weeds

Last summer, staff stood along the banks of creeks across Washington County with a peristaltic pump, bucket, a tube, and a small filter. From the outside, it looked simple. No large equipment, no marked survey plots, just water being pulled by a pump through a filter at the edge of the creek.

What those samples hold is less simple. Each one contains fragments of DNA moving through the water – material shed by plants, animals, and other organisms from upstream. This is environmental DNA, or eDNA. It gives us a way to detect species beyond what we can see through limited land surveys, acting as a record of what is living and moving through a system. A small sample, collected in a few minutes, can point to a much larger story about the Tualatin River watershed.

Why Use eDNA?

Surveying the waterways across Washington County takes time and resources and we currently have three team members that work in our Invasives Species division. Traditional field surveys rely on staff identifying species on the ground, which limits coverage. Some species are also difficult to detect at early stages or outside of peak growing seasons. This creates gaps in what we can confirm through observation alone.

eDNA helps narrow that gap. It allows us to detect potential species presence before or beyond what we can see. This is especially effective for aquatic plants and animals, where DNA moves more consistently through water. The goal is not to replace fieldwork, but to make it more targeted and efficient.

How eDNA Sampling Works

Staff collected samples from the mouths of twelve tributariesTributary A river or stream flowing into a larger body of water. flowing into the Tualatin River. While standing on the bank, they place a filter directly into the creek and pull water through it, aiming to collect about 1000 milliliters per sample. This captures DNA from organisms moving through the water at that location.

Once collected, the filter is preserved and sent to a lab in California for analysis. In addition to eDNA, staff record water quality data including temperature, pH, turbidity, conductivity, and dissolved oxygen. These measurements help provide context for interpreting results. Together, this information supports how we understand presence of various species across the watershed.

What We’re Seeing So Far

A total of 38 samples were analyzed across the sampling sites in late September 2025. Results show detections at different levels, including species, genus, and family. Some detections come with higher confidence than others, with many detections at 100%. Even with variation, patterns are clear enough to guide our next steps.

Several Early Detection and Rapid Response (EDRR) invasive species were identified. Priority areas for follow-up include Nyberg Creek, Chicken Creek, Fernhill Creek, Saum Creek, and Rock Creek. Detected groups include purple loosestrife, Ludwigia (evening primrose family), sulfur cinquefoil, and spurge species . In some cases, genus-level detection is enough for us to flag concern when no native species exist in that genus.

What Happens Next in 2026

eDNA results are used to guide field surveys. Staff will return to priority areas to look for invasive species on the ground. This may include revisiting known sites or reaching out to new landowners for access. The goal is to confirm presence and respond early where possible.

This approach helps focus time and resources. Instead of surveying areas randomly, we can prioritize where signals are strongest. That allows for faster response and more efficient use of staff time. It also improves coordination with partners and contractors already working in these areas.

An Ongoing Practice

eDNA water sampling builds on what we already know about invasive species in the Tualatin River watershed. As eDNA sampling continues alongside field surveys, it adds another way to trace patterns and guide where we focus next.

How You Can Help

Another Early Detection and Rapid Response (EDRR) tool in our toolbox is receiving reports from the public about our priority invasive plant and animal species. 

We recommend people report their observations to the Oregon Invasive Species Hotline at https://oregoninvasiveshotline.org/

You can find a list of our priority Invasive Species on our Weed and Pest Directory at https://tualatinswcd.org/species_categories/edrr/
Thanks for your help!