Lake Tahoe is famous for its remarkable clarity and striking blue color. As the largest alpine lake and second deepest in North America, it is designated an Outstanding National Resource Water by the state of California and a “water of extraordinary ecological or aesthetic value” by the state of Nevada. The Lake Tahoe Basin is a destination for millions of visitors annually and is home to approximately 60,000 year-round residents.
Between 1968 and 2000, approximately one-third of Lake Tahoe’s unique clarity was lost. To address this issue, the Lahontan Regional Water Quality Control Board (Lahontan Water Board) and Nevada Division of Environmental Protection (NDEP) collaborated to develop the Lake Tahoe TMDL. The Lake Tahoe TMDL is a science-based plan to better understand the causes of the loss in lake clarity, determine how much pollution needs to be reduced to reinstate historic clarity, and develop a workable, cost-effective implementation strategy. Now in the implementation and tracking phase, controls are being implemented to reduce pollutant loading to Lake Tahoe and Lahontan and NDEP are working closely with project implementers to track progress, report accomplishments, measure effectiveness and adaptively manage implementation efforts.
What Pollutants Are Causing Lake Tahoe's Clarity Loss?
The extraordinary historic clarity of Lake Tahoe is attributed to its extremely clean waters which allow sunlight to reach much greater depths than it reaches in most other water bodies. However, declining clarity over the past half-century is attributable to increased inputs of fine sediment particles (16 microns or less in diameter), and free floating algae fed by the nutrients nitrogen and phosphorus. Fine sediment particles scatter light, while algae absorb light. As pollutant inputs increase, light is increasingly scattered or absorbed and is unable to penetrate deeper into the water column. Consequently, clarity declines and Secchi depth measurements decrease.
What is the Strategy to Restore Lake Tahoe's Clarity?
Lake Tahoe TMDL science indicates that a 65% reduction in fine sediment particles, accompanied by reductions in nitrogen and phosphorous of 10% and 35% respectively, are necessary to meet the TMDL numeric target of nearly 100 feet. Approximately half these load reductions are needed to meet the Clarity Challenge, an interim milestone of 80 feet annual average Secchi disk depth to be realized by 2031. The Clarity Challenge is an important goal because once attained scientists can state with confidence the trend in clarity loss has been reversed and we are moving toward restoring Lake Tahoe's clarity.
Fine sediment particles have a greater impact on clarity than the algae fed by elevated nutrient concentrations, so although the TMDL program includes required reductions for Total Nitrogen, Total Phosphorus, and fine sediment particles, initial implementation efforts are focused on particle reduction. Furthermore, urban stormwater represents both the greatest source of these particles as well as the greatest opportunity to achieve needed load reductions. So while the restoration strategy includes efforts to reduce pollutants originating in forests, stream channels and atmospheric deposition, attaining the load reduction goals hinges on reducing fine sediment particles originating in urban areas and transported to the lake through stormwater runoff.
A California and Nevada version of the Final Lake Tahoe TMDL Report was approved by the United States Environmental Protection Agency in August 2011. Download the document adopted by California HERE. Download the document adopted by Nevada HERE.