Short answer: not very far, and the reach depends mostly on the tide at the time, not the river's flow. A tsunami is not a tidal wave, but the tide still sets how far the water gets.
The main study is Oregon State University's Hill and Kalmbacher work, published in the Journal of Waterway, Port, Coastal and Ocean Engineering (reported by OSU in 2015). It modeled a magnitude 9.0 Cascadia earthquake at the highest tide of the year:
- Mouth of the Columbia: the water rises about 13 feet.
- River mile 6, near Hammond: about 10 feet.
- River mile 12, within a few miles of Astoria: about 6.6 feet.
- River mile 25, near Welch Island: about 1.6 feet.
- River mile 50, near Longview, Washington: no measurable rise.
The Portland area and Bonneville Dam would see changes too small to measure. A separate DOGAMI Special Paper 51 modeling study says the tsunami can still be detected upriver to the Willamette confluence, arriving about 4 hours and 40 minutes after shaking starts, but it carries little energy above St. Helens and can be largely ignored above Longview. Flood-tide arrivals push furthest upriver.
These are models, so the exact numbers are estimates. The bigger danger is at the river mouth, in Baker Bay and Young's Bay, and at Tongue Point.
Also discussed here:
- Cascadia tsunami reach up Columbia River?
- A Subduction Zone Caught in the Act of Dying
