Having made the trip from the Tri-Cities to the coast and back several times I too can vouch for the definate possibilities of getting into some really rough water. Last June I was going down river, on my way ultimately to circumnavigate Vancouver Island and was held up for four days at Cascade Locks waiting for the wind to subside. The waves get very steep. I've buried the bow of my 37 several times and had green water all the way back to the dodger before it ran off of the deck. However with that said some of the best river cruising grounds in the world, IMHO are on the Columbia below Portland. It's another world altogether from that found above Portland. The little ports are quaint and sleepy all the way to Astoria and the town of Ilwaco just has to be experienced.
Going up river past Portland also affords one with a lot of unique experiences but you must have enough engine power to push past the fast currents just below the dams, especially Bonneville Dam, or you just won't make it. Once past The Dalles dam the scenery turns far more desert like but there are still sites to be seen including a large herd of big horn sheep that hang out above John Day dam.
The Snake river is navigable all the way to Idaho as long as your mast isn't too high. 50 feet is just about the maximum you can pass safely but the desert climate is worth the trip.
If you come up, stop in and say hello to us at the Walla Walla Yacht Club at river mile 312 in the Wallula Gap. There is someone home almost all the time and the guest dock is always open.