What was “the first of two pivotal setbacks” McCandless encounters that lead to his death?

I think that would be when Chris comes across a three-acre lake covering the trail, where before there was only ice and small ponds. He manages to climb around this, but when he reaches the Teklanika River, he finds what was easily crossed in April is now a rushing torrent which would surely drown him, so he turns back, hoping if he waits it out it will again become crossable.