U.S. Geological Survey: Development to blame for Tahoe’s loss of clarity | TahoeDailyTribune.com

U.S. Geological Survey: Development to blame for Tahoe’s loss of clarity

In a study of 14 Lake Tahoe watersheds, the U.S. Geological Survey has confirmed what environmental advocates have charged for many years – development is largely to blame for the lake’s loss of clarity. Still, the study suggests that urbanization is not the only reason the lake’s clarity is fading. Mother Nature has had hurried along the murkiness in her own way.

Since 1978, scientists have collected samples from 14 streambeds around Lake Tahoe, testing for concentration levels of nitrogen, phosphorus, iron and suspended sediments – components believed to be the cause of the lake’s loss in clarity, which has declined at the rate of 1 foot each year for the past 30 years.

The study showed that areas with paved roads, buildings and parking lots accelerated the transport of sediments carried to the lake’s shore by not allowing runoff to filter through the ground water cycle. It also discovered that some creeks kick in silt loads from natural events.

“We’re looking to see where the loading is coming from, and the four top streams for sediment loading are the Upper Truckee River, Third Creek, Blackwood and Ward creeks,” said Tim Rowe, a USGS hydrologist. “It kind of solidifies what everybody was already thinking but some of the nutrients are coming from some of the undeveloped areas like Third Creek, which has an old avalanche area that’s contributing to the sediment loading.”

Rowe added that a landslide at the headwaters of Ward Creek also makes for silty waters.

All the creeks’ loading rates increased the closer the water samples were taken to the lake, where development has spurred the increase.

“Incline Creek for example has a ten-fold increase as you go downstream,” Rowe said.

The study, which will be continued by the USGS, has been a cooperative effort of the Tahoe Regional Planning Agency and the Lahontan Regional Water Quality Control Board, regulator of water quality on the California side of the Lake Tahoe Basin.

Lahontan said it would use the report in its plan to limit the amount of sediment entering the lake.

“These kind of studies will help us determine how much nutrient load has come in via the tributaries and knowing where the load enters the lake will help us define where to focus our management efforts,” said Robert Erlich, a Lahontan environmental specialist.

A copy of the report’s fact sheet is available from the USGS, Branch of Information Services, PO Box 25286, Denver, Colo. or call (800) 435-7627. Specify the report number as FS-138-00.

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