Is the lake safe?

Spotting guide

What does toxic algae look like?

Looks like paint or pea soup

Treat any suspicious bloom as toxic

How do you spot a toxic algae bloom?

Toxic blue-green algae often looks like green paint spilled on the water, pea soup, or a scum or foam along the shore. It can also be blue-green, brown or red, or grow as dark mats on rocks and the bottom. You can't tell whether a bloom is toxic by looking, so stay out of anything that looks like this.

  • Paint-like streaks or a thick surface scum
  • Water that looks like pea soup or is unusually discoloured
  • Foam, mats or crusts of dried algae on the shore
Can you tell toxic algae by eye?
No
Common colours
Green, blue-green, brown, red
Where it gathers
Downwind shores and coves

The signs of a toxic algae bloom

Blue-green algae are cyanobacteria, tiny organisms that multiply fast in warm, still, nutrient-rich water. When they bloom they can change how a lake looks within a day. According to the CDC, blooms can appear as foam, scum, mats or paint-like coatings on the surface, and can turn the water green, blue, brown or red.

On the surface

  • Paint-like streaks or swirls of bright green or blue-green.
  • A thick scum that can look like pea soup or green latex paint.
  • Foam or froth along the shore, sometimes with a blue or turquoise tint as the algae die.
  • Small green dots or flecks hanging in the water, like grass clippings stirred into it.

Below the surface and on the shore

  • Cloudy green water where you can't see your feet when standing knee-deep.
  • Dark brown, black or green mats on rocks or the riverbed, common in rivers and creeks.
  • Dried crusts of green or blue-green algae on the shoreline or rocks, which can still be toxic.

Many blooms also smell earthy, musty or rotten, and dead fish can be a sign of one.

Where blooms gather on a lake

Wind pushes surface blooms towards the downwind shore, so scum often piles up in coves, along docks and boat ramps and at swimming beaches on one side of a lake while the other side looks clear. Calm, hot spells after heavy rain are prime time, because runoff carries the nutrients blooms feed on. A lake can look clean in the morning and have a scum by afternoon, so check again each time you visit.

Common lookalikes that are usually harmless

Not every green lake is a toxic bloom. These are common and usually harmless, though you still can't be sure without a test.

  • Green algae forms stringy, hair-like strands or slimy mats you can lift out on a stick. See blue-green algae vs green algae.
  • Duckweed is a tiny floating plant with separate round leaves, often covering ponds like a green carpet.
  • Pollen leaves a yellow or yellow-green dusty film in spring that gathers along the shore.

If you see a bloom

Stay out of the water, keep children and dogs away, and don't touch the scum. Take a photo and note where it is, then report it to your state's environment or health agency. Many blooms on this site were first reported by members of the public. If you or your dog had contact with the water, rinse off with clean water straight away.

Lakes with an advisory or recent report now

The latest reports from states that publish live data. See every lake on the US toxic algae map.

Questions

Can you tell if algae is toxic by looking at it?

No. Toxic and non-toxic blooms can look exactly the same, and the same bloom can produce toxins one day and not the next. Only a lab test can tell, so treat any suspicious bloom as toxic.

What colour is toxic algae?

Usually bright green or blue-green, but blooms can also be brown, red, white or almost black. Some look like green paint on the surface, others turn the whole water column cloudy green.

Does toxic algae smell?

Often, yes. Blooms can smell earthy, musty or like rotting plants or sewage, especially as they die off. But a lack of smell doesn't mean the water is safe.

Can toxic algae be under the water?

Yes. Some blooms grow below the surface or as mats on the bottom of rivers and lakes, where they are hard to see. These mats can break off and wash up on the shore, where dogs may eat them.

Who should I tell if I see a bloom?

Report it to your state's environment or health agency. Most have an online form or hotline, linked from the state pages on this site. Reports from the public are how many blooms are first found.

Sources

Health information on this page is based on: the US Centers for Disease Control and Prevention and the US EPA. Lake data comes from official state records, last checked on 24 October 2026.

This guide is for information only and isn't medical advice. If you feel unwell after contact with lake water, contact a doctor or Poison Control on 1-800-222-1222.

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