Why do you believe great civilizations developed in river valleys?

As you read the first lesson, you will discover that several successive empires developed in Mesopotamia, the land between the Tigris and Euphrates Rivers in what is now Iraq and along the Nile River in what is now Egypt, several Asian civilizations developed along the Yangtze and Yellow Rivers in ancient China and the Indus River in ancient India, and successive civilizations developed in Mesoamerica (Central America). While not all great civilizations developed in river valleys, many did, especially in the early stages of human civilizition. Why do you believe such great civilizations developed in these river valleys?