Rivers

Rivers are filled with water from rainfall running directly off the land, from melting snow or ice or from a spring bubbling out water that is soaked into the ground.

High up into the mountains near their source, rivers are usually small. They tumble over rocks through narrow valleys which they carved out over thousands of years.

All the rivers in a certain area, called a catchment area, flow down to join each other, like branches on a tree. The branches are called tributaries. The bigger the river, the more tributaries it is likly to have.

As rivers flow downhill, they are joined by tributaries and grow bigger. They often flow in smooth channels made not of big rocks but of fine debris washed down from higher up. River valleys are wider and gentler lower down, and the river may wind across the valley floor.

In its lower reaches a river is often wide and deep. It winds back and forth in meanders across floodplains made of silt from higher up.

Rivers wear away their banks and beds, mainly by battering them with bits of gravel and sand and by the sheer force of the moving water. Every river carries sediment, which consists of large stones rolled along the river-bed, sand bounced along the bed and fine silt that floats in the water.

The discharge of a river is the amount of water flowing past a particular point each second. Rivers that flow only after heavy rainstorms are "intermittent". Rivers that flow all year round are "perenial" - they are kept going between rains by water flowing from underground.