California’s waterways offer plenty of recreation opportunities this summer, but it’s important to also remember that with warm water comes the potential for toxic algal blooms.

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.
Information about caution, warning, and danger advisory signs. More information (also in Spanish) is available at the HABs portal.
Reservoir | Status |
---|---|
Lake Oroville | No Advisory |
Lake Del Valle |
Caution Advisory |
Clifton Court Forebay | No Advisory |
San Luis Reservoir | No Advisory |
O'Neill Forebay | No Advisory |
Quail Lake | No Advisory |
Pyramid Lake | No Advisory |
Castaic Lake | No Advisory |
Silverwood Lake | No Advisory |
Lake Perris |
Warning Advisory at Moreno Beach |
Los Banos Creek Reservoir | No Advisory |
Maggie Macias, Information Officer, Public Affairs, Department of Water Resources
(916) 820-7662 | maggie.macias@water.ca.gov
California’s waterways offer plenty of recreation opportunities this summer, but it’s important to also remember that with warm water comes the potential for toxic algal blooms.
Lake Oroville Community Update for May 5, 2023.
The State Water Project’s (SWP) reservoirs located throughout California kick off this summer with high water levels. When recreating at SWP facilities, you should always keep water safety as a top priority and stay alert of cold, fast, and fluctuating water conditions.
Lake Oroville Community Update for April 28, 2023.
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).