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 culprits are erosion and sedimentation. After a wildfire, there are few plants and trees left to hold the soil in place in a burned area. When it rains, soil and rocks are swept downstream causing sediment that settles in dams and debris basins. Steep slopes, such as SoCal’s San Gabriel Mountains, are especially vulnerable to being wiped bare by post-fire rainfall. The San Gabriel Mountains are one of the most erodible mountain ranges in the world and spit out millions of tons of debris annually that roar downhill in destructive muddy flows.
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. One-third of those watersheds will see sedimentation rates double.
Sedimentation is worrying for a number of reasons. As dirt builds up behind dams, reservoirs can hold less water for drinking or flood control, and public works projects need to be put in place to empty the dams more frequently.
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.
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.
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. Fighting both erosion and fire is a huge battle, said Pestrella. But ongoing sediment management is crucial to flood risk management and water conservation for Los Angeles County.