Forest Service Studies Soil Erosion After Fires

By creating their own artificial rainfall, from light showers to heavy downpours, Forest Service researchers are studying the potential for erosion in areas damaged by fire.

Pete Robishaud, Forest Service researcher at Moscow, uses a rain simulator to determine how much water, if any, soaks into the ground and how much runs off because the soils have become water repellent. The amount of water running off gives a hint of the erosion to come.

High density or extremely hot and destructive fires cause hydrophobic or water-repelling soil. When rain falls on that kind of soil, it doesn’t soak into the ground. It runs off the bare hillside and carries dirt with it.

Gov. Phil Batt, on a helicopter tour of forest areas damaged by last summer’s fires on the Payette and Boise national Forests, last month was shown an area just north and east of McCall where the soil was destroyed several inches deep. Foresters said it will be a century before the sterile soil recovers.

The rain simulator measures the amount of runoff and the sediment that comes down with it. It also measures soil moisture below the surface, which rarely increases because of the repellent layer on top.

Boise National Forest officials want to stop extensive erosion, like that at Louise Creek recently, by applying what they learn from the rain simulator to rehabilitation and fire recovery efforts.

“We want to be able to better predict when the erosion might occur and how intense it will be. That way we can determine which treatments to use when helping the land recover,” said Leah Jaurros, soils scientist with the forest.

Spring and summer rains have washed out several streams in the forest. That becomes apparent when Idaho rivers, normally clear, run like chocolate milk.

On the Payette Forest, advances in tree seedling production and planting have allowed foresters to replant over half the tree plantations burned last summer.

Jim Davis, silviculturist with the McCall Ranger District, said 2,200 acres have been planted so far, double the usual rate. In the Corral, Blackwell and Chicken fires, 274,000 acres burned, including 4,000 acres of small tree plantations.

About 4,500 acres of young plantation trees were destroyed on the Boise National Forest.