The book I’m looking for would have been written before 1990. It would be fiction it would be about a prehistoric tribe that believes that although there were once other people (the lake people or the people across the lake) they are the only people left (they’re either ‘the people’ or the people of the forest, forest people, etc.). One of the main characters would be a girl named Zillah or a variant thereof. She has an overbearing father and at the end of the book she either dreams she sees or actually sees a person in a boat floating towards her on the lake.