In this passage, the man's reaction can best be described as
Read the passage from Part 1 of "To Build a Fire."
Once, coming around a bend, he shied abruptly, like a
startled horse, curved away from the place where he
had been walking, and retreated several paces back
along the trail. The creek he knew was frozen clear to
the bottom-no creek could contain water in that arctic
winter-but he knew also that there were springs that
bubbled out from the hillsides and ran along under the
snow and on top the ice of the creek.