From Into the Wild, what forced Chris to return to the bus after deciding to leave the wilderness and rejoin civilization?
After living in the broken-down bus for almost four months, Chris found himself at peace and decided that he had realized the personal growth he needed to return to society. Aside from the failure to properly prepare a moose, thus losing all the meat, he had lived more-or-less comfortably, but with Fall approaching, he hiked towards the road to hitch a ride back to civilization. Unfortunately, he found that the Teklanika River flooded:
...sixty-seven days earlier... it had been an icy but gentle knee-deep creek, and he'd simply strolled across it. On July 5, however, the Teklanika was at full flood, swollen with rain and snowmelt from glaciers high in the Alaska Range, running cold and fast.
(Krakauer, Into the Wild, Amazon.com)
Chris returned to the bus to wait until the river subsided, but found himself unable to hunt and gather enough food to keep himself healthy. He slowly starved to death, not knowing that he still had had options to survive. A hunting cabin near his bus contained food, although most of it had spoiled, and if he had walked upstream he would have found shallower waters as well as a hand-powered cable-car. However, Chris believed himself capable of surviving the remaining months, and so made no other attempt to escape.