What Pollutants Are Causing Lake Tahoe's Clarity Loss?

Lake Tahoe's extraordinary clarity is attributed to its extremely clean water that allows sunlight to reach much greater depths than it reaches in most other water bodies. Declining clarity over the past half-century is caused by increased inputs of fine sediment particles (16 microns or less in diameter), and free floating algae fed by the nutrients nitrogen and phosphorus. Fine sediment particles scatter light, while algae absorb light. As pollutant inputs increase, light is increasingly scattered or absorbed and is unable to penetrate deeper into the water column, clarity declines. 

What is the Strategy to Restore Lake Tahoe's Clarity?

Lake Tahoe TMDL research found that a 65% reduction in fine sediment particles, accompanied by reductions in nitrogen and phosphorous of 10% and 35% respectively, are necessary to meet the TMDL numeric target of 97 feet. Approximately half these load reductions are needed to meet the Clarity Challenge, an interim milestone of 78 feet annual average Secchi disk depth to be realized by 2031. The Clarity Challenge is an important goal, as once attained scientists believe the trend in clarity loss has been reversed and we are moving toward restoring Lake Tahoe's clarity.

Fine sediment particles have a greater impact on clarity than the algae fed by elevated nutrients, so while the TMDL program includes required reductions for total nitrogen, total phosphorus, and fine sediment particles, initial implementation efforts are focused on particle reduction. Urban stormwater represents both the greatest source of clarity-reducing particles and the largest opportunity to achieve needed load reductions.  Although the clarity restoration strategy includes efforts to reduce pollutants originating in forests, stream channels and atmospheric deposition, meaningful clarity improvements hinge on reducing fine sediment particles from urban stormwater runoff.

See Our Partners below for more information on the types of projects implemented to improve lake clarity. 

Lake Tahoe's clarity is measured in open waters using a Secchi disk - a circular, white disk resembling a dinner plate - that is lowered slowly in the water. The depth at which the disk is no longer visible is recorded as the Secchi depth. The clearer the water is, the deeper the Secchi disk can be seen. Standardized methods for measuring Secchi depth at Lake Tahoe have been applied since the late 1960s. The University of California Davis' Tahoe Environmental Research Center (UCD TERC) performs approximately bi-weekly measurements throughout the year and reports the annual average Secchi depth in the annual State of the Lake Report.

Annual Lake Clarity Update - July 8, 2021

The University of California, Davis Tahoe Environmental Research Center (TERC) and the Tahoe Regional Planning Agency reported today that Tahoe's clarity averaged 62.9 feet in 2020. Because lake clarity measurements vary from day to day and year to year, managers and scientists remain focused on long-term trends as an indicator of the lake’s health. Measurements show Lake Tahoe’s annual clarity has plateaued over the past 20 years. Despite this progress, summer clarity continues to decline by over a half-foot per year.

The discrepancy between TMDL implementation progress and clarity measurements suggests other factors may be affecting the rate of clarity improvement. To investigate this further, TMDL Management Agencies and TRPA commissioned a panel of scientists – through the Tahoe Science Advisory Council – to review available clarity data.

The research team’s report reaffirmed the main drivers of clarity loss: FSP and algae continue to be the main issues affecting Lake Tahoe’s clarity. This analysis warrants a continued focus on reducing fine sediment and nutrient loads. The investigation also bolstered past research on the impact of climate change to the lake, which includes altered precipitation and snowmelt patterns, increasing lake temperature, and impediments to deep lake mixing. Mixing in late winter can bring cold, clear water up from deep in the lake, which improves clarity. In 2020, mixing was extremely shallow, contributing to the lack of improvement.

Click here to access the full UCD TERC press release

Click here to access the Science Council report

Restoring historic clarity within Lake Tahoe is anticipated to require 65 years. To ensure implementation proceeds efficiently, it is essential the Lake Tahoe TMDL Program operates under a formal process over this time. The TMDL Management System is a coordinated set of procedures that enable effective and transparent adaptive management of Lake Tahoe TMDL implementation. These procedures enable program adjustment in response to changing political or economic environments, new relevant scientific or technical findings, challenges identified by implementing partners, or unforeseen future condition caused by climatic, geologic or wildfire events.

Stakeholder engagement and interaction is critical for the success of the TMDL Management System. Stakeholders, including funders, implementers and scientists all play an important role in providing input and feedback to improve program operations, and thereby ensure clarity restoration proceeds in an efficient manner and expenditures of public funding on water quality improvements are justified.

Click this link to view products resulting from implementing the TMDL Management System annual process.

TMDL Program Managers

The Lake Tahoe TMDL Program is collaboratively managed and administered by the Lahontan Water Quality Control Board (Lahontan Water Board) and Nevada Division of Environmental Protection (NDEP). The Lahontan Water Board and NDEP are primarily responsible for overseeing TMDL implementation - which includes tracking and the reporting pollutant load reduction progress. 

Urban Implementing Partners

Urban Implementing Partners are public entities responsible for undertaking projects and performing actions that reduce pollutant loads entering Lake Tahoe from stormwater runoff originating from within their respective jurisdictions. The California Department of Transportation, City of South Lake Tahoe, and El Dorado and Placer Counties in California, and the Nevada Department of Transportation and Douglas and Washoe Counties in Nevada implement controls in the form of roadway operations and maintenance, stormwater treatment facility construction and maintenance, and/or parcel-based best management practices (BMPs). Urban implementers document their actions through a comprehensive pollutant tracking and accounting system known as the Lake Clarity Crediting Program (Crediting Program). Results are reported on the Urban Uplands Results page. 

Non-Urban Implementing Partners

Non-Urban Implementing Partners include local, state, and federal land and natural resource management agencies. California State Parks, California Tahoe Conservancy, Diamond Peak Ski Resort, Heavenly Ski Resort, Homewood Mountain Resort, Nevada Division of State Lands, Nevada Division of State Parks, and the United Stated Forest Service Lake Tahoe Basin Management Unit implement water quality and other multi-objective improvements through the Environmental Improvement Program (EIP). Activities to address non-urban sources are tracked using a set of relevant Non-Urban Performance Metrics (PMs) that quantify the extent of the activities undertaken to improve water quality. This approach does not report estimated pollutant load reductions associated with these activities. 

Coordinating Partners

TMDL Coordinating Partners are those agencies and organizations that provide resources or services to assist in implementation of the TMDL and assess implementation progress. Coordinating Partners include: Nevada Tahoe Conservation District, Tahoe Regional Planning Agency, Tahoe Resource Conservation District, Tahoe Science Advisory Council, and United States Environmental Protection Agency.