why did he have to be ''keenly observant'' about changes in the creek?

The man needed to be "keenly observant" so he wouldn't step into one of the springs that never froze no matter the temperature.

"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. He knew that the coldest snaps never froze these springs, and he knew likewise their danger. They were traps."