TMDL Projects - Clear Lake Nutrient Control Program and TMDL

To request a copy of documents listed on this page, please contact Jessica Rader at or (916) 464-3199.

Clear Lake is the largest natural freshwater lake in California and one of the oldest lakes in North America. Harmful algal blooms (HABs) have been observed in Clear Lake throughout the 20th century with blooms well documented and described since the early 1970s. In 1986, Clear Lake was added to the Clean Water Act Section 303(d) List of Impaired Water Bodies because of algal blooms that impacted recreational uses. Existing study results and data analyses at the time suggested that controlling phosphorus is the best approach for addressing the HABs. The Central Valley Regional Water Board approved a Basin Plan amendment on 23 June 2006 for the control of nutrients in Clear Lake. The U.S. Environmental Protection Agency subsequently approved the control program for Clear Lake as a TMDL on 21 September 2007. The focus of the control program is reducing nutrients entering Clear Lake by including load allocations for point and nonpoint sources to limit the loads of phosphorus entering the Lake.

Information Regarding the Status of Load Allocations

Through 2019 and 2020, Central Valley Water Board requested additional information and issued Enforcement Orders to responsible parties named in the Clear Lake Nutrient TMDL. The information provided from responsible parties was used to determine whether the parties are in compliance with the TMDL and summarized in the Technical Memorandum released in September 2021.


  • 2016 Enforcement Order Issued to Caltrans
  • Caltrans Response to 2016 Enforcement Order

Clear Lake MS4s (Lake County, Clearlake, Lakeport)

On 12 October 2023, the Central Valley Regional Board adopted Resolution R5-2023-0047, which amended Time Schedule Order R5-2019-1005-001 to revise requirements for Task 4 and extend the Task 5 deadline to September 2028. The amended Time Schedule Order is R5-2019-1005-002.

Irrigated Agriculture

Lake County

U.S. Bureau of Land Management (USBLM)

U.S. Forest Service (USFS)

Blue Ribbon Committee for the Rehabilitation of Clear Lake

The Blue Ribbon Committee for the Rehabilitation of Clear Lake, created by Assembly Bill 707 (Aguiar-Curry, Ch. 842, Statutes of 2017), was formed to make recommendations that will rehabilitate and restore Clear Lake. The Committee consists of 15 members, which includes representatives from tribes, UC Davis, the Central Valley Regional Water Quality Control Board, and members appointed by Lake County with expertise in agriculture, economics, environment and public water supplies. The Central Valley Regional Water Quality Control Board representative is Jennifer LaBay. The alternate representative from the Board is previous Board Member Robert Schneider.

More information regarding the Blue Ribbon Committee for the Rehabilitation of Clear Lake, along with Committee documents, is available on the Committee’s website.

Environmental Drivers Study in Clear Lake

The Southern California Coastal Water Research Project (SCCWRP) was contracted by the State Water Resources Control Board and the Central Valley Regional Water Quality Control Board to lead a two-year study (2020-2022) investigating environmental drivers contributing to cyanobacterial blooms and toxin production in Clear Lake, Lake County. The project consisted of developing and implementing a field study monitoring plan and identifying the specific triggers of cyanotoxin production. A total of thirteen lake-wide surveys were conducted in 2020 and 2021 on Clear Lake to compare the biological and physio-chemical parameters that influence the extent, duration, and microcystin concentrations of cyanobacterial blooms. The results of this study will be used to develop recommended approaches to manage and mitigate cyanobacterial blooms in Clear Lake and identify next steps for the Clear Lake Nutrient TMDL.

The final technical report for the study is available on the SCCWRP website or upon request.

Clear Lake Watershed Activities and Resources

Clear Lake is the subject of many programs and activities that are vital to the health and safety of the watershed. The efforts in Clear Lake are not limited to the Clear Lake Nutrient TMDL Program, and the Tribal-led monitoring program, local and State agencies working on water quality issues, and studies conducted on the lake provide more insight into the condition and complexities of Clear Lake.

Clear Lake Cyanobacteria and Cyanotoxins Monitoring Program

  • The Big Valley Band of Pomo Indians and Elem Indian Colony began a Cyanobacteria and Cyanotoxin Monitoring Program in Clear Lake in 2014 to address the need for regular and active monitoring in the watershed. The program monitors over twenty sites across Clear Lake for toxins produced by cyanobacteria. To find more information regarding the program, the most recent cyanotoxin monitoring data for Clear Lake, and cyanobacteria impacts to Tribal water uses, please visit the Big Valley Band of Pomo Indians Program web page.

County of Lake Water Resources

  • The County of Lake Water Resources is responsible for the preservation, health, and growth of Clear Lake. For more information regarding cyanobacteria, recent harmful algal blooms (HABs), or to report a bloom, please visit the Lake County Clear Lake Cyanobacteria web page.

Clear Lake Drinking Water

Clear Lake Studies

  • Under the Blue Ribbon Committee for the Rehabilitation of Clear Lake researchers from UC Davis Center for Regional Change (CRC) and UC Davis Tahoe Environmental Research Center (TERC) are conducting research to guide the Blue Ribbon Committee in improving the environmental quality and economic outcomes for the communities surrounding Clear Lake in Lake County, California. The TERC study is collecting data to understand processes in the Clear Lake watershed that are negatively impacting the rehabilitation of lake water quality and ecosystem health. This data will inform local and State decision-making. Check the UC Davis Clear Lake and TERC Clear Lake Website for more information on the studies.

Available Documents

  • Clear Lake Nutrient TMDL Update
    The Water Quality Control Plan for the Sacramento River and San Joaquin River Basins was amended for a Program of Implementation for Clear Lake Nutrients. A 5-year update to the Clear Lake Nutrient TMDL has been developed to consider information that has been developed and determine whether the phosphorus load and waste load allocations should continue to be required or if some other control strategy or approach is more appropriate.
    • Comments Received:
      • Essential Public Information Center
      • Big Valley Rancheria
      • Lake County
    • Response to Comments
  • U.S. EPA Approval Letter for the Total Maximum Daily Load (TMDL) to address nutrients in Clear Lake, 21 September 2007
  • Basin Plan Amendment for Control of Nutrients in Clear Lake Adopted by Regional Board on 23 June 2006
  • Amendment to the Water Quality Control Plan for the Sacramento River and San Joaquin River Basins for the Control of Nutrients in Clear Lake, 23 June 2006
  • Total Maximum Daily Load for Nutrients in Clear Lake, Lake County, California, 1 December 2004
    • Technical Report, by Tetra Tech

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