Fluvial flood is one of the most widespread and damaging natural hazards globally, claiming thousands of lives each year. Flood risk depends not only on the frequency and magnitude of flooding but also on factors such as riparian population exposure and flood protection capacity, giving rise to marked differences across regions and rivers of varying sizes. Yet existing research and flood management practices have tended to focus on large rivers while paying insufficient attention to the numerous and widely distributed small rivers; their flood risks and protection gaps remain poorly understood.

To address this problem, a team led by Academician Chen Deliang and Professor Lu Hui from the Department of Earth System Science (DESS) at Tsinghua University integrated high-resolution hydrodynamic modeling, population projections, and existing flood protection data to systematically assess future flood risk and protection needs along U.S. rivers of varying sizes under projected climate change and socioeconomic development. The study shows that small rivers host substantial riparian populations, yet their existing flood protection standards are the lowest among all river categories, leaving nearby residents exposed to disproportionate flood risk.

The study further finds that by mid-century, small rivers are more sensitive to climate-driven increases in flood magnitude, with projected increases in annual maximum daily discharge generally exceeding those of large rivers (Figure 1). Meanwhile, small rivers have low baseline flood protection, so climate change will further erode their protective effectiveness. In contrast, large rivers typically feature higher flood protection standards, and their future flood risk growth is driven primarily by rising riparian populations and expanding exposure.

Figure 1. Projected changes in annual maximum daily discharge by mid-century relative to the historical period, across rivers of different sizes.

Building on these findings, the researchers estimated the flood protection investment required to restore mid-century flood risk along rivers of varying sizes to historical levels (Figure 2). The results show that among all river categories, small rivers require substantially higher investment per unit river length than large rivers, reaching approximately USD 4 million per kilometer (in 2005 dollars). This indicates that, despite their smaller scale, small rivers — owing to weak existing flood protection and greater climate sensitivity — may actually incur higher future adaptation costs.

Figure 2. Flood protection investment required to restore mid-century flood risk along rivers of varying sizes to historical levels.

These findings reveal a significant mismatch between future flood protection needs and existing flood protection levels across rivers of varying sizes, with small rivers facing a particularly pronounced protection gap. The study recommends that future flood management planning and funding allocation give greater weight to long-overlooked small rivers and their riparian communities, prioritizing improvements in flood protection standards and climate adaptation capacity. Such measures would reduce flood exposure and casualty risk, narrow protection gaps across regions, and enhance overall regional resilience.

The study, titled "Future flood risk concentration for residents near small rivers across the United States," was published online in Nature Sustainability on July 10, 2026.

Jiang Ruijie, a postdoctoral researcher in the Department of Earth System Science (DESS), Tsinghua University, is the first author, and Professor Lu Hui is the corresponding author. Co-authors include Academician Chen Deliang (Xinghua Distinguished Professor at Tsinghua University), Professor Yang Kun, and Professor Guan Dabo; Professor Dai Yamazaki of the University of Tokyo; Professor Hiroshi Cho of Kumamoto University; Assistant Professor Zhao Gang of Tokyo University of Science; and Professor Wei Jiahua and Professor Tian Fuqiang from the Department of Hydraulic Engineering at Tsinghua University. The research was supported by the Fundamental Science and Interdisciplinary Research Program of the Ministry of Education, the National Key R&D Program of China, the National Natural Science Foundation of China, the Carbon Neutrality and Energy System Transition Program, and Tsinghua University.

Full-text Link:  https://www.nature.com/articles/s41893-026-01896-7