I can't help much as I have not kayaked the Rio. All I've got for you is third-hand info. I had thought about kayaking the river a time or two and my recollection is that there are some obstructions in the river that require portaging. You might research that.

Also, I remember a warning of the following: I guess the border is in the middle of the river, so if the Mexican National Police see you in the river they can pretty much detain you at will and say that you crossed the border. I don't know if this happens, has happened, etc. I just remember reading the admonition and warning.

Finally, take water with you. Without researching first, don't filter the water in the river for drinking. I recall reports of heavy metals being dumped on the Mexican side of the river up near El Paso. The best PUR filter isn't going to protect you from that.

Finally, do you kayak much? Are you comfortable making headway up river? For how long and how far? Does the canyon flood easily with late summer and early fall thunderstorms? The park is desert but it does rain there and flash floods occur in the park. The walls of the canyon trap water and make the water rise quicker and faster.