It’s a calm morning on the lake. The sun is just starting to rise, and the water is still—except for a line of fluffy white foam trailing along the shoreline. It almost looks like someone poured soap into the water overnight.

So what is it?

In most cases, foam on the surface of a lake or river is a natural occurrence, caused by organic compounds released from decomposing plants and algae. These compounds reduce the surface tension of water, allowing bubbles to form more easily when the water is agitated by wind or wave action. The foam is often pushed by wind or current into patches or long lines—called “foam lines”—near the shore.

While it may look strange, naturally occurring foam is not usually harmful. It tends to be off-white or brownish in color and will often have an earthy or fishy smell. It may stick to the shore, collect near piers or in eddies, and change in volume depending on conditions.

However, not all foam is safe. If you notice bright white foam with a perfume-like scent or rainbow sheen, it may indicate pollution (like detergents or sewage overflow). Additionally, foam can sometimes be associated with Harmful Algal Blooms (HABs), especially if the water is also discolored or has a strong musty or rotten smell.

Exposure to HABs can cause rashes, eye irritation, respiratory symptoms, or more serious health issues for people and pets. Always avoid contact with water that looks suspicious and do not let dogs swim in or drink from foamy or discolored water.

Want to know how to spot harmful algal blooms and what to do if you think you’ve seen one? Visit our Harmful Algae page for photos, safety tips, and reporting tools.