Stay Away from Harmful Algal Blooms in California Waterways


Algal blooms can be small blue-green, green, white, or brown particles in the water.

Algal blooms can be small blue-green, green, white, or brown particles in the water. DWR/2019

California’s lakes, rivers, and streams are a great place to cool down as the temperatures heat up. However, as the warmer weather approaches, so does algal bloom season on our waterways. 

Blue-green algae (cyanobacteria), which naturally occur in waterbodies, can grow very rapidly into an algal bloom due to factors such as warm water temperature, calm conditions, and certain nutrients in the water.

While some algae are harmless, certain types can produce toxins that can make people and animals sick. There is no way to tell if an algal bloom is toxic just by looking at it, so it’s best for swimmers and boaters to keep away from algal blooms.

The Department of Water Resources regularly tests State Water Project (SWP) reservoir recreation areas for algal toxins. When tests are positive, signs are posted on site at the reservoir and online. Once toxic algae are found, testing continues weekly and algae levels/warnings are not reduced until lab results show the water has been below the current advisory level for two weeks.


What are signs of Harmful Algal Blooms (HABs)?

            Small blue-green, green, white, or brown particles in the water

            Streaks in the water that look like spilled paint

            Mats, scum, or foam at the surface or along the shoreline

            Odors described as gasoline, septic, or fishy


What are the dangers?

Some algal blooms can produce toxins that are harmful to people and pets. Dogs and small children are most likely to be affected by HABs due to their smaller body size and probability to play in the water for longer periods.

Exposure can cause eye, nose, mouth or throat irritation, headache, allergic skin rash, mouth ulcers, vomiting, diarrhea, and cold- and flu-like symptoms. For dogs, symptoms can include vomiting, diarrhea, weakness, difficulty breathing, seizures, or even death.

How can I be exposed to algal toxins?

Exposure to HAB can happen during swimming or other water contact and when individuals accidentally swallow lake water. During waterskiing and jet skiing, the toxins can become airborne and inhaled.

People can also be exposed to toxins by eating shellfish and fish from affected waterbodies.

Dogs or other animals can become ill if they eat scum or mats in the water or on the shore, drink the water, or lick their fur after going into the water.


How can you protect yourself from HAB?

            Follow posted advisories

            Stay away from algae and scum in the water and on shore

            Watch children and pets closely

            Do not let pets and other animals go into the water, drink the water, or eat scum and algal accumulations on the shore

            Do not drink the water or use it for cooking

            Wash yourself, your family, and your pets with clean water after water play

            If you catch fish, throw away guts and clean fillets with tap water or bottled water before cooking

            Avoid eating shellfish from affected areas


More information available at:

DWR’s Algal Blooms website.