Climate change and dangerous waters

Learn how to avoid harmful algal blooms


Fall is a wonderful time for walks along a lake, river, or other waterfront. But, partially due to climate change, it’s also when toxic blue-green algae blooms are most likely to occur in state waters.

Cyanobacteria blooms along the shores of a Bateman Island in the Columbia River.

“Data suggests that the effects of climate change, including warmer temperatures and fewer days without clouds, lead to an increase in harmful algal blooms (HABs),” said Joan Hardy, Ph.D., a DOH expert on HABs. “We are more likely to see more toxic blooms as warm weather and droughts intensify.”

Toxins from blue-green algae blooms, also known as toxic cyanobacteria blooms, can sicken people and animals. Understanding why they happen and how to avoid them is important.

Most blooms occur in September and October. But toxic blooms happen year-round. A combination of warm temperatures, sunlight, and nutrient-rich waters can cause cyanobacteria to reproduce rapidly, or “bloom.” A bloom can cloud a clear lake, pond, or ditch in a few days. The toxins in blooms are especially dangerous to children and can cause severe reactions in adults.

Harmful algal blooms are in the news a lot recently. At least 7 pet dogs died this summer after encountering blooms:

Blooms sickened other dogs, too. Water testing is ongoing in the Columbia River near Tri-Cities to ensure the drinking water supplies remain safe.

Most blooms aren’t toxic, but it’s best to learn how to avoid them.

How to spot a bloom

HABs can look like paint spilled on the water or grass clippings. They can be green, blue-green, reddish or brownish. Blooms can be near the shoreline or cover large or small areas of a lake or other bodies of water. You can’t tell if a bloom is toxic by simply looking at it — it needs to be tested, frequently. Some blooms can test safe one day, then toxic the next. Toxins can also remain present in the water even after a visible bloom dissipates. That’s why water testing is required for two weeks before reopening an area after a toxic bloom.

Water system operators around the state regularly treat water for microorganisms, like cyanobacteria, to keep our drinking water safe.

When in doubt, stay out.

When you see something that looks like a bloom, give it room:

  • Do not swim or waterski in the water.
  • Do not drink the water.
  • Keep children, pets, and livestock out of the water. Toxins clinging to fur can poison pets when licked off. Always rinse pets that have been in a lake or river with a suspected bloom. When outdoors, don’t forget drinking water for your pets to help them avoid drinking from potentially toxic blooms.
  • Hunters need to be cautious because hunting dogs often become sick from entering fall blooms.
  • Clean fish well and discard the guts.
  • Avoid areas of scum while boating.
  • You can get help with bloom problems from the Department of Ecology.
  • Watch for warning signs indicating harmful algal blooms.

Stay safe and help us test the water.

If you see a lake with a potential bloom, report it to your local health department or the State Department of Ecology. You also can learn to test an algae bloom.

The Washington State Toxic Algae website lists lakes that tested higher than what’s considered safe values. Check it to see if your lake or stream is on the list of potentially unsafe bodies of water.

For data on how climate change affects Washington, visit our Washington Tracking Network page. Also, learn what you can do about climate change to help keep Washington healthy.

Enjoy our state’s beautiful waterways. But when it comes to toxic blooms, when in doubt, stay out.

More Information

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