San Luis Reservoir and O'Neill Forebay Open with Algal Advisories: Public Urged to Avoid Water Contact


Individuals preparing for some early morning fishing on O'Neill Forebay in Merced County, California.

Sunset at O'Neill Forebay in Merced County.

SACRAMENTO, Calif. –State Water Project facilities at San Luis Reservoir are open after closures due to nearby fires. However, a harmful algal bloom warning advisory has been issued at O’Neill Forebay and caution advisory is in effect at San Luis Reservoir in Merced County.  The Department of Water Resources (DWR) urges the public to avoid physical contact with the water at these two locations until further notice due to blue-green algae.


Boating is allowed, but swimming and other water-contact recreation and sporting activities are not considered safe under the warning advisory due to potential adverse health effects.


Advisories are based on the potential health risks from algae. Exposure to toxic blue-green algae, also known as cyanobacteria, can cause eye irritation, allergic skin rash, mouth ulcers, vomiting, diarrhea, and cold- and flu-like symptoms. Pets can be especially susceptible because they tend to drink while in the water and lick their fur afterwards.


Bloom conditions can change rapidly, and wind and waves may move or concentrate the bloom into different regions of the reservoir. The algal bloom can accumulate into mats, and scum, form foam at the surface and along the shoreline, and range in color from blue, green, white, or brown.


State guidelines on cyanobacteria and harmful algal blooms recommend the following precautions be taken in waters impacted by blue-green algae:


  • Take care that pets and livestock do not drink the water, swim through algal blooms, scum, or mats, or lick their fur after going in the water. Rinse pets in clean water to remove algae from fur.
  • Avoid wading, swimming, or jet or water skiing in water containing algae blooms, scum, or mats.
  • Do not drink, cook, or wash dishes with untreated surface water from these areas under any circumstances. Common water purification techniques such as camping filters, tablets, and boiling do not remove toxins.
  • Do not eat mussels or other bivalves collected from these areas. Limit or avoid eating fish. If fish are consumed, remove the guts and liver and rinse filets in clean drinking water. No fish should be consumed under a danger advisory.
  • Get medical treatment immediately if you think that you, a family member, friend, pet, or livestock might have been poisoned by blue-green algae toxins. Be sure to alert medical professionals to the possible contact with blue-green algae. Also, make sure to contact the local county public health department.

For more information can be found on our algal blooms and advisories web page.

Visitors are advised to follow CA Department of Public Health guidelines, as well as Merced County guidelines, to ensure the health and safety of employees and visitors. For California Parks updates on San Luis Reservoir, visit the State Parks website. For more information about how DWR is continuing critical operations of State Water Project facilities during the COVID-19 health emergency, please visit the DWR website.





Maggie Macias, Information Officer, Public Affairs, Department of Water Resources

(916) 653-8743 |