Long-term monitoring to quantify the success of river restoration through dam removal in the Paulins Kill NJ, USA

Dams degrade water quality and alter flow, thermal and dissolved oxygen regimes, sediment transport, and habitat connectivity. As a result, dam removals are increasingly used as an effective means of river restoration. However, studies on the multifaceted effects of dam removals are limited. We used comprehensive (multiple physical and ecological parameters) long-term monitoring to study the effects of the removal of the Columbia Dam on the Paulins Kill, New Jersey, USA. We performed seasonal continuous (May-September) instream monitoring of temperature, dissolved oxygen, and turbidity at sites upstream, downstream, and within the dam impoundment. We also collected macroinvertebrates at these sites as a biological indicator of water quality. Sampling spanned all stages of the dam removal with data collected 2 years pre-, 2 years during, and 5 years post-removal (n = 9 years total). Finally, we sampled migratory American eel ( Anguilla rostrata ), sea lamprey ( Petromyzon marinus ), and American shad ( Alosa sapidissima) to determine whether they returned to the river post-dam-removal. Our hypothesis was that the removal of the Columbia Dam would improve water quality in the impoundment and downstream of the dam, which would in turn be beneficial for aquatic organisms. Using a ‘differences-in-differences’ approach, we were able to quantify changes in water quality related to removal stage, supporting a return to natural stream conditions post-removal. Temperature and DO at the downstream site were more similar to the values at the upstream reference site after the dam was removed, with relative temperature decreasing 0.29°C (95% CI: -0.38, -0.20) and relative DO increasing 0.14 mg/L (95% CI: 0.04, 0.22). Though macroinvertebrate scores declined at the downstream site during the dam removal, scores quickly returned to pre-removal levels (‘Good’ and ‘Excellent’), supporting the restoration of healthy water quality downstream of the dam after removal. Finally, upstream of the former dam site, we documented increased densities of juvenile American eel, sea lamprey adults, larve, and redds (i.e. nests), and the presence of adult American shad. Overall, our data support the idea that dam removal can be an effective tool for successful river restoration, with some parameters improving quickly, and some steadily improving over time.