It’s a book I read in the 1970s (but it may have been published much earlier).  The central character is a family’s pet–a cat I think, but I’m unsure.  Maybe a dog.  The family lives on a river, maybe the Mississippi.  (My family was living on the river at the time, and the story may have been presented as a “local” tale.)  By accident one day the pet floats away on a raft.  Far downstream it comes ashore.  The heart of the story is akin to Lassie-Come-Home:  the pet struggles to return to the family.  In the closing pages I believe it does return to the house–but the family is just leaving and somehow there is no reunion.  Possibly the pet is even in the yard as the family packs out, and it hears the son’s voice as he laments that the pet never got back.  I could be imagining that last bit, but I’m pretty sure there’s no reunion.  I recall weeping at the near-miss, but all the other details are fuzzy.