Over in Tilden Park, Lake Anza looks like it has a big algae bloom, but it's fine to swim in. The algae there are not releasing toxins.
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But at Lake Temescal, it's a different story. The water looks clear, but it's tested positive now for toxins.
About 100 kids at summer day camp were disappointed that their summer afternoon swimming plans at the beach are cancelled indefinitely.
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"Everybody was all excited yesterday. They were in the water yesterday when the Rangers came over and put up tape and said we wouldn't be swimming for the rest of the summer," explained Shayna Cureton, the summer camp director
"When we found toxins above certain limits, we decided to close the beach to protect people," said Hal MacLean, a water management supervisor.
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Like quarry lakes in Fremont, Lake Temescal is off limits. Toxins in the warm stagnant water can cause gastrointestinal illness in people. If dogs swallow the contaminated water, they can die.
"You cannot let them go in the water. You cannot let them drink the water it's a real dangerous situation," Sonya Ginsburg, from Orinda.
Ducks still paddled in the water and fish splashed around. On the shore there was this eerie band of yellow warning tape circling the abandoned shoreline.
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"It's mother nature I guess. I don't know if there's anything they can do to alleviate it or not," said Ed Cranston, Oakland.
But East Bay Regional Parks District is going to try. "The end of this week or early next week, we might be using a product to destroy the blue green algae cells," explained MacLean. "It's like hydrogen peroxide."
If it works, the beach could reopen in a few weeks. Despite the rains and runoff, the algae reappeared. If the bloom continues it could be closed all summer, just like it was last year.