Water Quality

To reduce nutrient and sediment loads, for surface runoff, groundwater and atmospheric sources to meet 1967 to 1971 levels of algae and water transparency measured in Lake Tahoe.

Average Secchi Disk Transparency

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 was established by the State of California in the early 1970’s, and also is codified in the Bi-State Lake Tahoe TMDL.
Human and Environmental Drivers - 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.

2015 Threshold Evaluation

Status

- Somewhat Worse Than Target
- In 2015, the annual average Secchi depth was 22.3 meters (73.2 feet), a decrease of 1.4 meters (4.6 feet) from the previous year. However, the reader is cautioned from placing too much importance on this year-over-year change. This amount of change between years is not extraordinary for the annual average Secchi depth. In 2014, an increase of 2.3 meters (7.5 feet) was observed from the previous year. The 2015 annual average Secchi depth is 75 percent of the adopted clarity standard which also reflects a status of somewhat worse than target. The five year average Secchi depth of 22.3 meters (73.2 feet), is 94 percent of interim target of 23.8 meters (78 feet) for 2026, reflecting a status of somewhat worse than target.

Trend

- Little or No Change
- 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 (see figure above), 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 (TERC, 2014). 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 (TERC, 2015). Factors that have contributed to slowing the rate of decline and recent improvements in lake transparency likely include, Regional Plan program and policies, Lake Tahoe TMDL implementation actions, the drought-induced stream flow and stormwater runoff reductions, a 2014 decline in small algal cell the concentration, and the shallow lake mixing depth during winter 2013/14 (TERC, 2015).

Confidence

- Moderate
- View Details Below

Status is assessed by direct comparison of the most recent annual average value to the established interim target. Trend is determined using a generalized additive model (GAM). Since about year 2000, the rate of change in Secchi depth has decreased and recent Secchi depth measurements have been better than predicted by a long-term linear trend. Therefore, estimating the long-term trend using a linear estimate of change is no longer applicable to describe the observed pattern. Instead, a trend line was generated using a GAM, a more sophisticated statistical approach that shows the non-linear aspect of the observed trend in the data. The GAM permits a nonlinear relationship by fitting a smoothing function, which allows the trend analysis of recent years to be controlled more by recent measurements. The purpose of a GAM is to maximize the quality of prediction of a dependent variable from various distributions, by estimating unspecific (non-parametric) functions.

Trend

By: Data Average Method
Lake Tahoe is designated an Outstanding National Resource Water and a “Waterbody of extraordinary ecological or aesthetic value” by the states of California and Nevada, respectively, for its world famous clarity and striking blue color. Over the past half century however, clarity has significantly diminished. The Lake Tahoe TMDL Program seeks to effectively guide efforts to restore historic clarity within the lake so people may once again be able to see to depths of nearly 100 feet. 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. The trend line (dashed line) is determined statistically using a general additive model (GAM) and used to assess long term trend in clarity. The TMDL Clarity Challenge target is a five year average of 23.8 meters (78 feet) by 2026. The five-year running average (red line) between 2012 and 2017 was 21.3 meters (70 feet).

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

Lake Tahoe is designated an Outstanding National Resource Water and a “Waterbody of extraordinary ecological or aesthetic value” by the states of California and Nevada, respectively, for its world famous clarity and striking blue color. Over the past half century however, clarity has significantly diminished. The Lake Tahoe TMDL Program seeks to effectively guide efforts to restore historic clarity within the lake so people may once again be able to see to depths of nearly 100 feet. 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. The trend line (dashed line) is determined statistically using a general additive model (GAM) and used to assess long term trend in clarity. The TMDL Clarity Challenge target is a five year average of 23.8 meters (78 feet) by 2026. The five-year running average (red line) between 2012 and 2017 was 21.3 meters (70 feet).

