Where the red fern grows: Why Eugene’s Delta Ponds turn red each year

Those driving along the Delta Highway north of the Valley River Center Mall may notice a unique quirk of the Delta Ponds this time of year: they are red.

The red stuff adorning the 150-acre waterway site isn't algae or autumn leaves or dye, but an aquatic fern. Waterway enjoyers shouldn’t be alarmed, though, as the fern is actually beneficial to the local ecosystem, according to local experts.

Lauri Holts, the Parks and Open Spaces ecologist for the city of Eugene, said this red-brown aquatic fern, called Azolla, is often an attention-grabber for Eugenians who notice the ponds' change.

"We get calls every year … people think paint’s been spilled," said Holts, who oversees all city-owned waterways including ponds, parts of the Willamette River and Amazon Creek. "We get a lot of people that want to know if we're going to control it and and I say, no, it's natural. We wouldn't want to use a bunch of chemicals. It's just going to come back, so there's no reason to do that. And it has ecological benefits."

What is that red stuff?

Holts said Azolla, also known as "duck fern" or "mosquito fern,” is a genus of plants that can be found worldwide.

"It likes slow, quiet ponds. It doesn't like a lot of wind or a lot of current," Holts said. "It goes dormant in the winter, like a lot of plants do, and and then in good condition, once it's warmed up, and if it gets a lot of sun, it can grow."

Azolla is eaten by wildlife, especially ducks and geese. Holts added that the fern has a symbiotic relationship with a cyanobacteria (blue-green algae), which allows it to use nitrogen from the air, providing nutrients for the fern and providing a home for the bacteria. Holts said there are many examples of humans over centuries using this symbiotic relationship in farming.

Additionally, while it is also known as "mosquito fern" for its potential to reduce mosquito populations, Holts said that may be a bit misleading. While, yes, the aquatic fern can reduce mosquito populations, it is only if it covers 70% or more of the water, reducing the surface area in which mosquitos can lay their eggs.

Holts said it also helps combat climate change by processing carbon dioxide and providing oxygen and shade for the aquatic life living below it.

"It's a really fascinating plant with a lot of potential," Holts said.

Where the red fern grows

While some areas of Delta Ponds may see no Azolla, other parts are completely covered. Holts said it can grow quite rapidly in the right environment.

The ponds to the east of the Delta Highway appeared to have more of the aquatic fern growing in late September and early October. The northeastern-most pond near the northbound Goodpasture Island exit was the most red in color.

Holts said that with more sunshine, this Azolla turns more red. She said the northeastern-most pond turns dependably bright red year after year.

Should my pet eat that red stuff?

While Azolla itself has good nutrients, Holts still discouraged Delta Ponds visitors to avoid allowing pets to drink from the water.

"There could be something else in there," Holts said. "I caution people — I have dogs — to not let their dogs (drink it). A vet would do the same."

She said most people might not be able to identify harmful algae blooms or other toxins in bodies of water, so it's better to avoid drinking from public waterways, especially in urban areas.

History and future of Delta Ponds

Holts said Delta Ponds' unique history started out as a part of the Willamette. Where the Delta Highway and Delta Ponds are now used to be part of the Willamette River up until the late 1800s.

Over time, human use and urbanization changed the face of the area. The ponds and islands we see today were formed by gravel mining that took place in the 1950s and 1960s.

"A lot of the gravel that came out of these ponds was used to build, probably Delta Highway and Beltline," Holts said.

The city of Eugene has owned the ponds since the 1970s. For years, Holts has been in charge of keeping an eye on Delta Ponds and ensuring the nature is thriving alongside human use.

"Delta ponds is a natural area that's near and dear to my heart, because I started working on it 20 years ago," Holts said. "A lot of the plants and animals that are still here would have been thriving back in that day, when (the Willamette) did flow through here … How can I make sure we're taking care of it in a way that the plants and animals that are here can continue to thrive, given all the different changes that we've had?"

Now, the ponds are a popular destination for residents enjoying the nature and wildlife observations the site provides.

Currently, the city is developing a 1.4 mile Delta Ponds Trail Loop around the ponds on the west side of the highway.

The loop is funded in part by a $39.5 million bond that was passed in 2018 . Nearly complete, the city expects the project to be completed sometime this fall.

Miranda Cyr reports on education for The Register-Guard. You can contact her at mcyr@registerguard.com or find her on Twitter @mirandabcyr .

This article originally appeared on Register-Guard: Where the red fern grows: Why Eugene’s Delta Ponds turn red each year