Algae Warning Issued in Humboldt and Mendocino Counties

Potentially toxic Anabaena growing in a bed of decaying Cladophora in South Fork Eel River side water in Phillipsville on Aug. 1.
Potentially toxic Anabaena growing in a bed of decaying Cladophora in South Fork Eel River side water in Phillipsville on Aug. 1.

Press release from Humboldt County Department of Health and Human Services:

Typically, algae warnings come out between late July and early August, coinciding with low flows and sustained high temperatures in the inland areas. These factors, coupled with drought conditions, may cause blue-green algae to grow earlier than usual.

Human activities can have a big effect on nutrient and water flows in rivers, streams and lakes. Nutrients found in fertilizers, animal waste and human waste can stimulate blooms. Excessive water diversions can also increase water temperatures and reduce flows. People can take the following measures to prevent algal blooms in our waters:

  • Be conservative with the use of water, fertilizers and pesticides on your lawn, garden or agricultural operation.
  • Avoid nutrient runoff by recycling any “spent” soil that has been used for intensive growing by tilling it back into gardens, or protect it from rainfall.
  • Create shade and filter out nutrients by planting or maintaining native plants around river banks.
  • Inspect and pump out septic systems every three to four years.
  • Prevent surface water runoff from agricultural and livestock areas.
  • Prevent erosion around construction and logging operations.

Blue-green algae can be present in any fresh water body. It looks like dark green, blue-green, orange or brown scum, foam or mats floating on the water. Most blue-green algae does not affect animals or people, however, warm water and abundant nutrients can cause blue-green algae to grow more rapidly than usual. These floating algal masses or “blooms” can produce natural toxins that are very potent. Dogs and children are most likely to be affected because of their smaller body size and tendency to stay in the water for longer periods of time.

While the presence of blue-green algae toxins has been previously confirmed in some water bodies within Humboldt and Mendocino counties including the South Fork Eel, Van Duzen and Trinity rivers, it is difficult to test and monitor the many miles of our local rivers with conditions that may vary. Most algal blooms in California contain harmless green algae, but it is important to stay safe and avoid contact.

To learn more about the occurrence and appearance of blue-green algae on the South Fork Eel River, see the Eel River Recovery Project Toxic Algae Factsheet .

DHHS, Mendocino EH, and NCRWQCB officials recommend the following guidelines for recreational users of freshwater areas:

Keep children, pets and livestock from swimming in or drinking water containing algal scums or mats.
Adults should also avoid wading and swimming in water containing algal blooms. Try not to swallow or inhale water spray in an algal bloom area.
If no algal scums or mats are visible, you should still carefully watch young children and warn them not to swallow any water.
Fish should be consumed only after removing the guts and liver and rinsing fillets in tap water.
Never drink, cook with or wash dishes with water from rivers, streams or lakes.
Get medical attention immediately if you think that you, your pet, or livestock might have been poisoned by blue-green algae toxins. Be sure to tell the doctor about possible contact with blue-green algae.
Join or support one of the many watershed and river organizations.
For more information or to report unusual blooms or conditions occurring within Humboldt County, contact Humboldt County Environmental Health at 707-445-6215 or 1-800-963-9241. Photos of suspected blooms can be emailed to

To report unusual blooms or conditions occurring within Mendocino County contact 707-234-6625.

More details about blue-green algae are available at the California Department of Public Health’s website.


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