Connecting Actions to Outcomes

Actions

Intermediate Results

Outcomes

The Regional Plan requires the use of best management practices (BMPs) for new residential and commercial development, and BMP retrofit regulations for developed properties. For example, section 60.4.6.A.1 of TRPA Code requires properties be able to infiltrate the 20-year, one-hour storm into groundwater. The Regional Plan is also designed to limit growth and shift development from sensitive to less sensitive lands. All of these requirements contribute to reducing fine sediment and nutrient runoff from developed areas. The Regional Transportation Plan complements these by encouraging use of public transit and alternative transportation modes, and reducing reliance on private automobile. Water quality mitigation fees, collected on projects that create new cover, support erosion and storm water pollution control projects. Projects completed by EIP partners since between 2009 and 2015 have:
• Restored or enhanced 27,150 linear feet of stream channel.
• Retrofitted 120.55 miles of road and decommissioned an additional 7.4 miles of road.
• Restored or enhanced 120 acres of disturbed forested uplands.
• Inspected 108.72 miles of unpaved non-urban roads and maintained 98.2 miles.
• Issued 18,076 BMP certificates to developed commercial, multifamily and single family residential properties.
• TRPA’s grant funded Stormwater Management Program (SMP) focuses compliance and maintenance verification activities on priority commercial and large multi-family residential properties in coordination with local jurisdictions. In 2015, the SMP notified 2,441 parcel owners with BMP Certificates issued more than five years ago that maintenance was due and re-issued 186 BMP Certificates following maintenance verification.
• Completed street sweeping on 24,644 miles of roads.

The TRPA Stormwater Management Program leads broad professional and public education including annual BMP trainings for contractors, local jurisdictions and real estate professionals, articles in “Tahoe In-Depth” mailed to all property owners, and public workshops and events to increase BMP awareness and promote proper design, installation and maintenance. Public outreach and educational campaigns (such as the “Take Care” campaign) highlight for residents and visitors what they can do to maintain a healthy environment including BMP completion. Between 2012 and 2015 the South Tahoe Environmental Education Coalition delivered 36 educational programs and reached nearly 30,000 individuals.

The Lake-Friendly Business Program highlights and encourages patrons to visit businesses that are doing their part to help protect Lake Tahoe by installing and maintaining their water quality BMPs. There are currently over fifty Lake-Friendly businesses in the Region.

The TMDL Management System Handbook guides the actions of agencies in the Region to reduce inputs of nutrients and sediments into Lake Tahoe (Lahontan and NDEP, 2014). As part of the TMDL implementation, each jurisdiction in the Region prepares a load reduction plan (pollutant load reduction plans in California and stormwater load reduction plans in Nevada) that detail the steps to achieve the specified load reductions. The Lake Tahoe TMDL estimated that a 50 percent reduction in nitrogen load from urban sources (8 percent of the total nitrogen load) would be required to achieve lake clarity standards (Lahontan and NDEP, 2010b).

The 2015 TMDL Findings and Recommendations memo identified wintertime traction abrasives as a primary source of ultra-fine sediment particles (less than 16 microns in stormwater runoff) (Larsen and Kuchnicki, 2015a). Managers and heavy equipment operators in the Tahoe Region continue to adaptively manage wintertime traction application practices to reduce adverse environmental impacts while ensuring safe roads. In the 2015/2016 winter season this included treating roadways with brine solution prior to storm events, which prevents ice from developing on roads and can reduce prior dry salt applications by as much as 86 percent (Wigart and Ferry, 2015b)(Wigart and Ferry 2015b). El Dorado County, the California Department of Transportation and the City of South Lake Tahoe are utilizing new wintertime traction abrasives that contain 90 percent less ultra-fine particles compared to previously used materials and also break down less into fine fractions from vehicle traffic. This new abrasive is sourced from a native granite material rather than the previously imported non-native volcanic cinders (Wigart and Ferry, 2015a)(Wigart and Ferry 2015a).
Each year the actions of TMDL implementation partners are summarized and evaluated in the TMDL Performance Report (Larsen and Kuchnicki, 2015b).

