I learned to sail on the Columbia, it has its challenges, but is a great place to sail as long as you have a good motor too. (Or lots of time, for the die hard sailors) We bought a little 21 footer a few years ago and kept it in Saint Helens, OR. We sailed down to Astoria, and in to Portland, but never east of Portland. (No reason not to go east, just never got around to it.)
With the currents, tides (tidal effects can be noticed up into Portland) , and winds always running up or down river, rarely across, you can get some step waves and tough slogs back home if you are fighting all three at the same time.
We now have a larger boat in the Puget Sound and really enjoy sailing there more, there are few places to go and explore on the Columbia, the part of sailing we like best. But I think learning on the Columbia was very good for me with all the challenges it presents.
If you have any other specific questions, I would be happy to answer them if I can.
1964 Islander 32
Saint Helens, OR