Some valleys are almost flat, like a large saucer. Valleys of this type often have a very large river with many "tributaries" (streams that are like branches) running through them. The "tributaries" carry water from the hills or mountains that may be far from the main river. After heavy rain, lots of water rushes into the main river so that it rises and floods over the flat floor of the valley. When the flood waters spread, they drop lots of soil which has washed down from the hills. The soil that drops on the valley floor makes a flat flood plain. Valleys of this type are very useful for growing food crops. The widest valleys in the world are like this. Famous valleys of this type are the Mississippi-Missouri Basin in North America, the Amazon Basin in South America, the Lower Danube in Europe, the Ganges River Valley in India, the Nile River Flood Plain in Africa and the Darling River Basin in Australia. 