Major Floods: A New Study Reveals the True Power of Forests

A Reassessment of Traditional Measures

Forests are often credited with reducing the risk of flooding. However, for decades, the scientific community has debated the true extent of their impact, particularly during the largest and most destructive floods. This uncertainty may well be resolved thanks to recent work by a Canadian team.

A new study conducted by researchers at the University of British Columbia (UBC) suggests that the problem does not lie with the forests themselves, but with the way we have measured them so far. According to the authors, a widely used “before-and-after” comparative approach has led researchers to underestimate the role forests play in shaping flood risk over time.

The team suggests that instead of focusing on a few individual flood peaks, it is necessary to examine probabilities: how forests alter the chances that a flood will occur in the first place. From this perspective, forested areas could prove to be far more important than previously thought, even during major events.

The Methodological Flaw in the Comparative Approach

Many studies have attempted to answer a simple question: Do forests reduce flooding? The common approach has been to compare peak flows from individual storms before and after a change, such as logging, a wildfire, or land conversion. The study’s authors emphasize that flooding is not simply a matter of “before and after.”

Every storm is different, as are soil moisture, snow cover, and the way water moves through a watershed. If we focus solely on the peak flow of one flood and compare it to another, we may end up drawing the wrong conclusions about the effectiveness of forest cover.

Samadhee Kaluarachchi, a PhD student at UBC and the study’s lead author, explains this crucial distinction: “When we examine flood risk in probabilistic terms—how trees and forests change the probability of a flood—the picture changes. Forests are part of the solution, even for major floods.”

Rethinking the Link Between Forests and Major Floods

For decades, numerous articles have conveyed a familiar message: forests help reduce minor flooding in small watersheds, but they do little to mitigate major floods or protect large watersheds. These conclusions have shaped policy debates. If forests don’t help much during major floods, governments may treat them as a “nice bonus” rather than a serious flood management tool.

Samadhee Kaluarachchi and Professor Younes Alila of UBC argue that this widely repeated conclusion is partly a matter of measurement. They contend that the standard approach oversimplifies flood behavior and ignores the fact that flood risk actually concerns frequency and probability, not just the highest peak of a single event.

Professor Alila elaborates on their thinking: “When studies focus solely on the peak flows of individual events, they overlook how forests influence the broader distribution of flood risk over time. Our review shows that forests can alter the frequency and probability of floods, including major events.” He adds: “This does not mean that forests alone will stop catastrophic floods—but they can reduce flood risk at the source, making floods not only smaller but also less frequent in downstream cities and communities.”

The decisive influence of upstream on downstream

The authors do not claim that forests can replace dams, levees, or flood walls. Their argument is more fundamental: land cover in the headwaters influences the frequency of major floods downstream and their severity when they occur. They point to their own previous research in British Columbia showing that natural features—including forests, wetlands, and lakes—act as integrated flood-control infrastructure.

These natural features store water, slow runoff, and release it more gradually. This mechanism can reduce peak flows downstream and even alter the frequency with which certain flood levels are reached. The new paper asserts that this is not merely an interesting local observation, but a physical reality.

According to the researchers, this understanding of flood behavior deserves greater weight than the old “peak-to-peak” comparison method that dominates the field. Upstream land management therefore plays a decisive role long before the water reaches populated areas.

Land Management in the Face of Urban Risk

One reason researchers emphasize this point is that flood management policy often focuses on what happens near cities, where damage occurs. It pays less attention to what happens upstream, where flood risk can be amplified or reduced long before a river reaches homes and roads. Relying solely on technical defenses while ignoring land management upstream can lead to poor decisions.

For example, clearing forest cover in headwaters could increase the flood load placed on downstream infrastructure, making communities more vulnerable even if they invest in protective structures. Samadhee Kaluarachchi sums up this necessary approach as follows: “It’s about expanding the toolbox. Engineering infrastructure is part of the solution, but it cannot address the root causes of flooding.”

He goes on to emphasize the importance of ecosystems: “When land management and deforestation in headwaters increase the risk of flooding downstream, healthy forests and ecosystems must be a central part of flood management.”

Toward a New Assessment of Climate Risk

The authors call on researchers to change the way they conduct their studies and interpret the evidence. Instead of asking, “Has this single flood peak changed following logging?” they argue that studies should focus on how forests alter the probability distribution of floods over time. This shift in perspective could also influence policy.

If forests can reduce not only small floods but also the frequency of major events, this strengthens the case for upstream land management. Governments would find it harder to treat this aspect as separate from “true” flood control. Forests do not need to completely prevent a catastrophic event to be valuable: if they make damaging floods less frequent or reduce their intensity before rivers reach cities, this represents a significant reduction in risk.

Climate change is already intensifying extreme precipitation events in many regions. In this context, the authors—whose study is published in the journal Ambio—argue that it is time to count every tool capable of reducing flood risk. This includes those that work upstream, quietly, before the water ever hits the streets.

Source: earth.com

Major Floods: A New Study Reveals the True Power of Forests