Plateau streams among the healthiest in the county as others improve

Despite King County’s growth, it appears urban streams are becoming healthier, surprising local scientists.

It tracks that more development equals negative environmental impacts.

However, a recent study spanning 20 years has found that despite King County’s growth — the county population increased from 1.76 million in 2022 to 2.25 million last year, according to the report — many local streams have resisted “urban stream syndrome” and have actually improved over that time period.

The “surprising” findings in the September 2023 report, prepared by King County’s Department of Natural Resources, says that 25% of the 120 testing sites across the county improved with “statistical significance” over the last two decades, while only 3% of sites showed significant decline.

One of the metrics used to test the health of various streams in the county is called the Benthic Index of Biotic Integrity (B-IBI) score — or, in the words of lead scientists Kate Macneale and Beth Sosik, the “bug score”, because “macroinvertebrate communities are sensitive to the many stressors associated with the conversion of forests and other natural landscapes to urban lands,” the report reads.

The bug score doesn’t just mean there are a lot of bugs, but a diverse set of bugs — some predators, some prey, some “tolerant” taxa that can live in less-than-ideal conditions, and those that are more picky.

“Each bug has its own set of requirements, what it needs to survive,” said Sosik. “When you add all the community of bugs together… you get a fuller picture of what’s going on at that site.”

At the end of the day, the 120 testing sites showed an average of 9% increase in their bug score, and 16 watersheds in the county (out of 38) were found to have statistically-improving trends.

And while many other sites weren’t found to have improved in a statistically-significant manner, they are improving.

“That’s one thing that Beth and I were really encouraged by,” Macneale said. “We could see that many of these streams are going in the right direction, even if they’re not yet statistically significantly improving. They’re trending that way.”

Plateau streams, in general, consistently rated higher than many other areas across the county, in part because of the relative lack of development (and the open green spaces) around Enumclaw and Black Diamond.

However, one small watershed in the area was noted as the only one in the county to be in “significant” decline, which scientists can’t explain at this time.


Twenty-four of the county’s testing sites were in the north Enumclaw/Black Diamond area, known as the Newaukum Creek Watershed; the Puyallup White watershed, which divides the city of Enumclaw in half and envelopes Buckley, was not tested.

An ‘excellent’ rating is the highest a site could be, followed by ‘good’, ‘fair’, ‘poor’, and ‘very poor’. A site’s overall rating is an average of how the stream improved or degraded over the course of the study.

Of those 24 local sites, one was rated as “excellent” but its bug score didn’t significantly change in a positive direction over the course of the study. (There are only six other sites rated as “excellent”, all in southeast King County in the Tiger Mountain/Hobart/Maple Valley area.)

The ‘excellent’ site is part of the Newaukum Creek, to the northeast of Enumclaw.

Sosik noted that parts of the Newaukum Creek started out as “barely” as the “fair” rating when the study first started, “and then at the end of the study, be pretty consistently… what we could describe as ‘good’, even into ‘excellent’ conditions.”

One testing site was rated “good” with a significant positive change in its bug score; another 11 were rated as “good” but with no significant change.

There was also one site that was rated “fair” with a positive change, another “fair” site with a non-significant change, and one “poor” rating, again with a non-significant change.

However, the two most confusing sites on the Plateau were on Deep Creek, south of Black Diamond on Veazie Cumberland Road SE — they were the only two, not just on the Plateau but among all the testing sites in the county, to have their relatively high bug scores degrade in an extreme way.

According to Sosik, one of the Deep Creek sites actually started out with an ‘excellent’ rating, but are now in the ‘poor category’; the other only started as ‘fair’ but also ended up rating at ‘poor’.

“Those really tanked over the years,” she said.

Why this is, is not clear — Macneale said her team spent so much time trying to make sure the study was accurate, especially because the results weren’t what they expected, that explaining why more urban King County streams are improving — or in the case of Deep Creek, a testing site in unincorporated King County without much development in the area, was degrading — was not deeply explored.

“We spent all of our time figuring out this level, and the next step is the… why,” she continued.

Scientists with King County determine how healthy a stream is by the Benthic Index of Biotic Integrity score, or “bug score”, which rates how diverse a bug population is at a testing site. Photo courtesy King County

Scientists with King County determine how healthy a stream is by the Benthic Index of Biotic Integrity score, or “bug score”, which rates how diverse a bug population is at a testing site. Photo courtesy King County


While Macneale and Sosik’s study was great news about King County’s environment — or at least its streams — their report comes with a word of warning.

“While it is tempting to take this as evidence of successful local efforts to restore or protect King County streams, it is prudent to examine and evaluate the possible causes of this unexpected finding,” they wrote in the report.

In short, the county needs to find out why streams are becoming healthier and determine if this recent success can be repeated in the future — especially as King County’s population continues to grow and develop and the climate continues to change.

This could be quite the undertaking, as the reasons why stream health is improving are myriad.

Macneale and Sosik can only hypothesize that, in general, changes that affect King County as a regional whole, as opposed to changes at individual testing sites, are the leading drivers of local stream health and health improvement.

This includes air temperature changes, reduced fine sediment levels, altered hydrology, bug migration, and more.

Human stressors, or de-stressors, are also variables. Development and impervious land cover, for example, was hypothesized to degrade local streams, but more stringent environmental protections and habitat and hydrological restoration could be key to helping streams stay healthy, or become healthier.

Stormwater regulations specifically appeared to be a significant de-stressor; in fact, Sosik said that the data they collected suggests climate change takes a back seat to what King County has been doing in regard to environmental health, “even though we can’t exactly say, ‘here’s this one weird trick to restore your stream bugs.’”

For example, water basins where pre-stormwater pollution prevention programs were enacted generally started with a lower bug score at the beginning of this study than basins that had post-stormwater regulation developments, she said.

However, there’s no county database that records information about the effectiveness of stormwater programs, leaving this theory as just that, albeit a well-educated one.

While stormwater regulations appear to highly benefit local streams, a major stressor is pollution.

For example, 6PPD quinone, “one of the most toxic substances ever evaluated by the U.S. Environmental Protection Agency,” but is also used in car tires, KUOW reported in early August adding that 1% to 2% of tire weight is 6PPD.

Three Northwest Native American tribes have recently called for the ban of 6PPD-quinone because of how toxic the chemical is, particularly for migrating fish.

Insecticides can also be an big issue.

“Insecticides can impact fish, but oftentimes, in many orders of magnitude more toxic to bugs, as designed,” Macneale said. “There are more pesticides being used generally than we can keep up with toxicity monitoring, and that’s an ongoing concern.”

Who might research how climate change, pollution, or human intervention affects local stream health is up in the air — Sosik and Macneale said there are no “firm plans” to continue their stream health study or expand into researching the “whys” behind the “whats”.

“In the same way that the stressors that degrade a stream are a complex and varied network of factors, the sources of improvement to stream health may be equally complex and varied from one site to the next,” their report concludes. “We used a broad approach in selecting environmental variables to evaluate against B-IBI trends, and as a result we can only speculate if significant relationships truly represent meaningful relationships. However, we hope to use our exploratory findings as a framework for further investigation.”