Basics
Average Secchi Disk Transparency
Average Secchi Disk Transparency
Outcome
Average Secchi Disk Transparency
Meter (meters)
No
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.
This Indicator is reported in the following LT Info areas:
Status
Average Secchi Disk Transparency
By: Data Average Method

Annual average Secchi disk depth measurements recorded at the Lake Tahoe Index Station (1968 through 2017). Each annual value is an integrated average using 18 to 37 individual measurements taken throughout the year. The trend line (dashed line) is determined statistically using a general additive model (GAM). The five-year running average (red line) between 2012 and 2017 was 21.3 meters (70 feet), 2.5m less than the TMDL Clarity Challenge target of 23.8 meters (78 feet) by 2026. Data from the UC Davis – Tahoe Environmental Research Center.

 Transparency of Lake Tahoe as measured by the annual average Secchi depth.

Name Options
Data Average Method
Annual Average, 5-Year Average, Trend

Program Water Clarity

Approach

Measurements are taken in Lake Tahoe using a 25 centimeter, all white Secchi disk. The disk is lowered into the water column from a boat to a depth at which it is no longer visible by the observer, and then raised slowly until visible again. The midpoint of these two depths is called the Secchi depth. Between 18 and 37 individual Secchi depth measurements have been collected each year at an established index station. 

To download all of the water clarity data on this page please see Tahoe Open Data.

Partners

Associated Programs data not provided.

Lake Clarity Tracker icon
View in Lake Clarity Tracker

Average Secchi Disk Transparency is reported in Lake Clarity Tracker. Indicators in the Lake Clarity Tracker are used to demonstrate compliance with TMDL requirements.

Lake Clarity Tracker Indicator Data
Reporting Category
Water Clarity
View in Sustainability Dashboard

Average Secchi Disk Transparency is reported in the Sustainability Dashboard. Indicators in the Sustainability Dashboard focus on economic, environmental, and community health in the Lake Tahoe Region.

This indicator measures the annual average secchi depth in Lake Tahoe. The secchi disk depth is the standard gauge of Lake Tahoe's deep water clarity and has been used as a consistent measure since the 1960s.

Average Secchi Disk Transparency is include in the Threshold Dashboard. Threshold Indicators are evaluated against Threshold Standards every 4 years. Thresholds are environmental goals and standards for the Lake Tahoe Basin that indirectly define the capacity of the Region to accommodate additional land development.

2011 Evaluation

Status
Somewhat Worse Than Target
Trend
Moderate Decline
Confidence
Moderate

2015 Evaluation

Status
Somewhat Worse Than Target
Trend
Little or No Change
Confidence
Moderate

2019 Evaluation

The 2019 Threshold Indicator evaluation is not available yet. Check back soon for updates.
Key Messages
About the Threshold
Water Quality
Deep Water (Pelagic) Lake Tahoe
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 was established by the State of California in the early 1970’s, and also is 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 by absorption (Swift et al., 2006). The decline in transparency is a result of additions of fine sediment particles and 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 total load (Lahontan and NDEP, 2010a). Additional sources include atmospheric deposition (15 percent) and non-urban uplands (nine percent) and stream channel erosion (four percent) (Lahontan and NDEP, 2010a). Algal growth is stimulated by nutrient (nitrogen and phosphorus) loading from stream and stormwater runoff and atmospheric deposition (Lahontan and NDEP, 2010a). 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 (TERC, 2015). 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 (Winder et al., 2009). 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 alter the depth and frequency of mixing(Sahoo et al., 2015, 2013). Altered mixing regime may further influence the algal composition in the lake.
Standards
Numeric
The annual average deep water (pelagic) 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.
Transparency -­ Annual mean Secchi disk transparency: 29.7m (CA State standard) Clarity-­- Vertical Extinction Coefficient (NV State Standard)
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