First, I assume from what you said that you are going self-guided. I have lived in Alaska for 28 years and fly out with my raft to remote sites about every other summer. The biggest lesson I've learned, the hard way of course, is that you always need 3 things from a bush flight service. #1 is a flight service that will be honest with you and not fly you out to a river system that is blown out, that is, the water is too high and your fishing trip is shot before you start. #2 is one that won't take you to a river system that is too low and you have to walk your raft too much and #3 is one that will take you to and suggest alternative rivers to float because of either #1 or #2.
When you call a flight service, they need to be able and very willing and friendly and patient about talking to you about different options, time of year, species of fish, the right gear and/or flies, bait etc. to use, and water levels. If they are not able to do this in a professional, knowledgeable manner, find another flight service. Try Ptarmigan Air in Bethel. [url=http://listings.allpages.com/ak-0103541322-bethel.html:d3f71]http://listings.allpages.com/ak-0103541322-bethel.html[/url:d3f71] (this one has been used by my friend for many years and he has always been happy with them. That being said, ask the right questions.)
All flight services can fly to many different river drainages up here. Some of the drainages are very quick for the water to come up high and they take quite some time to clear out. This usually is because they draw water from such a big drainage area. The Kwethluk River can fall into this category. That doesn't mean you shouldn't float it and fish it, it just means that you want to have some other options long before you come up to go fishing. Some drainages are not affected as much because they don't draw water from as big an area. This can definitely be an advantage unless it hasn't rained much, the water is too low, and you get to walk your raft for a day or 2 or 3. (
Don't get fixated on "I've got to float this one river and that's why I came to Alaska". If you do and you don't have alternative plans, you might not have the trip that you wanted.
Dean