Rashi: The land was overturned via His Shelichus. He flooded a third of the world in the generation of Enosh.

Malbim: Hashem's actions cannot be reversed. The verse discusses a matter and its opposite, for one causes the other. Hashem withholds water in one place and it dries, and the vapors rise and go elsewhere. So in man, sometimes destruction is a reason for closing and guarding.