Suspended Sediment Concentration (Surface Runoff)
This indicator measures the concentration of suspended sediment in surface runoff. In an undisturbed watershed, the majority of stormwater is captured by vegetation and absorbed and filtered through the soil. Development, such as roads, driveways, and rooftops alter the watershed by creating impervious surfaces that prevent stormwater from infiltrating. Instead, stormwater runs over impervious surfaces, collecting pollutants such as suspended sediment as it travels, enters the nearest storm drain or stream, and ultimately ends up in Lake Tahoe. Pollutants, including suspended sediment, contribute to the decline in lake clarity. Landscape modification (e.g., impervious cover such as roads or residential and commercial development) influences the volume of runoff, erosion rates, and the ability of the watershed to retain sediment and nutrients. Urban growth control limits, best management practices (BMPs) to reduce nutrient and sediment discharge from disturbed soils, BMP retrofit regulations for developed properties, and limits on coverage all help to reduce suspended sediment from surface runoff. Lake Tahoe TMDL is the region’s science-based strategy to reduce pollutant loading and restore the historic clarity of the lake.
Status
Annual suspended fine sediment load reduction as measured by the clarity crediting program of the TMDL.
Evaluation Map
Lake Clarity Tracker Projects
2023 Evaluation
- Status
- Insufficient Data to Determine Status or No Target Established
- Trend
- Rapid Improvement
- Confidence
- Moderate
Applicable Standard
WQ22: Achieve a 90 percentile concentration value for suspended sediment of 250 mg/l in surface runoff directly discharged to a surface water body in the Basin.
Key Points
- Between 2020-2023 TMDL implementors (CalTrans, CLST, Douglas, El Dorado, NDOT, Placer, Washoe) collectively prevented 2.3 million pounds of fine sediment particle from reaching lake, an average of 570,000 a year.
- The Winter of 2022 - 2023 was a historic year in terms of snowfall and low temperatures, which led to long
and continuous periods of deep snow and cold temperature, that prevented full inspections necessary to award credits (2024 TMDL Performance Report). - Urban land uses accounts for over 70 percent of total FSP loading to the lake (Lake Tahoe TMDL). The goal of the TMDL is nearly 100 feet by 2076, with an interim target of the achieve an interim target of 78 feet of clarity by 2031. Acheiving the interim target requires reducing FSP by load in 2026 by 34% from the TMDL baseline.
- The TRPA Code of Ordinances provides specific direction to ensure that activities and development in the Region are compatible with the Regional Plan and support the attainment and maintenance of the Region’s shared goals for restoration and environmental quality as expressed in the threshold standards.
- The Tahoe Science Advisory Council recommended that the discharge standards WQ19-WQ32 are not suitable for threshold standards, but should be retained in the TRPA Code (currently at Chapter 60) as management restrictions on discharge
About the Threshold
- Fine suspended sediment (fine sediment particles) are the primary driver of the decline in lake clarity (responsible for 55 to 60 percent of the decline) and urban upland areas are the dominant source of fine sediment particles in the Region (Lahontan and NDEP, 2010a).
- Landscape modification (e.g. impervious cover such as roads or residential and commercial development or logging) influences the volume of runoff, erosion rates, and the ability of the watershed to retain sediment and nutrients. The concentration of sediment and nutrients in stormwater runoff is influenced by the type, magnitude, and location of landscape modifications. Concentration is further mediated by the extent to which practices to mitigate potential impacts are in place. A variety of natural factors also influence the concentration of sediment and nutrients in stormwater concentrations including climate, weather, landscape topography, and vegetation. The Lake Tahoe TMDL estimated that urban upland areas contributed 348 metric tons of fine sediment particles to the lake annually, 72 percent of the annual total (Lahontan and NDEP, 2010a).
Delivering and Measuring Success
Lake Clarity Indicators
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Fine Sediment Load Reduction
TMDL urban implementing partners have acheived greater than total fine sediment load reduction targets required by permits and agreements
- Forest Roads Decommissioned or Retrofitted
- Miles of Street Sweeping
Local and Regional Plans
Monitoring Programs
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Regional Stormwater Monitoring
The Regional Stormwater Monitoring Program measures pollutants in urban runoff to evaluate the effectiveness of pollutant control measures and track and report monitoring findings.
Rationale Details
- Insufficient Data to Determine Status or No Target Established. WQ19: establishes a goal of 90 percentile concentration value for dissolved inorganic nitrogen of 0.5 mg/l in surface runoff directly discharged to a surface water body in the Basin. The TMDL focuses primarily on estimating total load rather than concentration. Assessment of load reduction is governed by the TMDL’s Lake Clarity Crediting Program supported by the Regional Stormwater Monitoring Program. Assessment of the status of the threshold standard is not possible with the available information, but the information presented supports the conclusion that overall load is being reduced.
- Rapid Improvement, The Lake Tahoe TMDL establishes a series of increasing pollutant load reduction targets. Target attainment is verified through the Lake Clarity Crediting Program. Urban implementers document and report load reduction activities through the Lake Clarity Crediting Program which provides the standardized tools and protocols to consistently and transparently estimate, account, and report accomplishments. Lake Clarity Credit (credit) targets define load reduction milestones in California stormwater permits and Nevada interlocal agreements. Use the map and check boxes below to see the achievements relative to targets for each jurisdiction, state, and basinwide. To evaluate the effectiveness of management and track load changes over time, the crediting program modeling tools provide estimates on the average annual load that is verified through on-the-ground field condition assessment.
Confidence Details
- Moderate. The load reduction targets of the TMDL are designed to restore the long-term clarity of the lake. The TMDL crediting program is well-documented and science-based. Full details on implementation and crediting are available on the program dashboard: https://clarity.laketahoeinfo.org/Home/ProgramManagement
- Moderate. TMDL load reduction targets accelerate each year of implementation, building towards 34% fine sediment particle load reduction in 2026 from the 2004 baseline.
- Moderate.
Additional Figures and Resources
No photos available.
No documents available.