Thirty Year Project Provides Answers In Managing Runoff From Logging Operations

Thirty years ago, researchers began an experiment to study the impacts of logging in the Mica Creek watershed above the St. Joe River in Idaho.

Washington State University along with University of Idaho researchers have used this unique, historical dataset to develop a tool that could help timber operations choose the best areas to log to prevent flooding in downstream areas. With the model they developed and tested, they found that logging operations could cut peak flows up to 40% by doing thinning instead of clearcutting, and that logging in critical source areas could lead to greater flooding. Led by Jan Boll, professor in the Department of Civil and Environmental Engineering, the researchers report on their work in the February issue of Journal of Hydrology.

Researchers have known for many years that timber harvest has impacts on water runoff, water quality, and downstream flooding and that it can change the frequency and magnitude of flooding.

In agriculture, researchers have found that relatively small changes in farming practices can have an outsized impact on preventing erosion problems. So, for instance, practices such as reducing tillage or plugging gullies at their source can prevent up to 50% of soil erosion problems.

The WSU research team decided to study timber and forestry with the same lens as for agricultural areas to find critical source areas that might cause an outsized impact on downstream flooding.

“We wanted to know where in the landscape we find that run-off starts and where it tends to be most susceptible to erosion,” Boll said.

The team studied a research plot that the Potlatch Corporation established in 1991 to study the effect of their logging practices on runoff control and erosion. The Mica Creek watershed drains into the St. Joe River and the Coeur D’Alene Lake.

In undisturbed forests, water infiltrates into the ground, replenishes groundwater, and feeds back into streams. Trees act like giant pumps, sucking up water from soil and releasing it through evaporation and transpiration. The cycle dampens the effect of large storms, minimizes flooding, and assures cool water for fish in summer.

Once trees are removed, the soil stays much wetter, Boll says. When water from snow or rain is added to already damp soils, they don’t have much storage, and the water runs off.

For their work, the researchers found that soil saturation increased by 20% after timber harvesting, resulting in a higher risk of flooding after large rainfall events.

Clearcutting on south-facing slopes at medium elevations strongly increased the average melt rate, leading to potential flooding when temperatures rose and snow melted. Thinning instead of clear cutting reduced peak flow amounts by as much as 40%, and partial cutting at medium to high elevation and clearcutting at low elevation could mitigate the impact of timber harvest on snowmelt timing.

“We did as uncomplicated a modeling study as you can do and focused on what’s really important in those areas, in particular the effect of snow and snow melt,” Boll said. “We get really good results with a fairly simple modeling tool.”

The researchers hope the work could someday be used by logging operations to develop a vulnerability map that is based on their rainfall regimes and typical snow melt that would help them in their decision making as to which areas to log. While erosion and runoff might not directly cause flooding problems from the small Mica Creek watershed, the tool could be useful for other regions and downstream communities.

“If you knew where those hot spots and critical areas are, maybe you could rearrange your map, so on average, you don’t have too many of those areas in one season,” Boll said.

The work was funded by the US Forest Service.