Lake Anza closed to swimming due to toxic algae

Toxic algae can be highly visible, or not visible at all. In the photo above, algae can be seen at Lake Anza on Sept. 17. Photo: Sunshine Townsend/East Bay Regional Park District
Toxic algae can be highly visible, or not visible at all. In the photo above, algae can be seen at Lake Anza on Sept. 17. Photo: Sunshine Townsend/East Bay Regional Park District

The East Bay Regional Park District has just closed Lake Anza, in Tilden Regional Park, to swimming due to high levels of toxic algae, which can be fatal for dogs.

Park District spokeswoman Carolyn Jones said signs are going up now around the lake to alert visitors about the closure.

“Park District officials are very sorry to have to close Lake Anza,” she said. “It’s a beautiful time of year and a popular place to go swimming, but public health is the most important thing.”

Jones said park staffers test all the district’s lakes regularly, and discovered Thursday that toxic blue-green algae levels at Lake Anza were too high. They will now send samples to the EPA for further testing. Results for those tests should be in next week.


Toxic algae can be highly visible, or not visible at all. In the photo above, algae can be seen at Lake Anza on Sept. 17. Photo: Sunshine Townsend/East Bay Regional Park District
Algae at Lake Anza on Sept. 17. Photo: Sunshine Townsend/EBRPD

There is no indication regarding when the lake might reopen to swimmers.

Jones said dog owners should be particularly careful around the lake. Three dogs died earlier this year after drinking from water at Lake Chabot, where toxic algae has been present for more than a year.

Currently, the algae at Lake Anza is “really visible,” said Jones: neon green near the mouth of the creek.

“It looks like someone spilled paint on the water,” she said.

Jones cautioned, however, that this isn’t always the case. Not being able to see the algae does not mean conditions are safe.

Humans can experience skin irritation and gastrointestinal symptoms when exposed to the algae, which has been a significant problem due to the ongoing California drought.

Warmer-than-usual water related to the drought has caused outbreaks in numerous places in California, including the Russian River, Jones said.


According to the Park District, “Blue-green algae are natural organisms that are present in most lakes. Certain conditions – low water levels, limited water circulation, increased heat and light, among other factors – can cause the algae to bloom and, in some cases, release toxins. The most common toxins in the algae are Anatoxin-A, a neurotoxin, and Microcystin, which affects the liver. Scientists do not know what causes the algae to become toxic.”

Earlier this year, the Park District closed Quarry Lakes in Fremont, Lake Chabot in Castro Valley and Lake Temescal in Oakland for swimming due to the presence of toxic algae.

The Park District used chemicals to treat the algae in Lake Temescal, which brought down the levels but not enough. The agency has not yet decided whether to treat Quarry Lakes or Chabot, because the treatment is expensive and adds chemicals into the water.

Read more about toxic blue-green algae on the Park District’s website.

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