San Luis Reservoir Algal Bloom Increases to Danger Advisory


A drone view of San Luis Reservoir with the storage at 363,587 acre feet, 18 percent capacity, or 38 percent of historical average.

A drone view of San Luis Reservoir with the storage at 363,587 acre feet, 18 percent capacity, or 38 percent of historical average. DWR/August 10, 2021

SACRAMENTO, Calif.Today, the Department of Water Resources (DWR) urges people to avoid physical contact with water at San Luis Reservoir in Merced County until further notice due to the presence of blue-green algae. People should also avoid eating fish or shellfish from the lake.

This week’s lab results show an increase in toxin levels. A danger advisory was put in place today, and remains in effect for the entire lake until further notice. It is advised for people and pets to stay out of the water and avoid contact with algal scum in the water or on shore. Boating is allowed, but swimming and other water-contact recreation and sporting activities are not considered safe. For more information on a danger advisory, go to Harmful Algal Bloom website under Advisory Signs.

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. Keep pets away from the water.

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. 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:

Maggie Macias, Information Officer, Department of Water Resources
(916) 820-7662 |