Aligning dam removals and road culvert upgrades boosts conservation return-on-investment

Dams and road culverts fragment river ecosystems worldwide by restricting the movement of aquatic species. In many watersheds, a diverse set of actors coordinates the removal of these barriers. Non-governmental organizations often focus on small dams and road culverts, while large dam removal projects are coordinated by federal agencies or coalitions of partners. Here we evaluate the return-on-investment of these strategies by exploring a continuum of methods for selecting barrier removal projects, ranging from a focus on many small barrier removal projects to a few large ones. We used estimated removal costs of more than 100,000 barriers in the North American Great Lakes to construct economically realistic barrier removal scenarios. We then simulated the movement of stream-resident and anadromous fishes through model river networks with a few large dam removals, many road culvert retrofits, or a mix of both. We found that the strategy of removing both dams and road culverts had the greatest potential to benefit both stream-resident and anadromous fishes, but only when projects were aligned longitudinally within the river network. Our results demonstrate the importance of allocating conservation resources to both small and large restoration projects, and highlight a need for increased coordination and communication among the many different organizations investing in barrier removals. Our findings complement optimization approaches to prioritizing barrier removals by providing general guidelines for practitioners to follow when project selection must depart from a prescribed portfolio of projects.