photograph of algal blooms

Nonpoint Source (NPS) Program

To request a copy of documents listed on this page, please contact Karen Atkins at

Cyanobacteria and Harmful Algal Blooms (HABs) in the Central Valley


Cyanobacteria (commonly known as “blue‐green algae”) are small, photosynthetic bacteria that live in our freshwater, estuarine, and marine environments. These organisms have been living on earth for billions of years and are natural components of the aquatic ecosystem. They produce oxygen and serve as a food source for the lower food web.

photograph of Cyanobacteria Dolichospermum sp., Clear Lake 2017
Cyanobacteria Dolichospermum sp., Clear Lake 2017

Cyanobacteria can also be a public health nuisance. They can produce taste and odor compounds which affect the quality of drinking water and can multiply rapidly forming unsightly blooms in our local water bodies. Some cyanobacteria can also produce harmful compounds called cyanotoxins. When blooms produce toxins that pose a risk to humans, animals and the environment, they are known as cyanobacterial harmful algal blooms (HABs). Cyanotoxins pose risks to the health and safety of people and pets, drinking water, and people recreating in waterbodies affected by blooms. These toxins can affect our liver, respiratory and nervous systems, and accumulate in fish and shellfish to levels posing threats to people and wildlife. Human health effects from cyanotoxins include rashes, flu‐like symptoms, vomiting, and seizures.

Increased inputs of nutrients, like nitrogen and phosphorus, promote cyanobacterial growth and can lead to increased occurrences of HABs. Other favorable conditions that contribute to HABs include low water flows, stagnant and stratified waters, and increased intensity and duration of light and high temperatures.

What is the Central Valley Water Board doing about HABs?

Many of the Central Valley Water Board’s regulatory programs work to reduce nutrient inputs into the Region’s waterbodies. In addition to these programs, the Board participates in the state’s efforts to address HABs across California. The state developed a workgroup called the California Cyanobacteria and Harmful Algal Bloom (CCHAB) Network. The CCHAB Network includes federal, state, and local agencies, tribes, academia, and non‐governmental organizations working to develop a comprehensive coordinated program to address the causes and impacts of HABs in the state. As part of the coordinated program, the State Water Resources Control Board’s Surface Water Ambient Monitoring Program (SWAMP) developed the Freshwater HAB Program. The Central Valley Water Board participates in the statewide Freshwater HAB effort by:

  • Collecting information on blooms
  • Sampling and analyzing HABs
  • Providing information on blooms to local waterbody managers and health officers
  • Conducting outreach and education to the general public
  • Collaborating with academia and interested stakeholders to better understand the causes of HABs.
For more information on the statewide HAB program, visit the California HABs Portal.

HAB Evaluation

The presence of cyanobacteria has increased over the last few years in the Central Valley. CyanoHABs have been observed in several lakes and reservoirs, including the Delta. This has the potential to impact important drinking water supplies and popular water contact recreation areas. The increasing trend of blooms in waterbodies within the Central Valley highlighted the need for an assessment to determine bloom conditions and causes, and identify mitigation strategies to reduce the bloom frequency, extent and duration.

photograph of Cyanobacteria bloom at Lake Britton, 2017
Cyanobacteria bloom at Lake Britton, 2017

In response to this HAB increase, the Central Valley Water Board is undertaking a HAB Evaluation project to identify waterbodies within the Central Valley that are prone to HABs and prioritize them for future assessments. As part of the project, two waterbodies (Lake Britton and Clear Lake) were chosen for pilot assessments on the potential causes of blooms, and to identify potential mitigation actions that could be taken to reduce blooms from occurring. In addition, outreach and education materials are being developed to inform the general public on the causes of HABs and what actions they can take to help reduce them.

Work Products from this HAB Evaluation include:

Main Navigation

Contact Us