There are two takeaways from this story:
(1) always know the risks you are getting into - while she was an experienced outdoorsperson, the author clearly had little to know awareness of the dangers of moving water.
(2) statistically, things happen outside your control - and our natural response is to pretend it can't happen to us because we follow the rules, but that's not true. Sometimes lightning hits you, or the drunk driver slams into your car, or your foot slips in a place where there's no margin for error.
To me, (2) is the more important point, and can be lost in the harrowing story that she uses to make point (1). You can do everything right, whether that is in the outdoors or driving on a highway, and still become a fatality. When we pretend it couldn't happen to us, because we are better prepared, respect the environment, etc, we are kidding ourselves.
The outdoors may bring this into starker focus because the margin for error is lower, but the point applies throughout our lives.