Algal Blooms

A close-up photo taken near the Basalt Boat Ramp showing part of an algae bloom in the San Luis Reservoir.

A close-up photo of a blue-green algae bloom. DWR/2019

The California State Water Project (SWP), which is operated and maintained by the Department of Water Resources, spans 705 miles throughout California and offers a variety of recreational opportunities at several reservoirs. When blue-green algae, also known as cyanobacteria, are present in the waterways, DWR urges all recreational users to exercise caution and always avoid contacting algae. If the blue-green algae become a Harmful Algal Bloom (HAB), water activities will be impacted due to public safety concerns.

Blue-green algae are a natural part of the ecosystem. Algae can “bloom” or grow rapidly under ideal conditions, which include warm water temperature, calm conditions, and certain nutrients in the water. An algal bloom becomes harmful when it produces toxins.

Bloom conditions can change rapidly, and wind and waves may move or concentrate the bloom into different regions of a reservoir. Algal blooms can vary in characteristics from a variety of textures to colors as described below.

DWR monitors and regularly tests the SWP water for algal toxins. When tests are positive confirming a harmful algal bloom is present, advisories are posted on site, at the reservoir, and online. Once toxins are found, testing continues, and the posted health advisory and recommended precautions are not reduced until lab results show the HAB is declining and toxin levels have been below the level of concern for two weeks.

DWR urges recreational users, such as swimmers and boaters, to take the necessary precautions and always stay away from algae. While some algae are harmless, certain types can produce toxins that can make people and animals sick. There is no sure way to tell if an algal bloom is toxic just by looking at it, but there are indicators that a bloom may be harmful.

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
  • Animals can be especially susceptible to the toxins because they tend to drink while in the water and lick their fur afterwards
  • 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 activities and when they accidentally swallow lake water
  • During waterskiing and jetskiing, the toxins can become airborne in the water spray 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
  • Current Reservoir Status

DWR Videos

Advisory Signs

Information about caution, warning, and danger advisory signs. More information (also in Spanish) is available at the HABs portal.


Maggie Macias, Information Officer, Public Affairs, Department of Water Resources

(916) 820-7662

News Updates

Boaters launch from Castaic Lake Boat Launch Ramp at Castaic Lake, a reservoir formed by Castaic Dam on Castaic Creek, in the Sierra Pelona Mountains of northwestern Los Angeles County, California. Photo taken May 11, 2023.

Today, the Department of Water Resources urges people to avoid physical contact with water at Castaic Lake in Los Angeles County until further notice due to the presence of blue-green algae.

An aerial view of Silverwood Lake, a large reservoir in San Bernardino County, California on May 13, 2023.

The Department of Water Resources (DWR) is urging the public to avoid contact with water at Silverwood Lake in San Bernardino County until further notice due to blue-green algae (cyanobacteria).

An aerial view of Pyramid Lake and Dam on May 12, 2023.

Today, the Department of Water Resources urged people to avoid physical contact with water at Pyramid Lake in Los Angeles County until further notice due to the presence of blue-green algae.