Water is a problem for human communities when there is too little and when there is too much. Humans depend on the availability of an enormous amount of fresh water. In the 1930s, drought struck the American Great Plains, forcing three million people to abandon their farms and a mass migration to other areas of the United States. Chapter 16 discusses the need for fresh water but also the consequence of having too much water as rising sea levels and changing rain patterns threaten to flood heavily populated regions.