A river can be defined as any body of water flowing in one direction usually into a lake or the sea. The longest river on earth is the River Nile with a length of 4,160 miles although the River Amazon carries the largest amount of water, depositing 20% of all freshwater that flows into the sea.

Rivers are shaped by a combination of erosion and sedimentation which are influenced by the underlying geology, type of sediment present ,the flow rate and volume of water. A useful distinction to make is whether the river is perennial, with water being available all year round, or ephemeral - having a distinct dry season. In many hot countries rivers may be dry for the majority of the year and then have a huge amount of water flowing through them for just a brief period.