I thought you were wrong based on my memory of the 2011 Japan tsunami and the images we all watched as it raced inland for what seemed like forever, but according to wikipedia it only traveled up to 10 km (6 miles) inland.
So even the biggest tsunami the world has witnessed wouldn't come close.
Portland is ~80 miles inland from the pacific ocean - and the odds of a tsunami coming up the columbia river from the pacific, and still having much destructive power to do more than just flood, is again, basically nil - it would literally need to be a perfect storm for it to work.