Weird Brown Sludge on Oregon Coast / Washington: Turns Out It's a Good Thing

Published 11/24/25 at 7:55 p.m.
By Oregon Coast Beach Connection Staff

(Seaside, Oregon) - What’s that brown stuff on the beach? Oil? Sewage? (All photos courtesy Seaside Aquarium)

No, says Tiffany Boothe of Seaside Aquarium. She recently caught more of this oddball phenomenon – one that's mostly a north Oregon coast and south Washington coast thing.

It's just diatoms, she said.

“Diatoms are single-celled plants (phytoplankton) that are found in both fresh and salt water,” Boothe told Oregon Coast Beach Connection. “They are one of the most important food sources in the ocean. In the winter, spring, and early summer, diatoms rapidly multiply in the surf zone.”

This was actually a couple weeks ago – just before all the stormy action hit. When this period is over and you get calmer conditions again, these wild sight may show up once more.

The phenomenon is largely unique to the north Oregon coast and south Washington coast. In other areas, such as Newport, Port Orford or Coos Bay, brown waves are more likely the result of river mud stirred up by storms. Plus, it won't really look like this. It's mostly found from Seaside to Warrenton, and then the Long Beach Peninsula, but sometimes Cannon Beach and farther north into the Washington coast.

Boothe said diatoms absorb large amounts of nitrates and phosphates that are delivered to the ocean by coastal rivers, contributing to their population explosion. Everything in the ocean feeds on diatoms and other plankton, either directly or indirectly. Even great baleen whales, like the gray whale, filter plankton and diatoms as part of their diet. When the surf zone becomes really saturated with diatoms, they wash ashore. There's so many of them they change the colors of the waves.

To many it looks like oil or sludge in the water. Other times it can look a bit like chocolate, although not an appetizing kind. It's a good thing, however: a sign of a healthy ocean.

The material may look grimy, stain clothing, and leave streaks on the sand. Keith Chandler, manager of the Seaside Aquarium, said the appearance has alarmed beachgoers in the past, but educational signs have helped ease concerns.

When it gets all over your clothes or makes dark spots on the beaches, it's not like those zombie video games. It's simply a slightly smelly annoyance.

“It’s not really so much of a stain as it is a lot of the stuff accumulates on the beaches,” Chandler said. “The stuff, like the brown bubbles, feels a little oily, but that doesn’t mean it is oil. It just comes from a lot of it piling on the beaches. The tide will come and clean it out. Just like a ketchup stain – it comes right out.”

Brown stuff from a previous event

Diatoms are also the basic foundation for bubbles in the surf anywhere on the Oregon and Washington coast. That sea foam you find - sometimes in huge, snow-like clumps - is essentially made up of one kind of phytoplankton or another.

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