River

A river is a long body of water that flows. Due to gravity, rivers must flow from a higher elevation to a lower elevation. They often empty into an ocean, an inland lake or another river. Streams often feed rivers; such feeders are called tributaries. There are many rivers here on this earth. They consist of water, and hopefully not too much pollution.

The Nile, a river which has its basin in Egypt and Sudan, was considered the longest river in the world, until the Amazon river took its place as more accurate measurements were made, tracing the river back to its source in the Andes.