Preserving Lake Tahoe’s Clarity

Snow is removed in Tahoe Basin

Fortunately for parched California, this winter storm in the Sierra Nevada has been measured at 173% of normal. But the high snowpack brings special problems for the Tahoe Regional Planning Agency (TRPA): more stormwater run-off that transports nutrients and fine sediment that impair the Lake Tahoe’s clarity.

TRPA is unique among CALCOG members because of its unique mission to Keep Tahoe Blue. The lake is designated as an “Outstanding National Resource Water” by the U.S. Environmental Protection Agency, which also lists Tahoe as “impaired.”  The lake’s clarity—which has declined by as much as 30 percent—suffers from pollution from stormwater run-off. Although part of the problem is excess nitrogen and phosphorous (which induces algae growth), more recent research has identified the primary culprit as fine sediment run-off from infrastructure—like roads.

TRPA has been partnering with and encouraging local agencies to reduce fine sediment running off local streets and roads.   Although TRPA has several programs in place to protect the lake (see table), the one that has the most significant effect is the modification of road sanding practices.  Road sands become fine sediment when sands are crushed by car tires and are the primary source of fine sediments in the lake.

But that changed recently when the switch was made to grittier sand that does not disintegrate under a car tire. Now when sediment is washed into the lake, the heavier grains sink to the bottom—reducing the amount of ultra-fine particles responsible for the clouding of the lake by a whopping ninety percent. Still, more gains are made by pre-applying a brine solution to the roads to keep them warmer longer (which requires less sand), being more precise in applying the sands (in timing and location), and enhanced street sweeping after storm events.

The results are encouraging. Early water quality monitoring and testing have shown significant reductions in fine sediment.  But the real effect may not be known for years, as it takes time for the sediment to reach the center of the lake where official clarity readings are taken. Other TRPA Programs to protect Lake Tahoe clarity:

  • Wood Stove Replacement Rebate Program: Under this program, TRPA has partnered with local agencies to replace old wood-burning appliances (which result in less emissions from nitrogen that fuels algae growth)
  • Lake Tahoe Environmental Improvement Program: This program has delivered 729 miles of upgraded roadways, in addition to many other improvement projects
  • Total Maximum Daily Load: The TMDL looks at where pollutants are coming from and how to reduce the course of contamination over time
  • Best Management Practices: BMPs provide guidance for capturing stormwater runoff before it enters Lake Tahoe
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