MENDOCINO CO., 7/30/26 — Three dogs went into the South Fork Eel at Standish-Hickey State Recreation Area, outside Leggett, in the summer of 2002. Two started seizing within five to 10 minutes, and both were dead within 15 minutes.
Mendocino County environmental health officials sampled the water. A state animal-health lab in Davis found anatoxin-a — a nerve toxin made by algae — in the dogs’ stomachs, and concluded that was most likely what killed them. Two years later, another dog convulsed and died after swimming near Piercy, nine miles north of Leggett.
This month the state put two toxic algae advisories on the Navarro River, at another state park where people swim. They didn’t report toxin results. Turns out they don’t have to.
It’s not only the Navarro. Benthic mats are “responsible for several dog deaths in the Eel, South Fork Eel, and Russian Rivers,” the North Coast Regional Water Quality Control Board reported in 2022, counting 18 suspected or documented dog deaths in the region between 2001 and 2021. Every one came from a dog swallowing mat material, and anatoxins were implicated in all 18. On the Russian River, the board kept monitoring the site of a 2015 death for four years afterward.
Two advisories at a swimming hole
The state tracks blooms on a public map. Behind each advisory is a bloom report — a filed account of what somebody saw, where, and what the state recommended about it. Anyone can read them.
The first is dated July 16, at the park. The second is dated Monday, at the bridge just downstream. A member of the public filed the second one. On July 22 that person saw a mat of algae bigger than a football field, less than 10 feet from shore, and reported it to the state. Asked who else they had told, the person wrote that they didn’t know who to call, and that they didn’t think swimming there was safe.
The report from the bridge downstream mentions something the park report doesn’t: a suspected illness, reported and under investigation.
The state won’t say whether that illness is a person or an animal.
How to tell what you’re looking at
The algae behind these advisories doesn’t have to turn the water green. It grows in mats on the riverbed, stuck to rock and sand, and the water above a mat can look clear. Mats break loose, drift, and strand on the bank, where they dry into musty clumps.
Dogs get into them.
Anatoxin-a moves fast. Keith Bouma-Gregson, who studies algae for the U.S. Geological Survey’s California Water Science Center, puts it at “minutes to hours.” The dog is sometimes still at the water when it goes down, which is how owners know to blame the algae.
Other toxins take hours to days, he said: “You might be back at home, and then your dog’s suddenly sick, and you’re like, well, what could have happened?”
That speed also limits what a vet can do. “If a dose is large enough,” he said, “there’s not that much that can be done given how fast those toxins can act.”
Fiber good, slime bad
Sarah Ryan has run the Big Valley Band of Pomo Indians’ cyanotoxin monitoring program on Clear Lake since 2014. Asked how a person tells dangerous algae from the harmless kind, the tribe’s environmental director described a test that works with a stick. She was talking about floating material on a lake rather than riverbed mats.
Rivers and lakes are different problems. In a river the algae grabs onto rock, and the kind that makes the toxin favors the fast water. “Some of the algae that produces the toxins tends to grow, be found more in the riffles,” Bouma-Gregson said, and from there it detaches, drifts downstream and washes up on the bank. In a lake — Clear Lake, Lake Mendocino — the algae and its toxin are suspended in the water itself, which is why those blooms turn it green or leave scum on top.
“The way you can tell is if you put a stick in it … you lift it up, and if it hangs on the stick and it’s fibrous, then it’s green algae,” Ryan said. “But if you put a stick in it and you lift it up and it plops off and it’s slimy and then it just falls out, that’s cyanobacteria. And that’s the stuff that can be toxic.”
Green algae and cyanobacteria aren’t the same thing. The state’s own primer calls cyanobacteria “blue-green algae” and lists it separately from algae, and says both “have been around for billions of years and are natural components of ecosystems.”
Green algae is the harmless one. It’s what fish eat, Ryan said, and part of the food web: “There’s no problem really with green algae.”
Cyanobacteria is the one that blooms and makes the toxins.
Telling them apart by eye only goes so far. A single mat can hold many species at once, which the state says makes it “difficult to visually determine if toxin-producing species are present in a mat.” Mats come in “bright green, dark green, blue-green, orange, brown, maroon and more.” The state’s advice: “When in doubt, avoid touching algal mats.”
The toxin depends on the water. In rivers it’s anatoxin-a, which attacks the nervous system. In Clear Lake it’s mostly microcystin, which attacks the liver.
Nobody is required to test the water
Neither Navarro bloom report has lab data attached. The July 27 one says so outright: “Water testing was not conducted to measure toxicity of bloom.”
Asked whether testing is planned at either site and who would order it, the State Water Resources Control Board punted. It sent three links: its own program page, its own reports to the Legislature, and the response guidance it says it follows. Then it added a line of its own.
“Just to be clear, all guidance for response to hazardous algae blooms (HABs) is voluntary.” Voluntary covers the whole chain. Testing the water. Putting up a sign. Closing a swimming hole. The state writes the guidance and the agency that manages the site decides whether to follow it. Reporting a bloom is voluntary, too. Nothing goes on the state’s map unless a person sends it in.
What the state won’t say
Determining whether algae caused the illness is up to a panel of state agencies dauntingly called the Interagency HAB-related Illness Workgroup. It weighs interviews, environmental information and any medical or veterinary records it can get. A determination “can take weeks to months.”
The Voice asked the water board the simplest version of what happened near the bridge on the Navarro: a person or an animal?
“We don’t share details of the illness,” the board said, in written answers from staff relayed by spokesman Blair Robertson. It pointed to a federal database where past cases can be looked up.
California publishes those numbers eventually. Its illness tracking table sorts cases by county and by type: human only, human and animal, animal only. Through 2025, it lists Mendocino County at two human illnesses and zero animal, and Lake County, next door, at 16 human and 11 animal. The 2026 season isn’t included in the data yet.
If you’re on the river
Look at the bottom, not the surface. Clear water over dark green or brown mats on the rocks is the thing to avoid, and so is any dried clump on the bank.
Keep dogs out where mats are visible or stranded, and away from scum and washed-up piles. The danger is mostly in swallowing it — a dog that comes out with algae in its fur will lick it off. Rinsing the dog right away, Bouma-Gregson said, keeps it from ingesting the poison.
Don’t let dogs drink from the river. Don’t drink it yourself and don’t cook with it — “boiling water will not remove the toxins,” the state’s guidance says.
If a dog gets into it and then gets sick, get to a vet and say the word algae.
Blooms and illnesses can be reported at 844-729-6466 or cyanoHAB.reports@waterboards.ca.gov.
Both Navarro reports started that way.
It’s a damn shame that in the span of less than 200 years we’ve managed to turn what was once a beautiful paradise into a hellish land of toxins and trash. Great work everyone.