Not really a viable option for me.
Where would I place the ball? If it was at where the lake should be, I have not yet created the lake or determined its size and filled it up, so it would fall down into where the lake would be (an empty valley). Also, remember these are cubes/tiles, not actually terrain, so they are flat on the top, the ball would not actually move.
But even if all those issues got resolved, this is a 2048x2048 size map. Those are waaaaay too many entities to load (I only load large portions of the map for test purposes to see what it looks like, but at 3 fps), it would take way too long to do this at runtime for the player. Even if it were a small map that had 512 fps this aproach is too slow, but considering doing this at 3 or less fps is impossible.

I would prefer a solution programmed myself, not using somebody else's (physX) algorythim. And I am looking for game logic that I can program myself to do the calculation mathematically on my height data withought loading the level. which would be much faster, virtually instantaneous (in only 1 frame, even though it might be a relatively slow frame).

I actually never load the level, I use a FOV+LOS algorythm to only load a relatively small amount of tiles visible around the player. And I dont want to load all of them for my river/lake calculations either. The amount of tiles (4.2 million only counting the ground tiles) is crazy!

EDIT:
fov= field of view
los= line of sight