A source for info on the lower Columbia is this web site, which includes an interactive map of access points, campsites, and points of interest: http://www.columbiawatertrail.org/
There are a number of access points and campsites that are not shown on this map. Our put-in was at Rainier (Oregon) City Park, which is not on the water trail map, but is a good access point. It is across from Longview, WA.
Another source that is not good for practical trip planning info, and is geared toward power boaters, but has more in depth descriptions of sections of the river, including history, is here: http://www.oregon.gov/OSMB/library/docs ... e.pdf?ga=t
Camps along the river are not abundant, but there are some very nice ones. They can get busy with power boaters on summer weekends.
A key consideration is the shipping traffic. Large freighters glide by now and then. They are fast and quiet and you don't want to be in their way, but with a modicum of care they are not a big problem.
This is a big body of water and wind can be a big factor. Hopefully a good factor in an AI. But especially when the wind blows counter to the current it can whip up bigger and steeper waves than you want to deal with in an AI.
I'm far from an expert on the Lower Columbia, having done just two trips on it, but would be glad to try to answer other questions.
We also took our AIs up to Puget Sound (well, San Juan Islands actually) a few times this summer, C Lion. Loved it and hope to return next year.