Another way to look at it is that he knew there were unknown risks of various kinds, and he decided to prepare as well as he could and go anyway.
IIRC there was a ferry or similar a few miles upstream of where he was "trapped" (because he didn't expect the river to swell as it did) and died.
Learning about seasonal weather and checking alternate routes are a basic part of wilderness survival.
The only thing sadder than McCandless pointless depth is the people whose own lives are so devoid of meaning that they seek inspiration from a fool who killed himself.