Harmful blue-green algae can thrive in lakes, rivers and ponds

Blue-green algae, or cyanobacteria, are found naturally in lakes, rivers, ponds, and other surface waters. During summer and fall, blue-green algae is more abundant due to a combination of warm water temperature, high nutrient levels, and abundant sunlight.

High levels of blue-green algae, or cyanobacteria, increase the likelihood of producing harmful algal toxins, or cyanotoxins, in bodies of water. These algal toxins can threaten human health through insufficiently treated drinking water or eating contaminated fish. They can also be dangerous to people, pets, and livestock who have contact with the water. 

Recently, the California Dept. of Water Resources urged people to avoid physical contact with water in San Luis Reservoir in Merced County because of the presence of toxins produced by blue-green algae.

Accordingly, Valley Water’s reservoirs can experience cyanotoxin above the California Danger Advisory level for swimming and other recreation in and around the water, mainly near shoreline and boat launch areas.

On May 25, 2022, aerial view shows algae at O’Neill Forebay, a joint Federal-State facility and part of the State Water Project in Merced County, California. Algal blooms may contain toxins that can be harmful to people and pets. Florence Low / California Department of Water Resources.

While recreational activities such as boating are allowed in some of Valley Water’s reservoirs, swimming and/or wading is prohibited in all Valley Water’s reservoirs and percolation ponds. This restriction applies to people and pets. As such, Valley Water does not have a program to routinely monitor for algal toxins at reservoirs used for recreational purposes, but algal toxins are monitored regularly at Calero Reservoir supplying Valley Water’s water treatment plants.

All recreational activities at Valley Water’s reservoirs are managed by the Santa Clara County Parks and Recreation Department, and the two agencies are presently working together on informational signage regarding algal blooms that will be posted at Valley Water’s reservoirs. Valley Water is also diligently working with local agencies, regulators, and treated water retailers to educate the public about algal bloom issues.  

Valley Water does have a comprehensive algal toxin monitoring program aimed at protecting treated drinking water and public health. Our efforts include providing proactive water quality monitoring guidelines, sampling techniques, treatment strategies, and communication guidance in the event of a harmful algal bloom in any of our treatment plants, imported water sources, and the reservoirs that feed into our drinking water supplies.

For a list of frequently asked questions and answers related to algal toxins, please visit the Valley Water website. The California Dept. of Water Resources has a webpage with information about algal blooms.

For a list of available recreational activities at reservoirs in Santa Clara County, please visit the County Parks and Recreation website at https://www.sccgov.org/sites/parks/Pages/Welcome-to-Santa-Clara-County-Parks.aspx

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