Final answer:
McCandless' reaction emphasizes the smallness of the settlement compared to the harshness of winter.
Step-by-step explanation:
McCandless' reaction to the flash flood at Detrital Wash on page 29 of the book 'Into the Wild' can be summarized as emphasizing how small the settlement was compared to the harshness of winter. The passage describes how the flood swept away what little shelter McCandless had built, leaving him exposed to the elements. This highlights the vulnerability of the settlement and the harsh conditions McCandless faced in his quest for solitude in the wilderness.