PORTLAND, Ore. (KOIN) — The once-mysterious green “scum” that was first reported to be floating through Portland on the Willamette River earlier is still lingering in the area. However, local experts say that the species of aquatic plant isn’t harmful to the environment and should disappear with the next big rainstorm.

Oregon State University College of Agricultural Sciences Professor Emeritus Stanley Gregory told KOIN 6 News that there are a few common types of floating aquatic plants, known as macrophytes, that occasionally wash into the main stem of the Willamette. These plants include azolla, duckweed and watermeal.

“They will be flushed downriver with the next high flows,” Gregory said. “Of course, some will keep coming into Portland from upstream, but they usually are not very obvious during the higher flows of the fall season. Eventually their growth slows in winter, and they become less abundant even if the high flows do not wash them downstream.”

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  • river plants

These macrophytes, Gregory said, are a natural feature of the Willamette River and are not a sign of added toxins or pollutants in the water. Lamber Slough in the Fairfield area, for example, is known to contain “huge amounts” of these aquatic plants, he said. Additionally, they can help the local environment by providing an additional food source for animals living in the local floodplains.

“Animals in the water will munch on them,” Gregory said. “It’s part of the food web. Organic matter in the floodplains is part of the nutrient cycle.”

While he hasn’t had a chance to analyze the plants floating in the river, based on photos of the event, Gregory surmised that the species seen floating downriver is azolla — an aquatic fern often used in backyard ponds.

USGS research hydrologist Kurt Carpenter agreed with Gregory’s assessment.

“I saw some of these plants at Oaks Bottom over the weekend,” Carpenter said. “It’s water fern: azolla. It has that olive drab color, and velvety texture. [It’s] a vascular plant with roots that is very common. Also visible is duck weed or water lentils. Seeing these plants in the river is a sign that the river is coming up from the rains and plant materials are moving.”

With recent rains, Carpenter said that the aquatic plants have collectively washed into the Willamette from various tributaries, resulting in a noticeable summation of plants that have come together downstream.

“All of those tributaries could also have these floating aquatic plants, and we’re seeing the collective effect now,” Carpenter said. “Think of the miles these plants may have traveled, all the way from Eugene perhaps.”

During the fall, Gregory said that azolla can also take on a seasonal reddish color. Because the decorative water fern is often sold at nurseries for landscaping purposes, Gregory said that folks should take advantage of the opportunity and score some free water plants while they’re available.

“If you have a pond or water feature, go grab a cup of it,” he said.