Wildfires Threaten Urban Water Supplies, Long After the Flames Are Out

Wildfires Threaten Urban Water Supplies, Long After the Flames Are Out

The Cameron Peak Fire burned 208,000 acres, more than half of them in the Poudre watershed. Mr. Bowker and many others had spent the spring poring over maps, prepared by federal agencies following aerial surveys, that showed the severity of the burn over the entire acreage.

Devastation from wildfires is far from uniform. If a fire travels quickly, there may be little or no damage to the tree canopy, underbrush and soil, and the area may be at little risk of erosion.

At the other extreme, a hot-burning, slow-moving fire may burn everything — trees, underbrush, even the organic layer in the soil, creating ash and releasing nutrients. Waxy compounds in the leaf litter will vaporize and later be deposited on the soil, so that it repels water rather than absorbing it. That will increase runoff and erosion.

In the Cameron Peak Fire, more than one-third of the land suffered moderate to high burn severity. About 10,500 acres had been identified as high-priority areas that were likely to suffer severe erosion, Mr. Bowker said.

“We looked at where the major impacts are going to be in the watershed,” he told the volunteers, “and you’re standing on one of those today.”

In addition to staking the straw tubes, called wattles, across one hillside, the volunteers spread mulch and seed. “It’s not hard work, but it’s a lot of work,” said Tim Cochran, whose wife, Carol, and her family have owned the ranch for generations.

Nearby, another crew of volunteers was working on a slope where Ms. Astvatsaturova and others had determined that the water was likely to drain into a gully and run down in a deeper torrent that could worsen erosion. To slow that water, the team built a series of log-and-rock weirs, or dams, every 10 feet or so down the gully.

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