How Tahoe Measures Water Quality
Lake Tahoe’s clarity has historically been the bellwether indicator for water quality – and the health of the entire ecosystem. Lake Tahoe owes its remarkable clarity to several factors, including its great depth, volume, and the clean and clear runoff that flows into the Lake from the granitic soils and wetlands of its surrounding watersheds. Though Lake Tahoe’s waters remain pure compared to most water bodies, clarity has declined over the past 40 years.
Average Secchi Disk Transparency
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.
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.
What is Tahoe Doing to Improve Water Quality
Stormwater runoff from roads and dense urban areas, vehicle exhaust, altered wetlands and streams, and inadequate stormwater pollution control has significantly impacted Lake Tahoe’s famous clarity and the health of its watersheds. Many of these impacts occurred decades ago. Watershed restoration, air pollution controls, and aggressively implementing proper stormwater controls and best management practices are essential to restore the Lake’s clarity and the Basin’s wetlands and wildlife.
Miles of Roads Decommissioned or Retrofitted
Fine sediment loads entering Lake Tahoe are the primary cause of the Lake’s clarity loss, thus efforts to slow clarity loss are focused on fine sediment load reductions. Stormwater runoff from paved and unpaved roads in the Tahoe Basin is responsible for contributing about two thirds of total fine sediment pollution to Lake Tahoe. When roads are removed, taken out of use, or retrofitted with drainage conveyance and treatment facilities or source controls, the volume of stormwater runoff that leaves those roads is reduced, as is the pollutant load carried in the runoff. This, in turn, reduces the amount of these pollutants reaching Lake Tahoe, improving lake clarity.
This indicator measures the miles of city, county, state and US Forest Service (USFS) roads that are retrofitted, decommissioned or obliterated to reduce stormwater pollution through capital improvements.
Data displayed currently includes improvements accomplished by TMDL Urban Implementing Partners. TMDL Management Agencies are working on update to display only accomplishments completed on non-urban roads.
The amount of forestland roads that are retrofitted or obliterated to reduce stormwater pollution through capital improvements. This PM is reported in three categories of treatment priority based on water quality risk. Treating high-priority roads reduces stormwater pollution and cost-effectively improves the clarity of Lake Tahoe.
Parcels with Stormwater BMPs
Retrofitting public and private parcels with best management practices (BMPs) reduces pollutant concentrations of stormwater runoff from precipitation and reduces the volume of stormwater leaving those parcels. Reduced stormwater volumes result in less demand on public stormwater treatment systems and fewer fine sediment particles and other nutrients being delivered to Lake Tahoe. When fewer nutrients are available in the waters of Lake Tahoe, less algae can grow and clarity loss is reduced.
This indicator measures the number of developed parcels in the Tahoe Basin that are retrofitted with BMPs that remove fine sediment particles and nutrients. This indicator tracks private parcels and parcels belonging to large, public landowners.
Learn More/Get Involved
Check out the State of the Lake Report, produced annually by UC Davis Tahoe Environmental Research Center to learn more about Lake Tahoe’s clarity, physics, chemistry, biology and what affects it over time.
Everyone has a role to play in protecting the Lake Tahoe Basin. Science shows that implementing Best Management Practices (BMPs) on existing development is a critical step toward improving Lake Tahoe’s water quality and clarity. The BMP Retrofit Program represents part of the private contribution to the Lake Tahoe Environmental Improvement Program (EIP) Program. The Stormwater Management Program (SMP) staff provide free assistance to property owners, private businesses, and government agencies to advance effective BMP design and implementation on developed properties. Thousands of properties already have them in place, have you done your BMPs? Visit tahoebmp.org for more information.