

Frozen stream in Minnesota. Photo by: Tiffany Roufs.

Keeping forests standing would lessen both the number and size of spring floods in temperate regions, according to a new study in Water Resources Research, by slowing seasonal snow melts. In deforested areas, snow melts faster due to a lack of shade causing at least twice as many, and potentially up to four times as many, flood events. The new research highlights a largely unknown ecosystem service provided by temperate forests: flood mitigation.

“As soon as you get rid of the trees, the snow melts faster,” explains lead author Kim Green with the University of British Columbia in a press release. “It’s that simple.”

During the annual snow melt, nearby rivers see a sudden infusion of water potentially leading to flood conditions. The rate at which the snow melts is often as important as how much snow melts in total. Because snow melts faster when exposed to full snow (as opposed to under the shade of trees) it’s long been known that deforested areas cause larger floods than forested areas, but the study now adds that deforestation also substantially increases the number of floods.

By analyzing data from three creeks in British Colombia and one creek in Colorado, the scientists found that deforestation pushed 10-year floods (i.e. flood levels that occurred every ten years on average) to happen every three to five years. In addition, 20-year-floods began occurring every 10-12 years, and in one creek in Canada 50-year-flood began occurring every 13 years.

“Once you look at how the frequency has changed,” Green says. “you start to realize that deforestation has had a pretty dramatic effect on floods.”

CITATION: Green, K. C. and Y. Alila (2012), A paradigm shift in understanding and quantifying the effects of forest harvesting on floods in snow environments, Water Resour. Res., 48, W10503, doi:10.1029/2012WR012449.

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