Forest removal can turn rare floods into regular disasters

Forests do more than provide timber and shelter wildlife. They regulate water, stabilize soil, and buffer communities from natural disasters. But when forests are cut, the disruption doesn’t stop at tree stumps.

A recent study led by researchers at the University of British Columbia (UBC) reveals the alarming and long-lasting consequences of clear-cutting on flood patterns.

A once-in-70-years flood now hits every nine years. These findings come from one of the longest-running forest hydrology experiments in the world.

“This research challenges conventional thinking about forest management’s impact on flooding,” said senior author Dr. Younes Alila, a hydrologist in the UBC faculty of forestry.

“We hope the industry and policymakers will take note of the findings, which show that it matters not only how much forest you remove but also where, how and under what conditions.”

Landscape factors and flood risk

The researchers studied two neighboring watersheds in North Carolina. Both were clear-cut in the 1950s but responded very differently. One faced north, retained moisture, and received less sunlight. The other faced south and dried out faster.

“We found seemingly minor landscape factors – like the direction a slope faces – can make or break a watershed’s response to treatment,” said study first author Henry Pham, a doctoral student in the faculty of forestry.

In the north-facing watershed, floods became four to 18 times more frequent. Some flood volumes more than doubled. In contrast, the south-facing watershed showed almost no changes in flood behavior.

The same treatment led to wildly different results, emphasizing how orientation and microclimate drive watershed resilience.

Outdated flood models

Conventional flood models estimate runoff by applying a simple formula: cut X percent of trees, expect Y percent more water. The research team challenged this outdated thinking. They used a probabilistic framework across 70 years of data from the Coweeta Hydrologic Laboratory.

“This experimental evidence validates our longstanding call for better analysis methods,” said Dr. Alila. “When we apply proper probabilistic tools to long-term data, we find much stronger and more variable impacts than older models suggest.”

The study showed that forest removal doesn’t just raise flood averages – it changes how floods behave entirely. Rare events become frequent, and peak flood volumes rise unpredictably.

Forest removal and flood patterns

The team examined how floods behaved over decades after forest removal. The research revealed that altered flood behavior in the north-facing watershed remained extreme for many years.

This wasn’t a short-term shock but a deep, long-term shift in the watershed’s hydrologic response. Even 40 years after all the trees were cut, floods were still stronger and lasted longer than before.

Scientists think this happened because cutting the trees permanently changed how the soil holds water, how water evaporates through plants, and how soaked the ground stays after rain.

Past forest loss led to major floods

Dr. Alila noted that the findings can be applied to current and future flood disasters. British Columbia has many similar landscapes, including areas affected by the 2021 Sumas Prairie floods.

The team suggests that their models could be used to assess how clear-cutting contributed to these events and to predict future risks.

The researchers concluded that the big Texas floods, which occurred between 2015 and 2023, were intensified by changes in the land and heavy rain caused by climate change.

Cutting down forests long ago may have made the land weaker, which led to more damage when the floods developed.

Rethinking forest policy

The evidence is clear: where, how, and when we log matters. The old one-size-fits-all approach no longer holds. Forests are not simple reservoirs. They are complex systems shaped by slope, exposure, soil, and rainfall patterns.

“Our findings highlight how multiple landscape factors interact in complex ways. As climate conditions shift, understanding those dynamics is becoming increasingly important for forest and water management.”

By treating watersheds as living systems with unique personalities, policymakers can make smarter decisions. It’s no longer just about how many trees we cut – but about where and what happens next.

The study is published in the Journal of Hydrology.

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