Toxic Algal Blooms Found in California Lakes and Reservoirs, Officials Warn of Risks to People and Pets

The California Department of Water Resources (DWR) is urging residents and visitors to stay cautious around lakes and reservoirs where toxic algal blooms may be present.

Blue-green algae, also known as cyanobacteria, naturally occur in California waterways and are a normal part of aquatic ecosystems, according to the DWR. These algae can rapidly multiply and form blooms under normal conditions, such as warm temperatures, the presence of some nutrients, and calm water.

Although not all algae are dangerous, if the cyanobacteria become a Harmful Algal Bloom (HAB), it can produce toxins, resulting in health concerns for humans and pets and restrictions on water-based recreational activities, according to the DWR.

Bloom conditions can change fast, with the movement caused by wind and waves concentrating the bloom into different regions of a water body.

Algal blooms can differ in characteristics from textures to colors.

DWR, which operates and maintains the State Water Project’s network of reservoirs and waterways in California, regularly monitors and tests water quality for algal toxins.

When tests confirm the presence of a harmful algal bloom, the agency issues advisories on site, at the reservoir, and online. The health advisory and recommended precautions are maintained until test results show that HAB is declining and toxin levels are below concern for two weeks.

The following lakes and reservoirs are under advisories:

• Lake Del Valle: Caution advisory
• San Luis Reservoir: Warning advisory
• Pyramid Lake: Caution advisory
• Castaic Lake: Caution advisory
• Castaic Lagoon: Caution advisory
• Silverwood Lake: Caution advisory

DWR has advised recreational users, including swimmers and boaters, to avoid algae. Noting that some algae are harmless, DWR said that certain types of algae can produce toxins that can make people and animals sick.

While both toxic and non-toxic blooms can look similar, there are some indicators of a toxic algal bloom.

How to Spot a Harmful Algal Bloom (HAB)?

• Small blue-green, green, white, or white particles in the water
• Streaks in the water that resemble spilled paint
• Mats, scum, or foam at the surface or along the shoreline
• May smell like gasoline, septic, or fishy

Potential Health Risks

Authorities warned that exposure to HAB can cause eye, nose, mouth, or throat irritation, headache, allergic skin rash, mouth ulcers, vomiting, diarrhea, and cold- and flu-like symptoms.

Small children and pet dogs are often most likely to be affected by the toxic blooms, as they often play in the water for longer periods and due to their smaller body size.

Animals are particularly susceptible to the toxins as they also drink the water and lick their fur afterward.

In September 2024, two dogs died after swimming in a pool off the Eel River near Fernbridge in Humboldt County.

Safety Recommendations

• Avoid contact with algae and scum in the water and on shore.
• Watch children and pets closely.
• Ensure pets stay away from the water, do not drink the water, and do not eat scum and algae on the shore.
• Wash yourself, your family, and pets thoroughly with clean water after playing in the water.
• If you are fishing, do not consume the guts. Clean fillets thoroughly with tap or bottled water before cooking them.
• Avoid consuming shellfish from affected areas.