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Winter Average Secchi Depth

Status and Trend

Interpretation and Commentary

Winter Average Secchi Depth

Status: Somewhat Better Than Interim Target
Trend: Moderate Improvement
Confidence: Moderate

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  • Relevance - This indicator measures the winter average Secchi depth at the Lake Tahoe Index Station and provides a measure of Lake Tahoe transparency during winter months (December through March). Federal, state, regional, and local agencies have all adopted numerous regulations to protect Lake Tahoe’s renowned transparency. California has designated Lake Tahoe an Outstanding National Resource Water under the Federal Clean Water Act, and considers aesthetic enjoyment of the Lake’s clarity a primary beneficial use. Similarly, Nevada has designated Lake Tahoe as a “water of extraordinary ecological or aesthetic value.”  The protection of Lake Tahoe’s transparency is also a key component of the Regional Plan and priority focus of the Environmental Improvement Program; restoring Lake Tahoe's transparency is considered an important socioeconomic value.
  • Adopted Standards  - Winter (December – March) mean Secchi disk transparency: 33.4m (109.5 ft).
  • Indicator - Each annual value is the mean of 5-13 individual measurements taken at an established index station December through March. Individual measurements are recorded in meters.
  • Status – Lake Tahoe is considered an “impaired” water body under the Federal Clean Water Act (Section 303d). Lake Tahoe has not met the TRPA transparency Threshold Standard of 33.4 m (109.5 ft) since this standard was first adopted in 1982. However, the interim target of 24 m (78.7 ft.) has been met or exceeded numerous times over the period of record, including 2011. The interim target was identified in the 2006 Threshold Evaluation report (TRPA 2007). In 2011, the winter average Secchi depth was 25.9 m (84.9 ft), an improvement an improvement of 3.7 m (11 ft) from the 2010 value. However, the reader is cautioned from placing too much importance to this year-over-year change. This amount of change between years is not extraordinary for the winter average Secchi depth. Relative to the interim target, the status of winter lake transparency is “somewhat better than the target,” because the 2011 value is about 8 percent better than the interim target. However, the current indicator value (2011, 25.9m) is “somewhat worse” than the adopted Threshold Standard of 33.4m, measuring 22 percent below the Threshold Standard.

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  • Trend The line of best fit to describe the long-term trend was determined statistically using a general additive model (GAM). While lake clarity has improved for brief periods since 1968, the overall long-term trend had shown a significant decline. In the last 10 years, however, Secchi depth measurements have been better than predicted by a long-term linear trend. That is, the rate of decline in winter Lake transparency has slowed relative to the trend prior to 2000, and appears as though it may be transitioning to a slight long-term trend of increasing winter transparency (i.e. improving). The overall trend is now better represented by a curve (see figure below), rather than a straight line. This reduction in the rate of decline in Winter Lake Transparency over the last decade is the basis for assigning a trend of moderate improvement.


  • Status – 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 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. Jassby et al. (1999) evaluated the general precision of the method used at Lake Tahoe, and estimated the average precision based on the two observers was +0.027 m.
  • Long Term Trend – Confidence in the long-term trend between 1968 and 2011 is “moderate.” The long-term trend is estimated using a general additive model (GAM), which blends properties of generalized linear models and additive models. The purpose of a GAM is to maximize the quality of prediction of a dependent variable Y from various distributions, by estimating unspecific (non-parametric) functions. The intra-annual variability associated with each average winter estimate is expected as part of the normal ecosystem response due to year-to-year changes in precipitation, runoff, Lake mixing, and meteorology. There is a moderate level of confidence that the trend of improvement in winter average lake transparency observed since about 2000 will continue into the future. Continued monitoring is required to see how this apparent improvement progresses into the future.
  • Overall Confidence - The overall confidence in this indicator is “moderate” because there is high confidence in the condition status and moderate confidence in the long-term trend.
  • Human and Environmental Drivers - Water transparency in Lake Tahoe is almost exclusively the result of particles blocking light penetration either by scattering or absorption (Swift et al. 2006). Particles in Lake Tahoe are composed of both small, microscopic, free-floating algae (picophytoplankton) and fine sediment that is transported to the Lake with stream and stormwater runoff (Swift et al. 2006), or from atmospheric deposition (Lahontan and NDEP 2010). Excess nutrient (nitrogen and phosphorus) loading, which stimulates algal growth, also contributes to the loss of transparency (Lahontan and NDEP 2010). 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).
  • Monitoring Approach – Transparency measurements are taken in Lake Tahoe using a 25-cm, 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 5 and 13 individual measurements are taken throughout the winter period (December – March) each year to arrive at an estimate of the winter average Secchi disk depth. The measurements presented in this document are taken from the Index Station, where uninterrupted monitoring has taken place since mid-1967. Although this station appears close to the shoreline, it is >150m deep and is characteristic of open-water. Early studies by UC Davis show that this location is representative of the Lake’s deepwater condition (Goldman 1974).
  • Monitoring Partners –  University of California at Davis (Tahoe Environmental Research Center), Tahoe Regional Planning Agency and Lahontan Regional Water Quality Control Board.


  • Monitoring Plan
  • Conceptual Model


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Locations where UC Davis measures Secchi depth and other water quality parameters.Trend Charts

Trend Charts

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Winter average Secchi disk depth measurements (1968 - 2012). Each value is the mean of 5-13 individual measurements taken at the Lake Tahoe index station from December through March. The line of best fit was determined statistically using a general additive model (GAM). The standard deviations for each annual estimate and the average standard deviations are also shown. The 2011 measurement of 25.9 m (84.9 ft) is somewhat better than TRPA’s interim target of 24 m (78.7 ft). The long-term trend had shown a historically declining condition, but the trend has exhibited moderate improvement, particularly over the last decade (2002 – 2011). Data are from the UC Davis – Tahoe Environmental Research Center (TERC 2011a).

Additional Info


Additional Information

Last Updated on Monday, 11 March 2013 12:49