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.

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