Algal Bloom Season Begins on California's Lakes


Algal scum

Mats, scum, or foam at the surface or along the shoreline can be a sign of Harmful Algal Blooms (HABs).

California’s lakes, rivers, and streams are a great place to cool down as the temperature heats up this summer. But as the warmer weather approaches, so does the start of algal bloom season on the state’s waterways.  

DWR urges swimmers and boaters to take the necessary precautions and stay away from algae. 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.

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

There are multiple factors that can cause algae to grow, such as warm water temperature, calm conditions, and certain nutrients in the water. Blue-green algae (cyanobacteria) naturally occurs in lakes and can grow very rapidly to become an algal bloom.

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
  • Can have an odor 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 to toxic cyanobacteria, can cause eye, nose, mouth or throat irritation, headache, allergic skin rash, mouth ulcers, vomiting, diarrhea, and cold- and flu-like symptoms

How can I be exposed to algal toxins?

  • People can be exposed to the toxins during swimming or other water contact and when they accidentally swallow lake water
  • During waterskiing and jetskiing, the toxins can become airborne and be 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

What precautions should I take if I see a potential HAB?

  • Follow all 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

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