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