Clear-cut logging is making floods worse. But it’s not just about how many trees are cut

The 2021 floods in British Columbia’s Fraser Valley caused upwards of $14 billion in damages.

Since then, communities have brought lawsuits against logging companies and the provincial government , blaming them for logging practices that made flooding worse.

Over the past 50 years throughout B.C., clear-cut logging has been the primary method of timber extraction across watersheds of all sizes.

Floods are happening more often and are becoming more expensive, and unlike the storms that help trigger them, clear cutting and the associated cumulative impacts are something we can manage.

When watersheds are assessed for flood risk, the question often asked is a simple one: How much of the watershed has been logged? That may be the wrong question entirely.

For more than 100 years, the influence of forests on floods has been explained by the trees.

The forest canopy catches rain and snow, shades the snow from melting and transpires water back to the atmosphere.

Our new study shows that this is only a part of the picture and, if considered in isolation, can mislead forest managers.

Our study in Oregon, which has a similar climate to coastal B.C., suggests that the proportion of a watershed that is logged is not the only factor that determines flood risk.

The landscape and climate matter too.

We studied nine pairs of watersheds, with each pair made up of a logged treatment and an unlogged control that receive both rain and snow during the fall and winter.

In these forests, the largest floods are not produced by rain alone, but by warm rainstorms that melt snow.

A single storm brings its own water and releases the water in the snowpack at the same time.

It’s not just how many trees Through our research, a puzzle arose.