OSO — It has been nearly two months since the hill above Steelhead Haven collapsed and filled the valley of the North Fork Stillaguamish River. The mudslide buried the river, Highway 530 and homes under 10 million cubic yards of dirt and debris.
And the river has found its way through the debris.
A few miles downstream from the slide, the clear waters of a creek swirl into the steel-gray flow of the Stilly before being subsumed into the larger current.
This time of year, with the heaviest spring rains over, the Stilly should also be flowing clean.
“It shouldn’t look like wet cement,” said Peter Verhey, standing on a rocky bank where stream and river meet.
On Thursday, Verhey, a fish biologist with the state Department of Fish and Wildlife, was counting steelhead nests in the tributaries of the North Fork.
In normal years, he would expect to find steelhead redds up and down the Stillaguamish itself.
But this isn’t a normal year. While steelhead and other salmonids have made their way upstream past the slide area, the mudslide rendered six miles of river unsuitable for fish. Even if there was a redd in that stretch, the opacity of the water would make it impossible to find.
“We knew the fish will swim through dirty water to get somewhere, but it’s unclear if they’ll spawn in it,” he said.
This enormous disturbance of the North Fork’s natural course, volume and levels of sediment will have wide-ranging effects.
The initial blockage led to extensive flooding behind the slide, which abated only when work crews built a 2,000-foot berm, pumped out the “lake” behind the debris and dug a channel to help the river flow through more quickly.
Still, the river keeps changing, leading to uncertainty about what will happen when the heavy rains return in the fall.
Flooding is not an “if” question, but one of how much and when.
Snohomish County has been coordinating numerous agencies that are monitoring the river with an eye toward flood risks, said Steve Thomsen, the county’s public works director.
As the river explores a new course through the debris pile, it will take time for hydrologists to build a new flood model for the Stillaguamish.
“My intuition tells me it’s going to take a combination of modeling and getting through a flood season or two to have a better vision,” Thomsen said. “It’s a new hydrology thrown into the mix.”
Key to understanding flood risk is what’s known as an inundation model.
Debbie Terwilleger, director of the county’s Surface Water Management Division, said a new model will rely on assumptions: the width of the channel, the volume of water, the rate of flow. From that data, a map can be generated showing an area likely to be affected by a 100-year flood.
The data have been coming from multiple agencies. The transportation department has been modeling the river to plan how to protect the highway from future flooding. The Federal Emergency Management Agency has produced its own flood-model data.
In addition, federal agencies such as the U.S. Geological Survey, the U.S. Army Corps of Engineers and the Natural Resources Conservation Service have been contributing to understanding the river’s behavior, Terwilleger said.
The facts in the water keep changing, though. Most notably, the river continues to carve out a wider channel through the debris.
“The more the river widens itself, the more water it’s going to convey downstream,” Terwilleger said.
But it’s not that simple. The county plans to study how sediment moves through the entire Stillaguamish basin to get a sense of how quickly the debris pile is eroded by the river, where the sediment is deposited downstream and how that will affect flooding elsewhere in the river valley.
“If the river channel starts filling in, what sort of flooding risk will we see?” said Gregg Farris, the planning manager for the county’s surface water program.
“Because the river itself has been changing through the slide, that’s been a part of the challenge of modeling,” Farris said.
Add to this the uncertainty of whether the North Fork will stay in its course or shift to carve a new channel somewhere else.
In the past, the Stilly has not been content to stay put.
A notable example was a 1967 landslide at Steelhead Haven, which pushed the river channel about 700 feet to the south, destroying three fishing cabins.
Over the next 20 years, the river slowly moved back to the north until it settled into its most recent pre-slide channel. There it remained until March 22, when a wall of earth again pushed the river south.
“There seems to be some kind of attraction of the river to the north,” said Christopher Magirl, a research hydrologist with the U.S. Geological Survey.
Understanding how the river interacts with the slide deposits in the river valley will be important in assessing what it might do in the future.
“Five, 10, 15 years down the line, how stable might the river channel be in this large landslide deposit that now covers the width of the flood plain?” Magirl said.
