What a bloom looks like
The classic descriptions are worth memorizing because they are accurate. A cyanobacteria bloom most often looks like one of these:
- Spilled paint. The surface looks like someone poured out a can of bright green, blue-green, or turquoise paint and let it swirl. This is the single most reliable tell. Real algae almost never looks painted.
- Pea soup or split-pea soup. The water itself is thick, opaque, and green, as if the color is dissolved through it rather than floating on top.
- Surface scum, foam, or a film. A skin of scum, sometimes frothy, that piles up thickest on the downwind shore and in coves where the wind pushes it. It can look like a grass-clipping slick or a greasy sheen.
- Mats and streaks. Floating clumps or long streaks that can turn white, brown, purple, or even bright red as the bloom ages and dies. A dying bloom often smells foul.
Color is a clue but not proof. Blooms are usually green to blue-green, but can be tan, reddish, or milky. What they share is that the color is in or on the water as a suspension or scum, not made of structures you can pick apart.
Two quick field tests
The stick test. Dip a stick into the green and lift. Harmless filamentous algae drapes over the stick in stringy, hair-like strands you can lift out. A cyanobacteria bloom will not cling; it smears, breaks into flecks, or runs off like paint, because it is made of microscopic cells, not fibers.
The jar test. Scoop a jar of the water and let it sit out of direct sun for an hour or two. Many cyanobacteria float, so a bloom collects as a green ring or scum at the top. Sediment and most true algae settle to the bottom. This is a rough hint, not a lab result.
Neither test is definitive, and you should never touch a suspected bloom with bare skin. When the water looks wrong, treat it as unsafe and keep children and dogs well away.
Harmless look-alikes
Most green water is not a toxic bloom. These three are the usual false alarms.
Filamentous green algae
Stringy, hair-like, or cottony mats, often bright grass-green, that grow attached to rocks, docks, and the bottom and break loose in clumps. Picked up, it holds together like wet hair or cooked spinach. That structure is the giveaway: cyanobacteria falls apart. This kind of algae is a nuisance, not a toxin risk.
Tree and grass pollen
In spring and early summer, a fine yellow-green dust collects on the surface and along the shoreline, especially near pines. It looks powdery and dry, moves in sheets with the wind, and rinses off cleanly. It is not algae at all and poses no water-quality risk.
Duckweed and watermeal
Tiny individual green plants that carpet still water like a green blanket. Up close, each is a distinct leaf, and a handful lifts out as separate specks or plantlets rather than smearing. Duckweed is a plant, not a bloom. It can blanket a pond and look alarming, but it is harmless to swim through.
The rule that covers all of it
You do not have to be certain. If the water is scummy, discolored, painted-looking, or smells bad, stay out and keep pets out, whatever it turns out to be. Identification helps you avoid needless worry over pollen and duckweed, but the safe default when something looks off is always the same. For where the bloom risk is tracked by satellite, see the methodology and your local state page.