Climate Resilience Dashboard
EIP Project Tracker
Lake Clarity Tracker
Monitoring Dashboard
Parcel Tracker
Threshold Dashboard
Transportation Tracker
This indicator measures the concentration of nitrogen 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 nitrogen as it travels, enters the nearest storm drain or stream, and ultimately ends up in Lake Tahoe. Pollutants, including nitrogen, 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 nitrogen from surface runoff. The Lake Tahoe TMDL is the Region’s science-based strategy to reduce pollutant loading to Lake Tahoe and restore the historic clarity of the lake.
- Status
- Insufficient Data to Determine Status or No Target Established
- Trend
- Rapid Improvement
- Confidence
- Moderate
- In 2019, TMDL implementors (CalTrans, CLST, Douglas, El Dorado, NDOT, Placer, Washoe) exceeded their load reduction targets and collectively reduced average nitrogen concentrations in surface runoff by 11.7 percent from baseline levels.
- The targets for surface runoff and groundwater discharge articulated in water quality threshold standards (standards 19-32) were designed to reduce pollutant load and improve ambient water quality. 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 recommends that 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.
Lake Clarity Tracker Projects
- Increased nutrient input is primarily responsible for the cultural eutrophication of Lake Tahoe. The challenge is not unique to Lake Tahoe, increasing nutrient (nitrogen and phosphorus) inputs are considered a main cause of increasing primary productivity in temperate lakes globally (Conley et al., 2009). Lake Tahoe is an ultraoligotrophic lake and management goals include maintaining this status due to its historic, cultural, economic, and aesthetic value. Increased organic particles in the lake are also estimated to be responsible for approximately 25 percent of the loss of clarity (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 63 metric tons of total nitrogen a year to the lake, 16 percent of the annual total (Lahontan and NDEP, 2010a).
Lake Clarity Indicators
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Nitrogen Load Reduction
TMDL urban implementing partners have acheived greater than total nitrogen targets required by permits and agreements.
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.
- Because pollutant load is more important than concentration, the sampling design of the Regional Stormwater Monitoring Program supports the Lake Tahoe TMDL load estimation and load reduction tracking approach. Assessment of the status of the threshold standard is not possible with the available information.
- The Lake Clarity Crediting Program was designed to assess TMDL implementation by tracking pollutant load reduction in urbanized areas. Annual loading is driven by weather and hydrology. To evaluate the effectiveness of management and track load changes over time, the Crediting Program modeling tools provide average annual load estimates that are linked with on-the-ground field conditions to smooth the interannual variation associated with varying weather and hydrological conditions.
In 2019, local government and state highway departments reduced basin-wide nitrogen loading by 11.7% from 2004 baseline levels. Given load is a function of flow and concentration, the average annual load reduction estimated by the TMDL can be interpreted as an 11.7% reduction in the average annual stormwater runoff concentration.
- Moderate
- Moderate
- Moderate
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