Positive signs for Vashon watershed as stream health improves across King County

Of the two sites tested on Vashon Island, the stream health is either static or improving.

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.

Of the two sites tested on Vashon Island, the stream health is either static or improving.

You can read the results for yourself here.

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” life forms 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.”


Two of the county’s testing sites were located on Vashon: Christensen Creek near Christensen Cove, and Shinglemill Creek near Fern Cove.

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.

The Shinglemill site earned a “poor, non-significant” rating, meaning its bug score is poor but didn’t change significantly during the study period. Meanwhile, the Christensen site had a “good, positive” score, meaning its bug score was high and is only getting better.

Encouragingly, the entire island’s “subbasin” watershed trends are positive, according to a map and data of a “Regional Kendall test” from the research. That’s an indicator of the health density of the lifeforms on the island that scientists were looking at.


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 a 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.”