HomeMy WebLinkAboutArborist Report for permits1 | 200 Mill Ave S Report
200 Mill Ave S Tree Maintenance Project Summer 2026
Background: October 12, 2022, city tree TRE-24595 (black cottonwood) experienced
whole-tree failure and fell across the river, damaging several employee vehicles in the
parking deck of 200 Mill Ave S. On April 5, 2023, a contractor was hired to prune 5 riverside
trees to reduce load on the branches.
December 26, 2024, City tree TRE-24580 experienced whole-tree failure and fell across the
river, breaking through the glass of the building.
December 2025, the river experienced historic flood levels. Following the flooding, a
request was made to perform a tree risk assessment of the remaining black cottonwoods
on the riverbank.
Assessment: The Interim Urban Forester performed a Level 2 assessment on January 27,
2026. The trees have responded well to the pruning and show no noticeable signs of decay
in the canopy. However, the base of the trees presents some concerning signs. Flooding
deposited significant silt over the root plates. Previous assessments documented root
plate heaving due to sandy and silty soil conditions, and prior tree failures occurred at the
roots. Several trees exhibit large decay pockets at the base. Although some
compartmentalization is evident, the extent of decay cannot be confirmed without a Level
3 assessment. The tallest tree measures 150 feet, and the shortest measures 120 feet. The
distance between the riverbank where the cottonwoods are located and the building and
parking lot across the river is 105 feet. Tree measurements are provided in the table above.
Scope of work: The Interim Urban Forester mitigation recommendations include topping
the trees to a height of 90 feet and gently lowering debris into the Cedar River. Work will
occur between July 16 and August 31 in accordance with fish habitat protection guidelines
established by the Washington Department of Fish and Wildlife.
Branches will remain as intact as possible to function as large woody debris for habitat,
while being sized appropriately to avoid damage to the library over the Cedar River and
downstream bridges. The volume and size of debris are not expected to alter water flow or
Tree Species ID# DBH Notes
1 Cottonwood 24589 72 Reduce height to 90 feet
2 Cottonwood 24592 42 Reduce height to 90 feet
3 Cottonwood 24596 44 Reduce height to 90 feet
4 Cottonwood 24598 69 Reduce height to 90 feet
5 Cottonwood 24605 74 Reduce height to 90 feet
6 Cottonwood 24607 68 Reduce height to 90 feet
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the ordinary high-water mark. Debris will be lowered carefully to avoid disturbing the
riverbed.
Topping is recommended instead of removal for two reasons. First, retaining the root
systems will help stabilize the riverbank and reduce erosion. Second, the remaining stems
will continue to provide habitat and shade for aquatic and terrestrial wildlife.
Figure 1: A map of the proposed work with the trees to be pruned highlighted in blue circles.
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