those torrential rivers

I think it may be true that fortune determines one half of our actions, but that, even so, she leaves us to control the other half, or thereabouts.

And I compare her to one of those torrential rivers that, when they get angry, break their banks, knock down trees and buildings, strip the soil from one place and deposit it somewhere else. Everyone flees before them, everyone gives way in the face of their onrush, nobody can resist them at any point. But although they are so powerful, this does not mean men, when the waters recede, cannot make repairs and build banks and barriers so that, if the waters rise again, either they will be safely kept within the sluices or at least their onrush will not be so unregulated and destructive. 

The same thing happens with fortune: She demonstrates her power where precautions have not been taken to resist her; she directs her attacks where she knows banks and barriers have not been built to hold her.

Niccolò di Bernardo dei Machiavelli

Resilience: Hard-won Wisdom for Living a Better Life by Eric Greitens. Houghton Mifflin Harcourt. 2015. p.160