The bottom of the Grand Canyon should be mild in the fall, so I don't think that should be an issue on the river. However, I was on the waiting list for 10 years before I got my private permit to run the river. Commercial outfitters should have spaces for two on a fall trip. It's 18 days to do the whole thing on an oar raft. Less on a motorized raft, but less fun too. You can do half by hiking in or out at Phantom Ranch. I think some outfitters may helicopter people in or out, but I'm not sure if that is still permissible. A commerical river trip will be pricey, probably around $3k per person I think. It's definitely a great trip though and fall is a better time than summer. I ran it in July and it was 113 degrees at Phantom Ranch and about 120 degrees in the inner gorge. Jumping in to 48 degree water at those air temperatures is unnatural.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13