OP won't be surprised to know his example isn't well-expressed English. Better might be, perhaps,
People used to bathe in rivers. Now, they've changed their tradition - they're bathing in their homes, wasting a lot of water.
It's possible to say "The humans had forgotten the rivers" as a loose corollary/restatement of the above, but we wouldn't normally use the word humans like that unless it was in a context where humans were at least implicitly distinguished from non-humans (i.e. - extraterrestrial or fantasy beings, or animals).
But even then, using the verb to forget is rather florid/poetical/metaphoric - they haven't forgotten [that the rivers exist]; they just don't bathe in them any more. Personally, for most contexts I'd prefer...
"The people had forsaken the rivers" (OED: forsake = to abandon, leave entirely, withdraw from)
Regarding had forgot/had forgotten, it's worth pointing out that the former is now considered "non-standard", only occurring in dialectal or casual speech (the relevant OED entry for to forget says *Past participle forgotten /-ˈɡɒt(ə)n/ , (arch. and poet.) forgot /-ˈɡɒt/). Here's a chart to illustrate the shift....
