The Rim Fire burning through Yosemite National Park is now the largest ever seen in the region. The fire has so far torched 230 square miles of California forest; firefighters are making progress in dampening the flames. Unfortunately, just because the fire may soon be put out, it doesn’t mean that the charred region will be completely safe.
In the winter, burned forests collect far more snow than unburned forests, which is then stored in the snowpack for the following spring. Without a protective tree canopy to provide shade, the stored snow melts earlier in the spring, and is much more likely to melt all at once. A large wildfire in the summer, then, can set the stage for dangerous flooding the following spring.
It’s not only trees that burn during a wildfire, though. The soil itself turns into a patchwork of water-repellent burned soils and obliterated organic matter. Instead of soaking into the ground to replenish groundwater reservoirs, rain and snowmelt run across the burned land into streams and rivers to be carried downstream. This can lead to flooding – and even drinking water shortages – down the line.
The effect, says Boon, is most prominent in mountainous regions, where steep slopes and strong spring melt amplify the effects.
On top of flooding, charred forests can also set the stage for water quality problems. When the rains pick back up in in the land touched by the Rim Fire, the coursing water will carry ash and debris downstream. This isn’t just true for this fire: the same can be said for many of the other large wildfires currently burning across the country. In the case of the Yosemite fire, while authorities are worried now about the security of the Hetch Hetchy Reservoir, which supplies the bulk of drinking water to San Francisco, they’ll have to keep watching the reservoir even after the fire’s put out.
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