Forests mitigate flood risk at watershed scale

Clearcut logging leads to frequent, extreme flooding

New research measured the effects of logging on snowmelt-generated floods for two snow-dominated regions in British Columbia, according to the University of British Columbia (UBC).

“When 21% of trees in the watershed were harvested using clearcut logging, the average flood size increased by 38% in the Deadman River and a staggering 84% in Joe Ross Creek,” remarked first author Robbie Johnson, a master’s student in Applied Science in Forestry at UBC. “As well, floods that used to happen only once every 10, 20, 30, 40 and 50 years are all becoming much more common.”

Senior author and UBC Forestry professor Younes Alila explained, “Reduced forest cover leads to more snow on the ground and more solar radiation reaching the snowpack. With much less forest cover to catch and shade the snow, more of it melts faster, causing larger floods.”

The researchers confirmed the impact of clearcutting on floods is also influenced by specific local conditions. Water flow factors include the size of the watershed, topography, exposure to the sun, storage areas such as lakes, and the location of cut blocks (areas of land with defined boundaries that are authorized for harvest).

Their analysis revealed that clearcut logging induced large increases in the frequency of the biggest floods.

The study demonstrates forests’ ability to mitigate flood risk at the watershed scale, “especially since replanted forests in the cut blocks are not capable of providing the same hydrologic functions for decades.” Better forest management will reduce potential flood risks to communities and the environment. “Most importantly, (let’s) consider ecosystem-friendly alternative logging practices, such as thinning and selecting individual trees for harvest.”

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