See http://www.dpreview.com/ for everything you ever wanted to know about digital cameras. I think the EOS 350 is the lightest weight true SLR on the market. Do give a little thought to how you plan to keep it dry. I don’t think any of the SLRs are weather proof. Plus, look at the Olympus Stylus cameras (the 810 looks really good to me, but not available until March). Not as good as a SLR in the picture taking abilities, but far more robust than any SLR you can take! And the pictures you can take with such a small camera can be remarkable good.
Whatever you decide to take, figure out a way to tie the camera to your shoulder straps or something. Make sure you have enough mobility to use it, but short enough to keep it out of the river if you drop it. I managed to drop my Canon SD500 in the water on a January hike this year. First time I’ve ever dropped a camera in the water in ten years of hiking.
I only saw one person with a true SLR on my 2004 through hike. It was at the beginning of the hike. When I saw him later in the Smokies, he had ditched it in favor of a smaller point and shoot camera. I think they were both Nikons.
Bear Bag Hanger