[Gandalf: ]Long I fell [into the chasm of the Bridge of Khazad-dûm], and [the Balrog] fell with me.
LR III 5
According to Physics, their freefall velocity will increase with a factor of
g for every second they fall. So, if I see it correctly, by the time they reach the bottom, they would have enough of a velocity to squash them flatter than a pancake (even if it was water that they hit).
Retardation of freefall due to wings, anyone?