Wildfires can directly harm water supplies, such as burning down storage structures and melting pipes, but also indirectly harm by sending up clouds of smoke and loosening soil, which washes debris and ash into watersheds. Wildfires even in remote parts of the state can pollute water that ends up in city taps.
Water samples collected from drinking water in Santa Rosa after the 2017 Tubbs and 2018 Camp Fires tested positivefor benzene, a cancer-causing chemical. State water officials said the contamination most likely came from overheated pipe materials, as well as smoke and combustion byproducts.
The 2015 Butte Fire burned a 12,000-acre section of a watershed, then winter storms carried debris from the fire into the river and reservoir. The US Geological Survey estimates likelihoods and magnitudes of such post-fire debris flows.
And then there are megafires: Super hot and dry conditions combine to create a complex of super-intense fires. In 2020 California experienced megafires that burned three Sierra Nevada watersheds important to California’s water supply. Runoff, erosion and sediment could occur near areas with moderate and high soil burn severity, although effects may not be seen for several years.