Dave, once we get east of the Cascade Mountain range that splits WA almost on a direct North-South line we get into the rain shadow of the Cascades and that makes for a desert climate. Where we live in Pasco we get a whopping 8" of ran and 3" of snow per year.
Now, just for your viewing pleasure, here are some pics I took as we came through the Columbia Gorge where there are real live trees:
Much of the land along the river on the eastern side of the state is just too rugged to build on. There are a few places where there are towns along the river on the WA side, but they are few and far between once you get past the Cascades.
As you get further upstream, past McNary dam you'll find a lot of irrigated farm land where some of the world's best wine grapes are grown as well as fruit orchards. If you look on Google Earth and follow the river upstream from McNary Dam (upstream from Umatilla) you will see a lot of green land. That is all irrigated by water pumped up from the Columbia. In Google Earth you can also see the difference between the height of the river and the height of the tops of the mountains that line the WA side of the river.
If you're at all interested in geology, do some research on the Missoula Flood. It's an interesting look at how this land was formed by a massive flood that started in Montana and swept all the way to the Pacific. YouTube has some interesting videos on it.