SEATTLE — Diane Yeh, environmental aide for King County, used a hand tool to churn the mucky sediment on the floor of Thornton Creek, releasing writhing worms and pea-sized freshwater clams into a mesh net Tuesday morning.
The urban stream begins in Shoreline, weaving through neighborhoods where its sewage stench has, in the past, caught people’s attention and Northgate, where it was once buried below the mall parking lot before spilling into Lake Washington.
Despite the past century and a half of habitat degradation, recent studies led by King County environmental scientists suggest streams like this are more resilient than once thought.
The researchers have gathered stream bugs with nets and jars from more than a hundred sites across dozens of King County watersheds for the past two decades. The critters, sensitive to changes in their watersheds — like rising water temperatures, more pollutants or sedimentation often associated with forests leveled for apartments and fields paved for parking — can tell scientists a lot about stream health.
Surprisingly, some King County streams are bouncing back. About one-quarter of the 120 study sites that the county monitored are healthier now than they when the study began, while about 3% have declined in health.
Across the county, researchers saw overall improvement in the biological condition of streams or the type and number of stream bugs. They saw this improvement in 16 of 38 watersheds, while only one watershed, Deep Creek, declined.
The Coal Creek, Issaquah Creek, Little Bear Creek, Lower Cedar River and Lower Green River were among the watersheds that saw biological conditions improve over the past two decades.
“The reason that we kind of didn’t really necessarily believe our results initially, is that when you looked at the watershed scale, in many of these watersheds, you have increased development over the course of our study,” said Kate Macneale, a co-author of a recent report on these trends and an environmental scientist for the county.
County Executive Dow Constantine and Seattle Mayor Bruce Harrell celebrated the results at a small news conference Tuesday at Thornton Creek.
“The development that’s happened over the course of those last 20 years is less harmful, is less impactful than the old style of development,” Constantine said, as cars whizzed by on nearby 95th Street. “That has to do, I think, with regulations, with building codes, with new techniques that treat our natural environment more respectfully.”
For millennia, Coast Salish people stewarded the coniferous forests, freshwater highways and immense estuarine nursing grounds for juvenile salmon encompassing present-day King County. With the arrival of white settlers in the mid-1800s came the start of intensive extractive processes: forests leveled for timber, floodplains diked and drained for agriculture, rivers rerouted and dried up and industrial waste poured into the waterways.
By the 1920s, few lowland forests remained. By the 1950s, waterways across the county were full of sewage sludge, toxic and other unregulated pollutants. In the 1960s, stream protections began to slowly roll out.
Fast forward to the past few decades, where new regulations require the treatment of stormwater runoff and restrict development in urban growth areas. Coast Salish tribes have led — and been joined by local and state governments — investments in projects that return farmland and hotels to marshy estuaries, replant streamsides, restore rivers’ natural flows and restore spawning habitat.
From 2001 to 2019, impervious cover — or paved or impenetrable surfaces — increased from 0 to 12.7% across the studied basins. King County ballooned from 1.76 million to 2.25 million people from 2002 to 2021.
Scientists expected biological conditions in the studied streams to decline because of the growth.
Not according to the bugs, or in science terms: the benthic index of biotic integrity, or B-IBI, a measure of stream health based on the type and number of various benthic invertebrates.
“The best holistic view of how the stream is doing, what a stream is supposed to do, is from a critter’s perspective,” Macneale said.
It’s expensive and challenging to quantify everything going wrong in urban watersheds chemically, but longtime University of Washington professor Jim Karr led the development of an index for scientists to instead ask the critters, Macneale said. It’s intended to figure out: Are the critters that are supposed to be there — that we know should be there — there? And how are they doing?
Bugs are important for fish, too. Juvenile salmon rely on bugs to get fat before making their journey to the saltwater and eventually back to spawn.
If the bug community is showing a stream is unhealthy, fish probably aren’t going to be doing well there either.
But fish have their own set of requirements, so scientists can’t exactly say that if the bugs are showing it’s a high quality stream that the fish will be there too. Only the fish can tell you if there are toxic levels of certain chemicals or a lack of streamside vegetation contributing to unsustainable temperatures or insufficient woody in stream habitat.
While the bug study can paint a picture of stream conditions, it can’t explain why the stream conditions are that way. So scientists began to dig into what other environmental conditions have changed over this time that might help explain those trends and serve as questions for future testing.
With warming temperatures, they expected there would be more tolerant bugs and fewer that are sensitive to rising temperatures, but that’s not the pattern apparent in the data. Some of the bugs that are more sensitive to environmental stressors like heat, sedimentation and contaminants, like caddis flies and stoneflies, aren’t as abundant as they once were in many watersheds but are increasing in some watersheds across the county.
Meanwhile, more tolerant bugs, like flatworms and fly larvae, were declining, suggesting stream conditions were improving.
Climate change might have some influence on these trends, but it’s probably not the main driver.
The positive trends scientists are seeing are not happening in just a certain kind of site. It’s not happening in just urban or suburban sites. It’s happening in the forested, suburban and urban basins. All boats are rising at the same level, said Beth Sosik, water quality planner with the county and co-author of a recent report on the trends.
A different report from the county found when comparing basins with similar amounts of development, bug scores were higher in basins with more recent development, suggesting newer regulations may have helped protect and improve stream conditions.
A reduction in fine sediment impacts, water pollution and improved stream flows may be driving the trends, Sosik said. But not all streams are doing well. Urban development and climate change continue to threaten some sensitive bugs and overall stream health.
While scientists haven’t yet pinpointed what contributed to these improving conditions, some say the study suggests we shouldn’t give up on urban streams, and they are more resilient than previously thought.
“A cynical person could look at these trends and say, ‘Well, we just need to wait for the streams to get better,’” Sosik said. “My takeaway from this report is that this kind of this whole body of protective regulations and practices that we’ve had in place over the last 20 years is making a difference, and we need to keep it up.”
“It also tells me that that needle can be moved and so we should be doing more,” Sosik continued. “Urban streams can recover — more than we thought.”