LAKEPORT, CA., 9/3/26 — The State Water Resources Control Board has recommended a danger advisory, its highest warning for toxic algae, at Lake Pillsbury after sampling ahead of the Labor Day weekend found toxin levels above the state’s danger threshold, Lake County Health Services said in a notice. The reservoir sits behind PG&E’s Scott Dam on the upper Eel River and is the largest lake in the Mendocino National Forest. It has 31 miles of shoreline, according to the U.S. Forest Service.
The county said State Water Board staff collected a water sample at Lake Pillsbury Resort on Aug. 24 as part of a pre-holiday assessment and found high concentrations of cyanobacteria, the microscopic bacteria also called blue-green algae, and 23.84 micrograms per liter of microcystins and nodularins, the toxins the bacteria produce.
State staff saw visible blooms at the resort and at Pine Point, and satellite imagery showed cyanobacteria at multiple locations on the reservoir, the county said. People and pets were advised to stay out of the water until further notice, and the county said danger signs are expected to be posted around the lake.
Lake County said the State Water Board was notifying drinking water officials because a drinking water intake sits near the sampling location, and that boiling water does not remove the toxins and may even concentrate them.
The State Water Board’s advisory data, posted on the state’s open data portal and pulled Wednesday, show a caution advisory was recommended for the area around the resort Aug. 25 based on visual monitoring alone, and the danger advisory Monday, citing visual observations and high toxin concentrations. A separate state case file for the reservoir was opened Aug. 3 on a public report of a bloom near the Pine Point and Sunset boat launches, and notes that a suspected illness was reported. The state recommended a caution advisory for that area Aug. 27 based on visual observations and toxins.
A note in that file attributed to PG&E says camp hosts had sent photos of the reservoir showing dead fish. Because of observed animal deaths and satellite imagery showing elevated cyanobacteria, PG&E asked that all of its Lake Pillsbury recreation areas be posted with advisory signs, the note says. PG&E filed an application with the Federal Energy Regulatory Commission in July 2025 to decommission the Potter Valley Project and eventually remove Scott Dam.
A danger advisory is the highest of the state’s three tiers for planktonic blooms, the kind that float in the water. Under the posting guidance the State Water Board uses, caution signs go up at 0.8 micrograms per liter of total microcystins, or when scum or discoloration is visible or an illness is suspected. Warning signs go up at 6 micrograms per liter and danger signs at 20.
Mendocino County rivers also at risk
The North Coast Regional Water Quality Control Board said Aug. 27 that visual assessments and testing had confirmed potentially toxic algal mats in some of the rivers it monitors with tribes and counties. The rivers it monitors include the Eel, South Fork Eel, Navarro, Russian and East Fork Russian. The mats grow on the river bottom but can break loose and float or wash up on shore, where children and dogs can pick them up, the regional board said. The mats produce toxins that can make people and animals sick and pose a particular risk to children and dogs, the board said, and it is working with tribal and county partners to post Toxic Algae Alert signs where mats have been found.
In Mendocino County, the state’s data show algal-mat alert signs recommended for the Navarro River at Hendy Woods State Park and for the East Fork Russian River at an access point near Elledge Ranch Road. The Navarro advisory dates to July 27, when a bloom on the river bottom was observed at the bridge near the park and a suspected illness was reported. The state’s data list that case as under investigation, and a second entry for the park on Aug. 13 notes detached mats were observed. The East Fork Russian advisory was recommended Aug. 18 based on visual observations and toxins. The regional board said it also works with partners to monitor blooms that float in the water at Lake Mendocino and Lake Pillsbury. The state’s posted data listed no advisory for Lake Mendocino as of Wednesday.
Clear Lake not an option for Labor Day swims
Clear Lake carries danger advisories at Clearlake Keys and Lily Cove and caution advisories at some 20 other monitoring sites around the lake, according to routine monitoring results the state updated Tuesday. A caution advisory recommended Aug. 21 for a stretch of Putah Creek in Lake County, upstream of where it meets Dry Creek, also notes a suspected illness was reported.
Signs of blue-green algae poisoning include eye irritation, skin rashes, mouth ulcers, vomiting, diarrhea and cold and flu-like symptoms, and children and adults can suffer serious injury to the liver, kidney and nervous system if they swallow affected water, according to the California Department of Public Health. Dogs are at particular risk because they drink the water and can ingest algae while grooming, and in some cases exposure can lead to serious illness within minutes to hours, the State Water Board said in a statement it issued before Memorial Day. That serious illness can lead to death within hours.
The State Water Board recommends avoiding water that is bright green, blue-green or reddish, has paint-like scum or streaks, or has thick mats along the shoreline.
“People often have just a few seconds to decide whether the water is safe,” Marisa Van Dyke, a senior environmental scientist with the State Water Board’s freshwater harmful algal bloom program, said in the statement. “If the water looks unusual — discolored, cloudy, or covered in scum — the safest choice is to stay out.”
The regional board said anyone who develops signs of illness after contact with a bloom or mat should contact a health care provider or the California Poison Control System and mention possible exposure, and dog owners should seek veterinary care immediately if a dog shows signs of illness.
Current advisories are mapped at mywaterquality.ca.gov/habs. Blooms and suspected illnesses can be reported to the state’s harmful algal bloom hotline at 844-729-6466 or to CyanoHAB.Reports@waterboards.ca.gov.