Hi..
i am assuming you feel you dont already have something suitable for the trip,, so i would consider the trip and beyond.. ie, future use out of the bag/s you buy...
certainly you need to give thought to what the guys have already said about using a combination of dry bags and a daypack, or a larger load hauler and dry liners.. you will know how much walking there is to be done gear upi for this and what you will also get the most use out of after the trip..... if there is no real walking to be done take some ortlieb or similar dry bags, these are very comfy also when loaded, to relax back on in the boat on undemanding stretches of river..
i would always recomend something of a sturdy construction, ie, some army bergans, karrimor sabre range, etc.. they will give you the blend in colours if that matters to you, avoid much of the high street outdoor flyweight packs and materials. for more rugged use consider crux, macpac especially their aztec materials, very bombproof and weatherproof, fjallraven, karrimor sabres etc....
all depending on what you wish to spend... and wether you will get use out of it in the future .
fuel for thought.