In the book ALWTW is there any evidence through out the book that shows and explains how small events can lead to bigger changes?

Any war is not a small event. Civil War is what drives the narratives of both protagonists. The changes happen because violent conflict displaces Salva and complicates Nya's life. I suppose that Salva bringing clean water to Nya's village at the end of the story brings big changes to Nya and her family. She no longer has to walk half a day to find water.