The Snake/Columbia north of the Tri-Cities is basically an inland reservoir.

I started off with a small drift boat and had an electric motor. That will work for the dam forebays. I've seen guys fish the Reach in the fall with a gas kicker on a drift boat (I tried and failed). It's less than ideal for the Snake and Columbia, but it gives you the option of drifting the Ronde if you want some flexibility.

Otherwise, something like a 15ft Alaskan with a 25hp motor will get you going about anywhere. You could fish with less, but having a top speed over 20mph is nice covering ground. Aluminum is preferred for beaching on the rocks or primitive launches.

The potential for big water will coincide with the wind and long stretches for the waves to build, wind against current, or fishing below Wanapum.

Something like this would treat you well (if the condition is satisfactory): http://kpr.craigslist.org/boa/4799388011.html

Boats are like cars, you know best what you should get.

For reference, I have a 16ft 70's era starcraft with a 60hp tiller.