North Fork Skykomish River

In Fall 2024, we partnered with the Snohomish County’s Department of Conservation and Natural Resources to identify Knotweed along a 10-mile stretch of the North Fork Skykomish River, beginning at Jack’s Pass Bridge. To accomplish this, we captured high-resolution aerial imagery with a ground sampling distance (GSD) of 0.27 inches, produced GeoTIFF maps covering the river and its margins (ground less than 6 feet above river level), and placed sub-centimeter-accurate GPS pins on each identified Knotweed or Knotweed grouping occurrence.

Target area to be captured in aerial survey

This remote, obstacle-filled valley presented unique challenges for our team

Taking into account tree height, terrain angle, and the need to capture as much ground as possible beneath the canopy, this project proved more challenging than we initially expected. After several iterations, we came up with a plan that allowed us to fly above the treetops while still capturing imagery at a resolution high enough for accurate identification. Luckily no drones were hurt in the process.

An example of one of the many knotweed groupings we identified off the Skykomish river in the upper quadrant of the river

In total, we identified 51 high-confidence Knotweed locations and 106 additional locations that we flagged for closer examination this spring after leaf-out.

One example shot of our validation done in the spring of 2025.

Here we zoom in on one of the target identifications

A close up of the plant in question

As you can see this was a positive identification. We were able to go back and validate all of our findings from the first section, with only one turning out to be Alder upon further examination. We took over 10,000 photos with an overlap of 30 percent in each shot, meaning we were able to check multiple angles of the same area in question.

Four aerial shots taken of the same bank showing different angles of the knotweed sitting about 10 feet in on the bank

We’ve now surveyed 20 miles of the river capturing extensive visual evidence of the concentration of Japanese Knotweed. We’re excited to see how aerial surveys can help aid this project and hopefully provide more quantitative insight into the progress being made over time.