I was a dedicated waterfowler for years. I am WELL aware of what river ice flows, -20*C ambient air temp, and long swim distances will do to dogs.I think you’re underestimating the drive and determination of dogs. The saying “work like a dog” didn’t just happen one day, it was formed by years and years of dogs exhibiting great drive, often working themselves to death.
Even if the dog zig zagged, which it certainly would have, covering 100-200kms in 35 days is most definitely possible. Changing total distance to 200kms or 5.7kms daily only increases daily walking time to still only 1-2 hours. Again, very easy for a large breed dog.
Golden retrievers were specifically bred for swimming in water and are cold hardy. Most people with them have a hard time keeping them out of the water, even in the middle of winter. The rivers are irrelevant.
It seems impossible, until you look at the numbers. Then it seems very easy to travel that far in 35 days.
I also ran bears and cats with hounds for years, growing up in a hound household.
I currently have a heeler that spent his life working livestock until he went blind, that was lost for weeks at -35*C. That dog would travel 30+ km a day based on visual reports, but he stayed close to where he was lost.
The amount of ground a dog can travel in the course of a week can’t surprise me.
A dog trying on its own to cross a fast moving river that is hundreds of meters wide WOULD surprise me. A malnourished dog SURVIVING that swim is even more unlikely. I’d have an easier time believing that a golden eagle picked him up and flew him across the river, and gently put him down on the edge of the river valley.