Average Secchi Disk Transparency
This indicator tracks the transparency of Lake Tahoe as measured by the annual average Secchi depth at the Lake Tahoe Index Station. 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. The decline in transparency is a result of the additions of fine sediment particles and the growth of phytoplankton (algae). Drivers influencing the delivery of fine sediment and nutrients include urban development, anthropogenic and natural disturbance in the undeveloped portions of the watershed and local and regional climate. Many programs throughout the Tahoe Region are aimed at improving lake clarity such as stormwater reduction and stream restoration projects. Lake clarity has been measured at Lake Tahoe by UC Davis since 1968.
WQ1: The annual average deep water transparency as measured by Secchi disk shall not be decreased below 29.7 meters (97.4 feet), the average levels recorded between 1967 and 1971 by the University of California, Davis.
- In 2019, the annual average Secchi depth was 19.1 meters, a decrease of 2.5 meters from the previous year. The long-term decline in clarity was halted about 20 years ago, and annual clarity has remained relatively stable since 2000.
- In 2019, Total Maximum Daily Load implementors (CalTrans, Nevada Dept. of Transportation, City of South Lake Tahoe, Douglas County, El Dorado County, Placer County, and Washoe County) exceeded their load reduction targets. Collectively, implementers prevented roughly 477,000 pounds per year of fine sediment from reaching the lake, exceeding the target by 30 percent.
- Recent analysis by the Tahoe Science Advisory Council has highlighted emerging differences in seasonal clarity trends. While no significant trends were identified during most of the year, the Council found that clarity in July and August continued to decline over the last 20 years.
- 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.
Delivering and Measuring Success
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.
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.
UC Davis has monitored the clarity of Lake Tahoe with a Secchi disc since the late 1960s.