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Clear-cutting can make catastrophic floods 18 times more frequent with effects lasting more than 40 years, according to a new UBC study.

In one watershed, these extreme floods also became more than twice as large, turning a once-in-70-years event into something that now happens every nine years.

"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."

Same treatment, different floods

The UBC-led study draws on one of the world's longest-running forest experiments at the Coweeta Hydrologic Laboratory in North Carolina and is published in the .

The research team analyzed two adjacent watersheds, one north-facing, the other south-facing, that were both clear-cut in the late 1950s.

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

In the north-facing watershed, which receives less direct sunlight and retains more moisture, floods became four to 18 times more frequent. Average flood sizes increased by 47% compared to pre-treatment levels, and the biggest floods grew by as much as 105%.

In the south-facing watershed, the same treatment had virtually no impact on flood behavior.

Old flood models inadequate

Most conventional flood models use simplified assumptions: cut X percent of trees, expect Y percent more water runoff. But this study found that such models fail to account for extreme and erratic flood patterns that emerge after landscape disturbances.

"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."

In short, he adds, forest treatments don't just raise average flood levels—they can fundamentally reshape a watershed's entire flood regime, making rare and catastrophic events much more common.

The most concerning finding was that flood effects in the north-facing watershed persisted for over 40 years, confirming that forestry treatments can lead to long-term changes in a watershed's flood response, especially as climate change brings more extreme weather, putting downstream communities at greater risk.

Policy implications

The findings have immediate relevance for forest management practices, particularly in B.C. where there are similar terrain types and forestry operations in the form of clear-cut logging.

Dr. Alila noted that the model used in this study can be used to predict which parts of B.C. are currently more at risk of extreme flooding. It can also be used to investigate how much of the severity of the Sumas Prairie floods in 2021 and the more recent Texas floods can be attributed to global warming and/or land use and forest cover changes.

"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."