Researchers Look At Emerald Ash Borer Impact To Salmon, Steelhead Streams

The impending loss of Oregon ash trees due to an invasive bug spreading in the region is bad news for waters used by Willamette-Columbia system salmon, steelhead and their forage

In a recently published study, researchers looked at two streams on the north side of Portland – Columbia Slough and Johnson Creek – to quantify the makeup of their tree canopies, then modeled the impact that dieoff of the ashes due to emerald ash borers in the coming years would have on how much sunlight eventually bakes down on the streams.

“Our results suggest the decrease in canopy cover in riparian corridors and the resultant increase in solar loading due to the EAB-induced loss of Oregon ash will have significant impacts to water quality and aquatic habitat of the Columbia Slough and Johnson Creek,” write Dominic Maze, Julia Bond and Monte Mattson in the journal Biological Invasions.

They say that higher water temps both decreases how fast young salmonids can grow and increases their susceptibility to predation compared to juvenile fish growing in cooler waters.

And Maze and Bond, who work for the City of Portland’s Environmental Services bureau, and Mattson, on staff at the Clackamas Soil and Water Conservation District, say their work has implications well down the Willamette Valley, where Oregon ash trees are more predominate.

“EABs in the Pacific Northwest, west of the Cascade Mountain range, presents a forest pest worst-case scenario: the loss of a dominant and largely functionally irreplaceable riparian and wetland tree species that provides critical habitat for both ESA-listed and non-listed species and water quality ecosystem services,” the trio add.

The findings are a grim reconfirmation of a major article in the September 2025 article in Northwest Sportsman Magazine that looked at the convergence of surging Willamette Valley coho runs as well as Endangered Species Act-listed spring Chinook and winter steelhead stocks, with 2022’s discovery of EABs near Portland and last year in the bottoms of a relatively small stream that’s suddenly found itself host to tens of thousands of young coho.

But the impact from EABs won’t just be limited to those runs. The authors point out that streams like Johnson Creek and Columbia Slough also provide off-channel refuge for young salmonids coming down the Columbia River from Inland Northwest tributaries.

“Given Portland’s location at the confluence of the Willamette and Columbia rivers, all salmonids that spawn within the Willamette basin and Columbia basin upstream of Portland, pass through or adjacent to Portland. Accordingly, the Columbia Slough and Johnson Creek provide valuable habitats for all migrating salmonids, not only those that spawn or rear in these two waterbodies,” they write.

That young fish from beyond the Willamette use those streams was shown earlier this year during a presentation on 6PPD-q, a tire preservative that breaks down and can quickly kill some salmon species once it gets in the water. A map from Environmental Services illustrated where hatchery and unclipped Chinook and steelhead from the Mid-Columbia, Snake and Yakima basins had turned up, including in the slough and creek.

Even as there are some ways to mitigate the spread of emerald ash borers, the prognosis is very grim for Oregon ash trees, which have been dubbed a “wetland supertree.” Mortality rates seen in the Great Lakes states exceed 99 percent.

The authors of the paper call for prioritizing EAB surveys and early detection work, more studies on how the invasion will affect water quality and habitat across western Oregon and Washington, and say that local natural resources offices should think about how to replace Oregon ash trees, which will not be an easy task given the limited options due to riparian soil type and moisture.