REPORT: Efforts to reduce clarity-harming pollutants in Lake Tahoe remain on track

REPORT: Efforts to reduce clarity-harming pollutants in Lake Tahoe remain on track

Lake Tahoe’s iconic deep-water clarity is attributed to its pristine clean water which allows sunlight to reach much greater depths than possible in most other water bodies. Between 1968 and 1971, annual clarity levels averaged about 97.5 feet below the lake’s surface. By 2000, however, researchers found that the depth of visibility had declined by about one-third, largely due to fine sediment and other nonpoint source pollutants washing into the lake and clouding the water.

Upon discovering a decline in Lake Tahoe’s water quality and clarity over several decades, the Nevada Division of Environmental Protection (NDEP) and the California Lahontan Water Board moved quickly to develop the Lake Tahoe Total Maximum Daily Load (TMDL) program, a science-based plan to restore Lake Tahoe’s deep-water clarity to historic levels. To accomplish this, the TMDL team relies on local governments, highway departments, and public land and natural resource management agencies to implement pollution controls to reduce the amount of clarity-harming pollutants that wash into the lake.

Such efforts include:

• Improved road-sanding and sweeping to prevent abrasives from being ground down and entrained in stormwater runoff or into the atmosphere
• Constructing stormwater treatment facilities to treat and infiltrate polluted stormwater runoff
• Installing best management practices on developed lands, forest roads and facilities to minimize pollutants in runoff
• Restoring or rehabilitating channels to reduce erosion from streams 

The TMDL team releases annual reports to track and monitor progress, identify key trends, and inform key management and policy decisions using the best available science. The agencies recently released the 2021 Performance Report that summarizes accomplishments through 2020. The report found that pollution from light-scattering fine sediment in urban stormwater was reduced, on average, by over 523,000 pounds per year. Pollution from nitrogen and phosphorous, which contribute to algae growth, was also reduced by thousands of pounds per year, on average.

These results show that local governments, and California and Nevada transportation agencies are continuing to surpass the annual goals established to reduce urban stormwater pollution and improve lake clarity, and that efforts are on track to achieve the pollutant reductions needed to meet the Clarity Challenge goal of 78 feet of clarity (measured as a five-year average, by 2031). In time, the goal is for people to once again be able to see to depths of 97 feet.

“The reduction in fine sediment going into the lake last year is significant and speaks well of the effort and on-going commitment among our partners to restore Lake Tahoe’s crystal-clear water,” said Mike Plaziak, executive officer of the Lahontan Regional Water Quality Control Board. “Put in perspective, 523,000 pounds of fine sediment is equivalent in weight to over 180 small-sized vehicles.”

Unfortunately, in-lake clarity measurements are not tracking with the pace of TMDL implementation progress. Currently, annual clarity stands at about 63 feet, and measurements show the lake’s clarity has plateaued over the past 20 years. A more detailed look into winter clarity shows only slight improvement, while summer clarity continues to steadily decline at about one-half foot per year. The discrepancies between implementation progress and clarity measurements suggest other factors may be affecting the rate of improvement.

To assess clarity data trends and better understand why clarity improvement is not tracking with the pace of pollutant load reduction progress, NDEP together with the California Lahontan Regional Water Quality Control Board and Tahoe Regional Planning Agency, partnered with the Tahoe Science Advisory Council (Science Council) to carry out several research projects. The findings show that while the lake’s ecology is influencing seasonal and annual lake clarity trends, fine sediment and algae continue to be the primary factors affecting Lake Tahoe’s clarity. This outcome validates the current TMDL implementation approach focusing on load reductions of these pollutants.

Findings also corroborated and bolstered earlier research on the increasing role and impacts of climate change. Changes to watershed hydrology and internal lake dynamics, including changes to rainfall and snowmelt patterns, increasing lake temperature and reduced mixing that brings cold, clear water to the surface, are influencing seasonal and annual lake clarity values.

Looking ahead, partner agencies will continue to engage with the Science Council to fully understand the influence of other factors on clarity conditions, including water quality impacts from the Caldor fire and other fires that blanketed Tahoe in smoke for much of the summer. TMDL agencies will also continue to work with TSAC to identify solutions that may help speed up improvement of the lake’s clarity.

“The challenges in Lake Tahoe continue to grow every day,” said NDEP Administrator Greg Lovato. “The Nevada Division of Environmental Protection, together with our partners, remains steadfast in our commitment to restore and enhance Lake Tahoe’s iconic water clarity for generations to come. We look forward to building on the great progress that’s been made to address current and emerging challenges head-on by putting forward science-driven solutions to reduce pollution and protect the ecological health and natural beauty of the lake. I thank our partners on both sides of the lake for their continued collaboration and support to ensure a vibrant, sustainable, climate-resilient future for Lake Tahoe.”

Many projects on the Nevada side of Lake Tahoe have been funded by the NDEP 319(h) Nonpoint Source Grant program to support the goals of the Lake Tahoe TMDL Program.  A new Story Map resource was recently developed highlighting projects funded under the 319(h) grant program dating back to 2015.  Follow the link to learn about Nevada Tahoe Basin and Statewide projects that have mitigated nonpoint source pollution and resulted in fine sediment and nutrient pollutant reductions.

Additional information on the Lake Tahoe TMDL Program can be found on Lake Tahoe Info’s Lake Clarity Tracker (www.clarity.laketahoeinfo.org).