Despite increased understanding and adoption of nature-based solutions (NBSs) within urban and coastal areas, large-scale NBS for fluvial flood mitigation remain challenging to study and implement. A stronger evidence base is needed to identify critical research gaps and to best inform the design and deployment of NBS on the watershed scale. We synthesize evidence of the performance and co-benefits of NBS for fluvial flood mitigation based on a systematic review of 131 peer-reviewed papers worldwide, developing an Ecosystem Focus Type (EFT) to compare flood mitigation across large-scale NBS. While we find that NBS can mitigate fluvial floods across all EFTs, our study also highlights that inconsistencies in measurement methods, a dearth of empirical case studies, and large variability in reported values limit generalization and comparison across NBS. Co-benefits for fluvial flood NBS are numerous, but few are quantified, and study methods vary with regard to specific NBS. Social benefits of NBS, including benefits to communities most in need of support, are infrequently part of these studies. There is a clear need to develop common design and performance standards for large-scale NBS and for guidance on which measures are key to consider and monitor for flood mitigation and co-benefits. The success of large-scale NBS for fluvial flood mitigation will depend on research and practice guided by transdisciplinary systems thinking approaches that can deliver evidence-based, community-driven outcomes.
