• lake and reservoir water quality;
  • total maximum daily load;
  • lake restoration;
  • lake clarity model;
  • lake management;
  • climate change

[1] A quasi-two-dimensional lake clarity model (LCM) was developed to better understand the impacts of pollutant load on lake/reservoir water quality and to provide guidelines for lake/reservoir management and restoration. Though the LCM can be applied to any lake, the model was calibrated and validated using the available detailed data set of Lake Tahoe. The estimated and measured annual average Secchi depths demonstrate a very high degree of agreement with relative error of less than 6%. The sensitivity analysis was performed on those parameter(s)/load(s) found to have a large effect on lake clarity. Loading and settling rates of fine inorganic particles (<16 μm in diameter) were found to have the largest effect on lake clarity. Atmospheric load adds inorganic particles and nutrients to the surface layer, thus directly affecting lake clarity. Since maximum chlorophyll a concentration is observed in deep water (approximately 50 m below surface) during spring and summer, the Secchi depth (approximately 20–22 m) was found to be relatively insensitive to the change in chlorophyll a concentration in this deep layer. The model was used to simulate different load reduction scenarios to help answer questions such as how much load reduction is needed to restore Lake Tahoe's historic clarity of approximately 30 m and how quickly this change can be achieved? The LCM provides a firsthand scientific solution to managers that the historic clarity can be achieved if total reduction of nutrients and inorganic particles loads will be reduced to approximately 55% from all sources or approximately 75% from urban sources.