A water quality approach under the Clean Water Act helped established the Total Maximum Daily Load (TMDL) program for impaired bodies of water. A description of the state TMDL program can be found on the State Water Resources Control Board website.
"The TMDL calculates the maximum amount of a pollutant allowed to enter a waterbody so that the waterbody will meet and continue to meet water quality standards for that particular pollutant and allocates that load to point sources (Wasteload Allocation or WLA), and nonpoint sources (Load Allocation or LA), which include both anthropogenic and natural background sources of the pollutant." - United States EPA (2012).
The history of the Clear Lake Nutrient TMDL and downloadable PDF reports/updates can be found on the Central Valley Regional Water Quality Control Board's Clear Lake Nutrient TMDL webpage.