three dams would lead to improvements in water quality, biotic integrity, and resilience in lower Bushkill Creek. Sediment analyses and toxicity assays suggest that dam removal and sediment mobilization may route contaminated sediments downstream at concentrations that may harm more sensitive biota. However, macroinvertebrate mortality and behavior were not significantly different from clean water controls for the large majority of toxicity assays. All together, these results suggest that dams 1-3 are good candidates for successful stream restoration but that the removals would best be planned in a way that mitigates potential impacts of contaminated legacy sediment.