Forests May Reduce Flood Risk More Than Previously Thought

Forests are frequently recognised for their ability to lower flood risk. Yet scientists have spent decades debating the scale of that benefit, particularly when the largest and most damaging floods occur.

New research by scientists at the University of British Columbia (UBC) suggests that forests are not the issue; rather, it is the way their effects have been assessed. The authors say a commonly used “before-and-after” method has caused researchers to undervalue the part forests play in influencing flood risk over time.

Rather than concentrating on a small number of individual flood peaks, the team argues that researchers should examine probabilities: the ways forests affect the chances of floods occurring at all. Viewed through that lens, forests could be substantially more significant than earlier research has suggested, including during major flood events.

Measuring floods the wrong way

Many studies have sought to answer one straightforward question: do forests lessen flooding? Typically, they compare peak flows from individual storms before and after a change such as logging, wildfire or land conversion.

The authors contend that the flaw is that flooding does not follow a simple “before and after” narrative. Every storm differs, as do soil moisture, snowpack and the paths water takes through a catchment.

Comparing only the peak flow of one flood with that of another can therefore result in the wrong conclusion.

“When we look at flood risk in probabilistic terms – how trees and forests change the likelihood of a flood – the picture changes,” said lead author Samadhee Kaluarachchi, a Ph.D. student at UBC. “Forests are part of the solution, even for big floods.”

Rethinking forests and big floods

For many years, papers have repeated a familiar finding: forests reduce smaller floods in small basins, but have limited effect on major floods or large watersheds.

These findings have influenced policy discussions. When forests are seen as offering little protection during major flooding, governments may regard them as a “nice bonus” instead of an important flood-management measure.

Kaluarachchi and UBC professor Younes Alila maintain that this often-cited conclusion is partly the result of how floods have been measured.

In their view, the conventional method makes flood behaviour too simple and overlooks that flood risk concerns frequency and probability, rather than merely the highest point in one event.

“When studies focus only on peak flows from individual events, it overlooks how forests influence the broader distribution of flood risk over time,” said Alila. “Our synthesis shows forests can alter the frequency and probability of floods, including major events.

“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.”

Forests shape floods upstream

The authors do not suggest that forests could take the place of dams, dykes or floodwalls. Instead, they make a more fundamental-and, in some respects, more compelling-case: land cover in headwaters affects both the frequency of large downstream floods and their severity when they occur.

They refer to their earlier British Columbia research, which found that natural features such as forests, wetlands and lakes function as inherent flood infrastructure.

These features hold water, slow runoff and release water over a longer period. This can lower downstream peaks and may even alter the frequency with which particular flood levels are reached.

According to the new paper, this is not simply a noteworthy local finding. It reflects the physical behaviour of floods and should carry greater weight than the older “peak-to-peak” comparison approach that dominates the field.

Where land use meets flood risk

Researchers stress this issue partly because flood policy commonly centres on areas near towns and cities, where damage takes place. It gives less consideration to upstream activity, where flood risk may be increased or reduced long before a river reaches homes and roads.

They warn that focusing solely on engineered defences while overlooking upstream land management can produce poor decisions.

For instance, clearing forest cover in headwaters could raise the flood pressure on downstream infrastructure, leaving communities more exposed even where they have invested in protective structures.

“It’s about broadening the toolbox,” Kaluarachchi said. “Engineering infrastructure is part of the solution, but it cannot address the root causes of flooding.

“When land management and forest removal in the headwaters increase flood risks downstream, forests and healthy ecosystems must be a core part of flood management.”

A new way to evaluate forest flood risk

The authors urge researchers to revise both the way studies are designed and how evidence is interpreted. Rather than asking, “Did this one flood peak change after logging?” they say research should investigate how forests change the probability distribution of floods over time.

This shift in perspective could also affect policy. If forests can lower not only minor floods but also the frequency of major ones, the argument for upstream land management becomes stronger. Governments would find it more difficult to view it as separate from “real” flood control.

The experts say flooding has been assessed incorrectly. Forests do not have to prevent a catastrophic event entirely in order to have value.

If they make destructive floods rarer or lessen their force before rivers arrive in cities, that is a meaningful reduction in risk.

Climate change is already making extreme rainfall more intense in many areas. Against that backdrop, the authors say every measure capable of reducing flood risk should be considered, including those working quietly upstream before water reaches the streets.