Oh woe is me, I'll have to lose 2500 feet in 10 miles if I cruise at 3500 from Kelso to HIO. I don't know what you're planning to fly but that'll be about a 500 fpm descent at 120 kts.
Put down your GPS and look at a map. The Columbia river runs from Portland to Kelso. If you can't handle descending 500 fpm, just fly down the river and turn right before you get to PDX.