San Luis Reservoir Algal Bloom Increases to Warning Advisory


An aerial drone view of the Basalt Boat Ramp showing part of an algae bloom in the San Luis Reservoir. DWR/2019

An aerial drone view of the Basalt Boat Ramp showing part of an algae bloom in the San Luis Reservoir. DWR/2019

SACRAMENTO, Calif. – Today, the Department of Water Resources (DWR) is urging people to avoid physical contact with the water at San Luis Reservoir in Merced County 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. O’Neill Forebay remains free from algal bloom advisories. Warning signs are posted at the Basalt Boat Launch and Dinosaur Point Boat Launch.


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, visit the Algal Bloom web page.

Visitors are advised to follow California 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, go to 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.