Future flood risk concentration for residents near small rivers across the USA

Abstract

Climate change and socioeconomic development are projected to increase global fluvial flood risk. Most studies have focused on large rivers or aggregated regional assessments, leaving limited understanding of how flood risk varies across river sizes. By integrating high-resolution hydrodynamic simulations, demographic projections and current flood defence levels across the contiguous USA, our projections suggest that smaller rivers tend to exhibit greater sensitivity to climate change-induced increases in flood intensity by 2050. Meanwhile, flood defences along small rivers are currently the weakest, potentially placing nearby populations at disproportionately higher risk. Restoring future flood risk along small rivers to historical levels would require the highest flood defence investments among all river sizes—up to ~US$4 million (2005) per kilometre. The elevated flood risk for residents along small rivers appears to be primarily driven by increasing climate-related hazard severity and low defence levels, whereas flood risk along larger rivers is dominated by rising exposure. Together, these results indicate a potential mismatch between future flood defence needs and existing defence levels across river sizes, highlighting the urgent need to prioritize investments for communities along small rivers.