A 1999 report for the Corps of Engineers explored the river’s meandering as the factor that most strongly correlated to slide activity above Steelhead Haven.
The report found that while historical slides occurred in periods of very wet weather at both Steelhead Haven and Gold Basin, on the South Fork Stillaguamish River near Granite Falls, slides also didn’t occur during the wettest seasons.
Instead, a major factor contributing to slide activity appears to be the river’s steady nibbling at the “toe” of previous slides, the report found.
A 2004 plan developed by the state and county to control flooding cited those two landslide zones as major sources of sediment in the river. It suggested taking measures to control it. The report, however, considered flooding a greater risk to human life and property than landslides, and it considered damage to fisheries to be a more concerning outcome of slide activity.
Such assessments are being reconsidered after the Oso slide, with some officials seeing a need to more accurately identify the risks to human life and property in slide zones. For example, last week the U.S. Forest Service chose to keep the popular Gold Basin campground on the Mountain Loop Highway closed until an active slide above it can be thoroughly assessed.
The sediment issue is also getting a fresh look, not just in terms of flooding but also for what it might do to fish.
Pat Stevenson, the environmental manager for the Stillaguamish Tribe of Indians, said migrating salmon and steelhead have proven fairly resilient over years of slide activity on both forks of the river.
In the wake of the eruption of Mount St. Helens in 1980, the sediment running off the volcano was found to have harmed the gills of migrating fish. That sediment was often composed of tiny but sharp particles of volcanic glass.
The gills of fish in the Stillaguamish, on the other hand, have looked fine, Stevenson said.
The biggest problem with fish sampled from the Stilly, he said, is that their stomachs were empty.
“They weren’t able to find food because they couldn’t see it due to the turbidity,” Stevenson said.
The tribe will be monitoring returning fish throughout the summer and surveying the riverbed every week to see whether it is getting covered in silt. Many insects live in the gravel of the bed, and a blanket of silt would continue to deprive the fish of a key food source.
With the Stilly flowing at about 40 percent of its natural capacity through the slide area, Stevenson said, a longer-term worry is whether the channel needs to be widened or moved to increase its flow and control sedimentation.
That poses other problems, including that of money.
Thomsen said that just widening the river channel to approximately the size before the slide would involve excavating 600,000 to 800,000 cubic yards of debris.
If the transportation department’s $5 million contract to remove 100,000 cubic yards from Highway 530 is a benchmark of the costs for that kind of work, widening the river becomes a much larger problem.
“Where are we going to find $30 million to do a channel excavation, or should we?” Thomsen said.
“Solutions are easy to brainstorm,” he said. “Financing them are another kind of problem to deal with. This is a gargantuan project.”
In the meantime, the fish seem to have adapted to their changed environment.
Verhey and his fellow biologists have mapped two steelhead redds in a creek near Oso that had never hosted them before, presumably because spawning grounds elsewhere are harder now for the fish to get to.
Last week, he spotted the fluorescent ribbon another biologist had left behind to mark the redd’s location. Rain and higher creek flows have flattened the pile of gravel and most other signs of the spawning ground.
After surveying the creek, he was unable to find the second redd that his team had earlier spotted.
“That’s one of the potential drawbacks to the slide — that it forces the fish to look elsewhere” to spawn, Verhey said. “And we’re finding them elsewhere.”
The new spawning grounds might be a bit too warm for the fish or otherwise less than ideal. A real understanding of the effect of the slide might not come for four years, when the steelhead that were laid this spring, and which will hatch in the summer, will return to the tributaries of the Stillaguamish as adults.
In another creek, Verhey comes across a small school of what he thinks are coho fry. Coho, like steelhead, remain in their native creeks for about a year before heading out to sea, he said.
As he tries to get a closer look, the fry scatter, hiding in the shady gaps between the stones and pebbles on the creek bed.
In the murky Stillaguamish, a silty bottom will likely prevent that.
“That’s going to be one of the biggest challenges for this river,” he said.
Chris Winters: 425-374-4165,firstname.lastname@example.org.