River restoration through dam removal: Examining ecological responses to small dam removals across Massachusetts

Due to negative ecological impacts, costs of repair and maintenance, and safety risks, small dams (m)—ubiquitous across the United States (U.S.)—are being removed from streams. However, few dam removal projects include scientific study of ecosystem responses, raising questions about the ecological benefits and limiting predictions of project outcomes. The overall goal of my dissertation was therefore to develop a holistic understanding of ecological responses to small dam removals. I incorporated field sampling, existing data, and a practitioner survey to characterize responses of water quality (temperature, dissolved oxygen), benthic macroinvertebrates, and fish assemblages to small dam removals across Massachusetts. At 11 dam removal sites, I monitored continuous temperature (year-round) before and up to 5 years after removal, and at 10 sites, I monitored dissolved oxygen (DO; week-long deployments in summer months) before and 1 year after removal. Across sites, I observed reduced spring and summer temperatures, with recovery modulated by dam height and forest cover. Most sites recovered DO within a year after dam removal, and the magnitude of recovery was related to pre-removal impacts. Macroinvertebrate samples at 10 sites revealed that although short-term biotic impacts of removal varied depending on watershed characteristics, most formerly impounded and downstream reaches recovered sensitive taxa within 3‒5 years after removal, even at sites with passive sediment release. To assess changes in freshwater fish assemblages after dam removal, I used a long-term fisheries database to select 34 sites across Massachusetts. Increasing taxonomic similarities between upstream and downstream fish assemblages suggest restoration of stream connectivity is a major driver of post-removal fish assemblages across these sites. Lastly, I distributed a survey to dam removal practitioners across 14 states in the eastern U.S. and found that co-production of relevant research questions and accessible and interpretable research products may support the incorporation of climate change science more consistently and effectively into dam removal decisions. Collectively, this dissertation supports dam removal as a restoration tool that may provide a suite of ecological and climate benefits; however, restoration outcomes may be context dependent, and therefore, practitioners should consider site and watershed characteristics to set appropriate restoration targets