Small Rivers, Big Risks: US residents near small rivers face disproportionately high future flood risks due to climate change
An international team, including Kumamoto University and its partner institutions, finds severe flood risks and high climate-adaptation costs for U.S. small rivers.River floods are among the world's most destructive natural disasters. While larger rivers traditionally receive the majority of infrastructure investments, small river basins represent a growing, under-protected frontier of climate risk. Addressing this critical blind spot, an international research team comprising institutions from Tsinghua University, the University of Tokyo, Kumamoto University, and Institute of Science Tokyo, has systematically evaluated future flood risks and protection needs across different river scales in the contiguous United States.
The findings, published on July 10, 2026, in Nature Sustainability, reveal a startling reality: while small rivers are highly sensitive to climate change-induced intensifications, they currently possess the most fragile defense infrastructure (Fig.1). Consequently, millions of residents living near small rivers face disproportionately high flood risks.
Using high-resolution hydrological and hydrodynamic simulations, demographic projections, and existing flood defense levels data, the researchers calculated that maintaining current flood defense levels against mid-century climate projections will require massive investments—peaking at up to $4 million USD (in 2005 equivalent values) per kilometer for small rivers (Fig. 2). Because small river basins lack robust foundational defenses, their relative climate-adaptation costs are significantly higher than those of large river systems.
Professor Hiroshi Cho of Kumamoto University’s Faculty of Advanced Science and Technology, a member of the research team, emphasizes that the findings highlight a critical gap between current flood defenses and real climate risks. While the study examines continental rivers in the U.S., Japan faces a similar challenge despite operating on a much smaller geographical scale. The team calls on policymakers to move beyond a heavy bias toward large rivers and prioritize urgent funding and infrastructure upgrades for long-neglected small-river communities to build robust, climate-resilient societies.
Reference
| Authors |
Ruijie Jiang, Hui Lu, Deliang Chen, Kun Yang, Dai Yamazaki, Hiroshi Cho, Gang Zhao, Jiahua Wei, Fuqiang Tian & Dabo Guan |
| Title of original paper |
Future flood risk concentration for residents near small rivers across the USA |
| Journal | Nature Sustainability |
| DOI | 10.1038/s41893-026-01896-7 |