Arty: Only Proto-graveminds.
And they still can't go under a river. Te current would drag them away.
It depends; the current could serve to drag them in the direction they're aiming for. And not all rivers have strong currents all the time. What we do know is that any stagnant water should be possible for the flood to traverse, example = swamps.
The silver crest of the gleaming point of the tool wishes you freedom from a retched existence.