I think this question is hard if not impossible to answer on this SE format; after all we're outdoors men and women and not lawyers. I'd still like to offer some points, (though IANAL and don't even live in the US) as there are certainly several aspects to consider here:
I'm pretty sure what you're doing is perfectly legal, unless kayaking on the river was illegal in the first place (which I don't assume).
I hope it will never come to that, but: from the limited knowledge I have on the US law system I think it is sadly not impossible that somebody would try to sue you were anything to happen to them while accompanied by you. There is always a chance that someone will try to use such a situation to their (financial) advantage.
Making your community service 100% "liability-waterproof" might be infeasible (e.g. involve the people you're accompanying signing lots of forms etc.).
But, if your really wan't to be as sure as possible, you'd need proper legal counsel.
Quite apart from this I think what you're doing is generous and a very nice service to your outdoor community. If someone were to make you liable for such a service I'd personally consider them to be in the wrong. Keep up the good work. :)