Suggestion: Rivers through mountains

It would be nice to have rivers going through mountains (in a tunnel or a river that cuts a mountain in half). 
Similar to what the Terra mod does. 
I don't know if you would consider this to be in the scope of this mod but it would be really nice to see in a 1.16 world.
It would be in line with what Mojang seems to be going for in the future.

Thank you for the amazing work Corgi Taco.