I guess it depends on what state your in, but in Texas the law is that islands, sand bars, gravel bars, etc on rivers are public land.
There is a grey area thing about the high water mark and below it, but I just stick to island, etc.

There are areas in Texas where the landowners, and ranchers don't like people on "their river". They will come down with the shotgun and let you know your not welcome. It's good to know the rules and have phone #s of the area authorities.

One thing to consider when picking a camp spot is rising water, from up stream rains or dam releases.

I'm excited - next week I'm doing a 4 day 30-40 mile run down the mighty Brazos River.

I also do some kayak camping trips. I have left you ground dwellers though. After finding out about ahmmock camping I will never sleep on the ground again.

http://www.hammockforums.net/forum/

We offer multi-day guided fishing trips up here on the North Branch of the Susquehanna river and this year we will be able to provide a few tents and additional gear as needed, but have based our overnights on full rigged anglers coming out with their own gear.

Hank and I when we fish without clients hardly ever do anything under a two day trip no matter what. Its not worth it for us to just run out and fish(in our minds at least) so we go for a few days.....

Our camp spots are determined every spring and cleaned out and set up for the season on islands that are DCNR land and not used. There is NO water trail system up here so there is no designated camping islands and NO crowds, most of our camps are ours for the season without ever seeing another camper on the island.

We use platypus bags for water because they fold up to the size of a cd when empty and lay flat in the yaks when full. I did alot of moutaineering and back country skiing and ice climbing so we adapted the same gear and techniques that I used to our kayak camping.

Go lite and think twice about everything you pack, if there is a chance that you might not need it and its a luxury then leave it, every ounce of gear you don't need to take is that much more water and food you can bring.

I do not trust any backpacking filters anywhere in a river that sees run off from farms and industry. I use them, BUT only when I'm in the mountains and higher country like out west or up in NH and Vermont. They filter bacteria and such, but they do not filter toxins or chemicals that are in the water, if in doubt and you have to use river water then boil the ever lovin crap out of it, but still its what you can't see that can get ya.