We are all concerned about the general quality of the water, so bring lots of Diet Coke, I hear the carcinogens from Atlanta absorb the radioactive materials leaking into the river from Hanford. I grew up near rivers of the Midwest and Southeast and the Columbia is MUCH more pleasant to play in than most rivers I have seen or tasted. Eye protection - Sun glasses with UV protection is a good idea. I wear disposable contacts and the sunglasses help prevent spray from knocking out the contacts. Just ask my sister about her Maui Jims.
Some people tuck a thin line in the legging of their wetsuit to offer or get a tow.
It can be used to wrap up a sail, mast and boom. Or you and a friend can stretch it out and play chicken with local Kite-skiers. Each time you start a reach, look up and down river for those sneaky barges. Do not cross in front of a barge, they have the right away, they can't stop, nor do they want to stop. Don't make them drag you to Portland.