Study finds it's where you log, not just how much, that determines flood risk

A new study from researchers at UBC's Faculty of Forestry and Environmental Stewardship reveals that it's not just the amount of forest removed, but also the landscape's natural characteristics that determine flood risk. Published in Advances in Water Resources, the study examined data from nine paired watersheds in Oregon's coastal mountain forests and found that watersheds with flatter terrain, warmer temperatures, and less elevation variation are more vulnerable to increased flooding after trees are harvested.

Instead of focusing solely on the percentage of forest removed, the study emphasizes the importance of considering elevation, slope, road density, and snowmelt timing in predicting flood vulnerability. By analyzing watersheds with similar conditions, researchers identified that snow often melts at different rates across elevations, spreading runoff over longer periods in mountainous landscapes.

However, in flatter areas, snow melts more uniformly, causing large volumes of water to reach streams simultaneously. Removing forests in these regions can amplify flood peaks and significantly impact a watershed's flood vulnerability.

The study also highlights the role of fog in fog-prone forests. Tree canopies capture moisture from fog and release it to the ground, influencing snow accumulation and melt patterns. When trees are removed, this process is disrupted, potentially leading to more runoff during rain-on-snow events and increasing flood variability. This factor has been previously underestimated in assessing flood risk.

The findings have significant implications for regions like British Columbia, where forests experience similar weather patterns and complex coastal mountain landscapes. The research advocates for a more targeted approach to forest and watershed management, considering landscape features that naturally regulate water flow. By understanding how different watersheds respond to forest harvesting, managers can develop more effective strategies that account for the natural factors influencing flood risk.