Forests Reduce Major Flood Risk More Than We Thought

For years, scientists thought forests could only help prevent small floods in small areas, leaving cities vulnerable when major storms hit. Now researchers from the University of British Columbia say we've been looking at the problem all wrong.

The issue comes down to how we measure flood risk. Most studies have compared individual flood events before and after forest disturbance, like snapshots in time. But this approach misses the bigger picture of how forests actually change flooding patterns over decades.

"When we look at flood risk in probabilistic terms—how trees and forests change the likelihood of a flood—the picture changes," said Samadhee Kaluarachchi, the study's lead author and a PhD student at UBC. "Forests are part of the solution, even for big floods."

The research, published in the journal Ambio, explains that forests don't just make individual floods slightly smaller. They fundamentally reduce how often major flooding happens by storing and gradually releasing water over time. Think of forests as natural sponges that keep catastrophic floods from reaching cities downstream.

This matters because the old findings have influenced real policy decisions. Governments have been hesitant to protect upstream forests as part of flood prevention strategies, assuming they wouldn't help much during the big events that truly threaten communities.

Dr. Younes Alila, a professor at UBC's faculty of forestry, emphasized that while forests won't stop every catastrophic flood, they reduce risk at its source. "This doesn't mean forests alone will stop catastrophic floods—but they can reduce flood risk at the source, making floods not only smaller but also rarer in cities and communities downstream."

Previous research by the same team showed that British Columbia's natural landscape of lakes, wetlands and forests offers built-in flood protection. The new paper explains why this protective effect is scientifically sound and why it should be trusted over conventional methods.

The Bright Side

The discovery opens up new possibilities for flood management that don't rely solely on expensive infrastructure. While dams and dikes remain essential, protecting and maintaining healthy forests upstream could save communities millions in flood damage while preserving natural ecosystems.

The researchers are calling for a fundamental shift in how we evaluate flood impacts and urging policymakers to update guidance based on this more rigorous science. When land management decisions in headwaters increase flood risks downstream, forests need to be recognized as a core part of the solution.

"It's about broadening the toolbox," says Kaluarachchi. "Engineering infrastructure is part of the solution, but it cannot address the root causes of flooding." When we combine smart land management with traditional infrastructure, communities gain powerful protection against one of nature's most destructive forces.