Indicator
Average Secchi Disk Transparency
2019 Evaluation
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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. 

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Photo Credit: UC Davis

Status

Average annual Secchi depth from the Lake Tahoe Index Station as taken by UC Davis.

2019 Evaluation
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Status
Somewhat Worse Than Target
Trend
Little or No Change
Confidence
Moderate
Applicable Standard
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.
Key Points
  • 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.
Evaluation Map
Description

Lake Clarity Monitoring Locations

About the Threshold
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.
Water transparency in Lake Tahoe is largely controlled by particles 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.
Delivering and Measuring Success

Example EIP Projects

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.

Monitoring Programs

Rationale Details
In 2019, the annual average Secchi depth was 19.1 meters, a decrease of 2.5 meters from the previous year. The TMDL establishes goals and evaluates progress against the five-year running average to smooth annual fluctuations arising from natural variability in climate and lake conditions. The 2019 five-year average secchi depth was 20.5m (67.3 ft), 86 percent of the interim target of 23.8 meters (78 feet) for 2026, reflecting a status of somewhat worse than target. The 2019 five-year average secchi depth was 69% of the 2076 target of 29.7m (97.4 ft).
Since 2000, Secchi depth measurements have been better than predicted by the long-term trend of linear decline observed since 1968. Statistical analysis supports the observation that the decline in Lake Tahoe’s transparency has slowed since 2000, and the overall trend is now better represented by a curve, rather than a straight line. The line of best fit to describe the long-term trend was determined statistically using a generalized additive model (GAM). This reduction in the rate of decline in annual lake transparency over the last decade is a direct result of the improvement in the winter average Secchi depth. The mechanisms driving the improvement in winter Secchi measurement are not fully understood but are potentially linked with a reduction in fine particles from urban stormwater. The summer average Secchi depth has shown a consistent linear decline since 1967, albeit with considerable inter-annual variability exhibiting a 10- to 15-year cyclical pattern.
Confidence Details
High. There is high confidence in the status determination. Secchi depth measurements are used widely as a measure of water transparency in oceans and lakes; it is a highly reliable, relatively simple, and an inexpensive measurement of lake transparency. It is among the oldest limnological devices and was first used by Italian Professor P.A. Secchi in the 1860s.
Moderate. Confidence in the long-term trend between 1968 and 2019 also is high. The long-term trend is estimated using a generalized additive model, which blends properties of generalized linear models and additive models. The intra-annual variability associated with each average annual estimate is expected as part of the normal ecosystem response due to year-to-year changes in precipitation, runoff, Lake mixing, and meteorology. Future weather conditions, particularly extreme conditions (i.e., droughts and floods) can have a substantial effect on pollutant loading and lake transparency.
Moderate. Overall confidence takes the lower of the two confidence determinations.
Additional Figures and Resources

No photos available.

No documents available.