The Oso neighborhood built near the North Fork Stillaguamish River was called “Steelhead Haven” for a reason. The north fork is known as one of the best habitats in the entire Stillaguamish river system for steelhead spawning.

Nearly nine weeks after Steelhead Haven was destroyed by the devastating mudslide that took the lives of at least 42 people, officials are confident the river will eventually recover — even after it was dammed by the slide debris and created a new channel — though a full recovery could take years.

However, they are less sure about the long-term effects on fish — including three species listed as threatened under the federal Endangered Species Act — that spawn in and near the North Fork Stillaguamish River.

So far, biologists aren’t seeing higher rates of fish mortality or warning signs. But the slide has led to number of questions about the effect on aquatic life:

Will the fish be able to make it through the dammed area?

Will the fish go somewhere else?

Will the river’s turbidity — the cloudiness of the water — affect the fish, and if so, how?

“Some of these questions we are still in the process of answering,” said Jenni Whitney, a biologist with the state Department of Fish and Wildlife.

New channel

Some effects from the mudslide were immediate — changes to the color and depth of the river, for example. But it may take an entire spawning cycle of one to four years, depending on the species, for the full impact on fish to become apparent, according to Jason Griffith, a fish biologist with the Stillaguamish Tribe.

After the mudslide, debris blocked the river, leading to concerns about flooding upstream and the possibility of flash floods downstream. Hydrologists estimated the water upstream from the mudslide rose 10 to 12 inches every 30 minutes on the day of the slide.

Downstream, the river level dropped from 3.1 feet deep to 0.9 feet deep over the course of one hour. The river slowly carved out a new channel the next day that allowed water to pass through, relieving concerns about flooding.

Heavy silt made the river downstream turn what the Fish and Wildlife authorities described as “the color of wet cement” — even as far as Arlington, 14 miles away, said Bill Blake, a co-chairman of the Stillaguamish Watershed Council.

The river is still cloudy, but not nearly as dirty as it was immediately after the slide.

“It’s not what you want to see in the Stillaguamish this time of year,” Blake said. “Usually you can see the rocks at the bottom.”

The visibility is still being measured in inches, not feet, Whitney said. Data from monitoring sites sampled shortly after the mudslide showed increased concentrations of arsenic, copper, lead and zinc in the river below the slide area, all likely from mudslide material, according to the state Department of Ecology.

Effect on spawning

The river turbidity could affect aquatic life. Steelhead are in their peak spawning time, Whitney said. The fish have been swimming to a different part of the river, past the slide area.

Steelhead are anadromous, which means they move from a freshwater environment to saltwater, then back again. They spawn in freshwater, and the eggs incubate in the gravel of the riverbed. After the eggs hatch, the fish, called smolt at this stage, migrate to sea. When it’s time to spawn, the fish return to the freshwater they left as smolts. Steelhead, which are not officially classified as salmon, do not die after they spawn; other salmon species do.

Steelhead, along with bull trout and chinook salmon, are listed as threatened under the federal Endangered Species Act. Bull trout are primarily observed in the upper South Fork. Chinook salmon spawn in the Stillaguamish above the slide area from late August through October, so adults likely weren’t harmed, but the high sediment levels could lead to higher fry mortality, officials of the Department of Fish and Wildlife said.

“One of the obstacles is that the river is just so turbid, and there is so much sediment in it, but that doesn’t seem to be stopping them from getting to the clean part of the river,” Whitney said.

A concern has been that fish wouldn’t be able to swim through the slide area to get upstream. However, the number of egg nests in areas above the slide suggest the adult fish have been able to navigate their way up, Whitney said.

“They’re pretty tenacious fish.”

Weekly surveys are monitoring the number of fish by the number of spawning nests, called redds. The figures give biologists an idea how the data compare to previous years. Griffith, the fish biologist, said that depending on what they find, officials from the Department of Fish and Wildlife, the Stillaguamish Tribe and Tulalip Tribes will determine the best approach.

“It’s kind of an unknown what actions will be taken,” he said. “We still don’t know how the fish will react. We have to look at what the fish tell us.”

Paige Cornwell: 206-464-2530 or pcornwell@seattletimes.com