It’s not only trees — wildfires imperil water too

The Fourmile Canyon Fire, sparked by a backyard burn west of Boulder, Colorado, in 2010, caused $220 million in damage and destroyed 168 homes. It also scorched nearly a quarter of a watershed that supplies water to the nearby community of Pine Brook Hills. The problems didn’t end there: Long after the blaze was put out, intense rainstorms periodically washed sediment and other particles downstream, disrupting water treatment and forcing the local water district to stop pulling water from Fourmile Creek, leaving it reliant upon water already collected in its reservoir.

In forested watersheds — the source of 65 percent of the West’s water supply — trees, soil and leaf litter soak up precipitation like a sponge, then slowly release it to aquifers, streams and rivers. But wildfires can sear the soil, making it water-repellent, and incinerate stabilizing plant roots. “Then, when it rains, all that water gets transported right off the surface,” ferrying sediment, nutrients and debris downstream, says Jeff Writer, a hydrologist at the University of Colorado Boulder. Sometimes precipitation triggers deadly mudslides that destroy homes and bury highways. Sediment can also shroud streambeds and reservoirs, forcing managers to dredge or conduct other costly fixes.

Now, new research suggests that such water-quality problems might become more frequent across the West. Climate change is already causing a surge in wildfire activity. As a result, scientists expect to see a rise in erosion in most of the region’s watersheds in the coming decades. Sediment and ash running off burned hillsides into streams can clog reservoirs, smother fish and disrupt municipal water supplies.

In many places, however, water managers and other officials are already taking steps to prepare for both wildfire and its long-term aftereffects. For communities that rely on forested drainages for their water, “It is a key aspect of water supply and watershed protection to plan for a wildfire.”

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