More wildfires mean more dirt flowing into your drinking water supply

Water quality and supply in many western cities could suffer in the future as wildfires burn hotter and more frequently, according to a new study by the U.S. Geological Survey.

The culprit is erosion. After a wildfire, there are few plants and trees left to hold the soil in place in a burned area. So when it rains, soil and rocks are swept downstream. Steep slopes are especially vulnerable to being wiped bare by post-fire rainfall.

This process is called "sedimentation," and it’s worrying for a number of reasons. As dirt builds up behind dams, reservoirs can hold less water for drinking or flood control, and need to be emptied out more often. Dirt can also carry pollution, requiring extra water treatment.

Due to the increase in wildfires, almost 90 percent of watersheds in the West will see a significant increase in sedimentation by mid-century, according to the study, which appeared in Geophysical Research Letters. One-third of those watersheds will see sedimentation rates double.

Some cities are seeing the impacts of sedimentation now. Since the 250-square-mile Station Fire in 2009, the L.A. County Department of Public Works has seen a nearly 10-fold increase in the amount of sediment flowing into the reservoir behind the Devils Gate Dam in Pasadena, according to Mark Pestrella, the department’s director.

A charred landscape is left in the wake of the 250-square-mile Station Fire as firefighters work to complete the final defensible fireline to contain the massive fire in the San Gabriel Mountains on September 16, 2009 in the Angeles National Forest, northeast of Los Angeles.

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Devils Gate and other reservoirs and debris basins, which trap sediment but allow water to pass through, now need to be emptied out much more frequently than before the fire, he said.

In addition to more frequent, more severe fires, the type of precipitation falling on L.A. County is more apt to cause erosion. Rain is increasingly falling in fewer but more intense storms that pound the burned hillsides with more power.

"It’s just the perfect formula for increased sedimentation due to increased temperatures," Pestrella said.

While sedimentation is happening much more rapidly now, it certainly isn’t new to L.A. County. The San Gabriel Mountains, one of the most erodible mountain ranges in the world, spit out millions of tons of debris annually that roar downhill in destructive muddy flows that are powerful enough to take out houses.

After a disastrous flood in 1915, the L.A. County Department of Flood Control was created to build a system of dams and debris basins in the mountains and foothills to keep sediment out of the county's quickly expanding cities and suburbs.

Going forward, L.A. County Public Works hopes to combat sedimentation not just by removing more of it from behind dams and debris basins, but also by carrying out reforestation projects with the U.S. Forest Service in the San Gabriel Mountains. Thicker forests prevent dirt from flowing downstream in the first place. Pestrella said fighting both erosion and fire is a huge battle, said Pestrella.

"That is fighting Mother Nature," he said, "and Mother Nature is a large entity."