A 2011 analysis found diminishing returns from increasing storm retention capacity beyond the 20-year, one-hour storm, the TRPA infiltration requirement (2ndNature and NHC, 2011). The synthesis found that doubling retention capacity required to handle the 20-year, one-hour storm would increase annual retention by only seven percent and at a significant cost (2ndNature and NHC, 2011).
An interim target of 23.8 meters (78 feet) Secchi disk depth in 2031 has been established through development of the Lake Tahoe TMDL. The interim target has been adopted by the Lahontan Regional Water Quality Control Board, California Water Resources Control Board, Nevada Division of Environmental Protection, and the U.S. Environmental Protection Agency. Evaluation of this target is based on the five-year annual average Secchi disk depth. The five-year average Secchi disk depth for the period from 2010 to 2015 was 22.1 meters (73.1 feet). The interim target for the 2019 Threshold Evaluation Report should be continued progress towards achievement of the 2031 Lake Tahoe TMDL “Clarity Challenge.”
2031, the year identified in the Lake Tahoe TMDL “Clarity Challenge” (Lahontan and NDEP, 2014, 2010a). The TMDL estimates that the annual average Secchi depth standard (29.7 meters, 97.4 feet) would be achieved around 2076 if prescribed management actions are implemented and maintained (Lahontan and NDEP, 2010a). This estimate assumes that load reductions will slow after the first twenty years as load reduction opportunities become scarcer. The estimate does not account for impacts arising from global climate change or catastrophic events that may adversely affect clarity.

Applicable Standard

- 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)

Monitoring

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.

Locations

Monitoring locations for Secchi depth and other water quality parameters. The index station is located along the west shore.

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. Jassby et al. (1999) evaluated the general precision of the method used in Lake Tahoe, and estimated the average precision based on two observers was +0.027 m (Jassby et al., 1999). A recent analysis of annual average Secchi depth readings (includes water conditions down to a depth of approximately 20 meters in recent years) and the vertical extinction coefficient (a more sophisticated electronic sensor for measuring light levels down approximately 100 meters), has shown these two measures of light penetration in Lake Tahoe to be well correlated over the entire period of record (UC Davis - TERC, 2011).
- Moderate. Confidence in the long-term trend between 1968 and 2014 also is high. The long-term trend is estimated using a generalized additive model, which blends properties of generalized linear models and additive models. While the annual average Secchi depth in 2014 is highly encouraging, 2014 was among the driest years on record and should be considered in the context of three consecutive years of drought. It is still too early to determine if the recently observed increases are the result of actions taken to improve water quality or are primarily the product of recent weather. 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.

Recommendations

Daphnia are zooplankton that graze on phytoplankton. Reducing the abundance of phytoplankton in the water column increases clarity, and recent work in eutrophic systems has found that the loss of grazers contributed significantly to clarity loss (Walsh et al., 2016). Although early studies in Lake Tahoe suggested the loss of Daphnia did not significantly impact water quality due to the relative low Daphnia population (Elser and Goldman, 1991), increasing primary productivity in the Lake may require revisiting the working hypothesis that grazing does not significantly impact clarity. Recovery of Daphnia spp. and related filter feeding taxa like Bosmina have been documented in the Lake during years of high primary productivity (Byron et al., 1986) .

No changes recommended

No changes recommended

Implementation of the Lake Tahoe TMDL and associated EIP water quality improvement projects have primarily focused on the control of fine sediment particles and attached nutrients from urban areas. This implementation strategy is pragmatic in that it focuses regulations and management actions on the largest source of fine sediment particles and on factors most directly controlled by humans: control of urban stormwater runoff, improved road maintenance and watershed restoration. Recent science suggests that preserving and restoring Lake Tahoe’s clarity under a changing climate may require a greater emphasis on policies and management strategies that reduce the influx of nutrients from all sources (Sahoo et al., 2015). Further reducing the influx of nutrients into Lake Tahoe may require greater emphasis on the strategies and practices of the Regional Plan that focus on nutrient reduction, including stream environment zone restoration and reducing atmospheric deposition of nitrogen and phosphorus.

Historic Evaluations

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