Aftermath of the 2007 California Wildfires: Erosion, Flooding and Debris Flows

As I write this, the last of the nearly two dozen California wilderness fires of 2007 are mostly contained. Thousands of thankful residents who had their houses spared from fire are now safe to return home as the danger has apparently subsided. However, there are many more dangers that residents of the burned landscape will endure over the next several years because of these fires.

Catastrophic flooding, debris flows and landslides are particularly likely after large scale fires. This occurs because soil is exposed, no longer covered by trees and held in place with root systems. When hard rains come, this vulnerable soil is washed away along with debris and, oftentimes, homes.

This effect is further exacerbated because burned soil is more impermeable to rainfall than soil that has not been burned. Runoff from hard rain events is thus greater than in unburned areas causing flooding. Floods of this nature also contain debris from the fire and cause what are known as debris flows. Thus, even residents whose homes were spared by the fires stand to lose their houses to landslides and debris flows.

Aside from impacting human residents, the aftermath of devastating fires can influence animals as well. Flooding can change the landscape of streams and make them less habitable. Sediment transported in streams from severe erosion that may be expected after fires can kill wildlife such as fish. Post-fire flooding can also wash contaminants and other undesirable compounds that have been tied up in soil into water systems. For instance, manganese released from pine needles that had washed into a Colorado reservoir after a large fire in 1996 required that more chlorine be added to the water before being clean enough to drink.

There are several strategies that make attempts to minimize the impacts of the aftermath of fires. Adding mulch to the burned soil has been shown to be effective in minimizing erosion, but can throw off the nutrient balance of the ecosystem so is many times not desirable. Breaking up impermeable soils is often done mechanically or chemically and can reduce flooding. Seeding has limited effectiveness on fire-torched soils and has the potential to introduce non-native plants.

Placing debris dams and retention basins, straw bale check dams and silt screens throughout the burned area can slow water flow and trap sediment. This will help mitigate the impacts of post-fire flooding, but not eliminate them. Typically it takes several (5-8) years for plants to re-establish themselves to the point where they are protective enough of soil to prohibit erosion, flooding, and landslides. Until this occurs, areas ravaged by wildfire remain vulnerable to the aftermath.