MICHELE NORRIS, host:
Lake Tahoe's sapphire water is renowned for its remarkable clarity, but scientists worry that last month's South Lake Tahoe fires might have a lasting impact on the lake's health.
Tamara Keith of member station KQED reports.
TAMARA KEITH: Four days after the Angora fire lit up this side of a mountain and destroyed more than 250 homes, U.C. Davis biologist Brent Allen stirs his silver research boat through the still waters of Lake Tahoe.
Mr. BRENT ALLEN (Biologist, University of California Davis): We're pulling up to one of our research buoys out here in the middle of Lake Tahoe.
KEITH: To check lake clarity, Allen performs a test that researchers here have been using for the last 40 years. Using a hydraulic lift, he lowers a weighted 10-inch white disc into the water. And he keeps lowering it until he can't see it anymore.
Mr. ALLEN: Basically, you're measuring the light path down to the disc and back to your eye. And obviously, the more things that you have in the water column on the top of the water, you're not going to see it as far down.
KEITH: The reading is what Allen would expect for this time of year. The clarity is good. Lab analyses of water and air samples find similar promising news. The impacts of smoke and ash weren't as significant as feared. But Allen has no doubt Lake Tahoe and its famous clarity will eventually be affected by the blaze.
Mr. ALLEN: The lake responds to environmental change on the landscape. And the fire is a major environmental impact in the basin.
KEITH: He's concerned about run-off.
(Soundbite of rain)
KEITH: Several days later, hefty raindrops fall from a thundercloud parked over South Lake Tahoe. This afternoon thunderstorm is the first of many weather events expected to test the 3,000-acre burn zone. A survey of the area found that three-quarters of the soil is now hydrophobic. Meaning, water that would have once soaked in will just run right off.
Sue Norman is a hydrologist for the U.S. Forest Service in the Lake Tahoe basin.
Ms. SUE NORMAN (Hydrologist, U.S. Forest Service): Increases run-off. It's a domino effect that starts building and building. It can start really moving huge amounts of sediment if it's not controlled.
KEITH: The Forest Service plans to treat the charred hillsides with a mulch to try and limit run-off. But if sediment reaches the lake, it could seriously hurt Tahoe's clarity. So Norman is counting on a natural solution as well -meadows. As she trudges through knee-high grasses in a large meadow just downhill from the fire, she explains that meadows can work like filters.
Ms. NORMAN: This is where we're hoping a lot of natural treatment will occur if there are sediment and nutrients that come off the fire, the burned area.
KEITH: Angora Creek runs right through the burn zone, then meanders through this meadow before emptying into a river that feeds Lake Tahoe. Norman and her research team are taking water samples here and in several other spots throughout the watershed to see if nature can handle the run-off. A big rainstorm or rapid spring snowmelt could cost a lot of trouble for the lake, which has already seen its clarity drop an average of a foot a year for the last 30 years.
Ms. NORMAN: We are in the watershed that currently is the highest contributor of sediment and nutrients to the basin even before the fire. So we don't want to add anymore to the problem that already exists.
KEITH: Norman and others here don't really know what to expect. The severity of the fire's impact on the lake depends most on what Mother Nature brings in the coming months and years.
For NPR News, I'm Tamara Keith.
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