Watersheds that are flatter, drier and warmer are generally more vulnerable to increases in flooding after trees are harvested. Those with greater variation in elevation spread runoff out over time, reducing flood peaks.
This is the key finding of a new study from researchers at UBC's faculty of forestry and environmental stewardship that could help identify where logging is most likely to increase flood risk.
The study, published in Advances in Water Resources , examined data from nine paired watersheds in Oregon's coastal mountain forests. The researchers found that flood responses are shaped not only by how much forest is removed, but by the natural characteristics of the landscape itself, including elevation, slope, road density and timing of snowmelt.
"The question was never simply whether logging affects floods. It was whether we understood the conditions that make some watersheds more sensitive than others," said Dr. Younes Alila, senior author of the study and professor in the faculty of forestry and environmental stewardship at UBC. "By identifying the landscape factors that control flood response, we can move toward predicting where forest harvesting is likely to increase small, frequent peak flows as well as large floods."
Comparing watershed responses
By comparing areas that had been harvested with similar nearby forests that remained intact, the researchers identified the landscape features that influence how water moves through a watershed after disturbance.
In mountainous landscapes, snow often melts at different times across elevations, naturally spreading runoff over longer periods. But in flatter landscapes, snow can melt more uniformly, causing large volumes of water to reach streams at the same time. Removing forests in these areas can amplify flood peaks and play a major role in determining how vulnerable a watershed is after logging.
"By studying how different watersheds respond to forest harvesting, we developed a framework that shows why some landscapes are more vulnerable to flooding than others," said Nick Rong, lead author and UBC master's graduate. "This approach challenges the traditional method of assessing flood impacts based on the percentage of forest harvested within a watershed and instead considers the combination of climate, terrain and watershed features that influence flood risk."
Role of fog in flood behaviour
The study also highlights an overlooked driver of flood risk: fog. In humid forests, tree canopies capture moisture from fog and release it to the ground, while also influencing how snow accumulates and melts across the landscape.
When trees are removed it disrupts this process and can lead to more runoff during rain-on-snow events. This can make flood patterns more variable after logging – a finding that the authors say has been underestimated as a key factor in shaping flood risk.
Implications for B.C.
The research has important implications for B.C., where forests experience similar Pacific Northwest conditions – including rain-on-snow events that contributed to the 2021 atmospheric river flooding in Sumas Prairie and southwestern B.C.
"This is especially relevant in the province because the region's coastal mountain landscapes are complex," said Dr. Alila. "The same amount of harvesting can have very different consequences depending on where it occurs. We need forest management approaches that recognize the natural ability of landscapes to regulate water."
The authors are calling for a more targeted approach to forest and watershed management.
"This research reinforces the importance of managing forests at the landscape level rather than looking at individual harvest areas in isolation," said Kyle Bishop, co-author and PhD candidate in forest hydrology at UBC. "Every watershed responds differently to disturbance, and understanding those differences allows us to develop management plans that account for the natural features that influence flood risk."
