What kind of device could I use to float my dog down a river with me in my kayak?

What about just a lifejacket that will assist him in swimming along? With some type of harness I am sure he could be pulled along as well.

I'd be really leery of doing something like this. I think it's a really bad idea. If you have him on a rope or leash of some sort, and you get snagged on a branch, you might drown your dog, or tip you out of the kayak. Then you're in no position to help him, and maybe not yourself. And are dogs any good at endurance swimming? You'd probably be better off to take the dog out on a small lake or other flat protected water. You could use your kayak to paddle around that immediate area, but I wouldn't try to take him more than a hundred feet from shore. If he gets tired, how will you help him without flipping yourself? If you're holding him up out of the water, how do you paddle? If this is all going on when the wind or waves kick up, you might get blown out away from shore. There's lots of things that can go wrong here. If you have to take your dog with you, I'd think about doing it in a canoe, not a kayak. And only if the dog will sit still and not move around. I've been in a canoe with a big dog - it was an irish setter. And when he moved around to look over one side or the other, he almost upset us several times. He finally did when we came across some ducks. If you really want to do this, be sure both of you are wearing PFDs. Yes, they make them for dogs.

You could attach a blow up kayak for the dog, however, what about when you "roll", wouldn't your dog get dumped, and be in danger of death from the cold water, or being thrown against a rock? I live on the McKenzie River in Oregon, there are people taking dogs out, but they have life jackets on, and they are secured in drift boats. I have seen dogs go through the rapids with their owners, but they were smaller dogs, and essentially in the kayak with you. They make a product for humans, to be pulled behind a boat, but they lay on their stomach and hold onto a rope. The dog could lay on this thing, and you could attach the dog's life jacket to the cord that the person would normally hold.