Relevance

This indicator tracks the transparency of Lake Tahoe as measured by the annual average Secchi depth at the Lake Tahoe Index Station. To restore Lake Tahoe’s historic transparency and clarity and protect its special status designations, the states of California and Nevada collaborated to develop a water quality restoration plan and jointly administer the Lake Tahoe Total Maximum Daily Load Program (TMDL). The protection of Lake Tahoe’s transparency is a key component of the Regional Plan, and a priority focus of the Environmental Improvement Program. Restoring Lake Tahoe's transparency is important to maintaining both ecological function, and its values to local and regional economies as a recreational destination and drinking water source. This standard is also codified in the Bi-State Lake Tahoe TMDL.

Human and Environmental Drivers

Water transparency in Lake Tahoe is largely controlled by particles in the 1– 15 µm size range blocking light penetration either by scattering or absorption. The decline in transparency is a result of the additions of fine sediment particles and the growth of phytoplankton (algae). The TMDL estimated that fine sediment particles (FSP) are responsible for about two-thirds of the overall decline in transparency. The primary source of fine sediment particles in the lake is stormwater runoff, which accounts for 72 percent of the total load. Additional sources include atmospheric deposition (15 percent) and non-urban uplands (nine percent) and stream channel erosion (four percent). Algal growth is stimulated by nutrient (nitrogen and phosphorus) loading from stream and stormwater runoff and atmospheric deposition. Drivers influencing the delivery of fine sediment and nutrients include urban development (including the transportation network and vehicle density), anthropogenic and natural disturbance in the undeveloped portions of the watershed, and local and regional climate (especially wind and precipitation). Below average stream inflows and stormwater runoff due to the continuing drought are substantial contributing factors in the recent improvement of lake transparency. The composition of diatom communities also influences clarity. When communities are dominated by smaller size diatoms, clarity is reduced because smaller diatoms remain in suspension longer, thus continuing to scatter light and decrease clarity. Lake mixing also influences clarity. The deeper waters of Lake Tahoe are very clear. During mixing events, when deep waters are brought up the surface, clarity is often quite high. However, mixing also brings nutrients to the surface which promotes algae growth which can reduce clarity. Climate change has the potential to alter the depth and frequency of mixing. An altered mixing regime may further influence the algal composition in the lake. A 2022 Tahoe Science Advisory Council review of factors that influence clarity suggested that clarity was most impacted by particles in the 1.00–4.76 µm size range. The 2023 annual clarity report from UC Davis suggested future research explore the impact of microplastics on clarity. Work to date has confirmed the presence of microplastics in Tahoe, but additional research is necessary to understand the concentration in the size range that most impacts clarity. Research into the causes of the continued decline in summer clarity is ongoing, but preliminary work suggests that both climate change and ecological change are impacting clarity. The 2023 annual clarity report from UC Davis suggested future research explore the impact of microplastics on clarity. Work to date has confirmed the presence of microplastics in Tahoe, but additional research is necessary to understand the concentration in the size range that most impacts clarity. The difference in seasonal trends that the Tahoe Science Advisory Council first highlighted in 2020 continued in the last four years. While no significant trend is visible for annual clarity, summer clarity continues to decline and winter clarity is improving. Research into the causes of the continued decline in summer clarity is ongoing, but preliminary work suggests that both climate change and aquatic invasive species are impacting clarity.

Example EIP Projects

• Area Wide Stormwater Infrastructure

  TRPA assists local jurisdictions to plan and design area-wide infrastructure within key watersheds to enable more efficient and effective stormwater treatment for local neighborhoods.

• State Route 89 Water Quality Improvement Project - El Dorado County Line to State Route 28

  Caltrans completed this roadway retrofit project in 2017 to improve collection and treatment of stormwater runoff along 8.6 miles of roadway, some portions adjacent to Lake Tahoe.

Lake Clarity Indicators

• Fine Sediment Load Reduction

  Total Maximum Daily Load implementers collectively prevent roughly 477,000 pounds per year of fine sediment from reaching Lake Tahoe.

• Lake Clarity Credits Secured

  The TMDL's robust pollutant load reduction tracking system.

Local and Regional Plans

• Lake Tahoe TMDL

  The Lake Tahoe TMDL Program is a science-based plan with stormwater load reduction for implementers to reinstate historic clarity levels in Lake Tahoe.

Monitoring Programs

• Pelagic Water Quality (Clarity)

  UC Davis has monitored the clarity of Lake Tahoe with a Secchi disc since the late 1960s.