Why do you believe such great civilizations developed in river valleys, history help

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.  When we study the origins of Asian civilizations, we will discover that significant civilizations developed along the Yangtze and Yellow Rivers in ancient China and the Indus River in ancient India.  While not all great civilizations developed in river valleys, many did.  Why do you believe such great civilizations developed in these river valleys?