Beginning life in the mountains, rivers form from streams created through precipitation or springs of water that are sourced from groundwater that has percolated the earth. These streams, known as tributaries, then flow rapidly through V-shaped valleys, over rocky terrain and over rock edges as waterfalls. This is the first of three stages any river goes through and is known as the upper course, or youth.
By the second stage – known as the middle course, or maturity – many tributaries will have joined together to form the main body of water that makes up the river. The river meanders at a medium speed across narrow flood plains, which are areas of flat land lying either side of a river. Flood plains are formed when successive flooding causes sediment to be deposited on the banks.
As the river follows its course it carries with it a load, which is made up of rocks, stones, sand and other particles. It is the load that causes erosion as the materials crash against the banks of the river. The load is transported down the river in four ways, depending upon the size of the material.
Traction is the rolling of the largest particles across the riverbed, whereas saltation is the bouncing of those slightly smaller. Finer materials are carried along through suspension and some are dissolved within the water and are moved through solution.
The final stage of a river is the lower course, predictably sometimes known as old age. By this time the river has slowed considerably as it heads towards the sea across broad flood plains, finally ending at what is known as the mouth, where the river finally joins the ocean. Deltas are formed as the river then deposits its load.
Deltas, estuaries and the river mouth
The mouth of a river signifies the end of its course and is where the river meets the sea. The ‘D’ shaped area of sediment that forms at the river mouth is called a delta. Deltas are built up from the bed as the river slows and deposits its load as it reaches the end of its course. The river tends to split as it travels over a delta.
Estuaries are also found at the mouth of a river. In these areas the fresh water of the river meets and mixes with the salt water of the sea. Estuaries are affected by the tide, and the combination of salt and fresh water provides a diverse habitat for many plants and animals.
Oxbow lakes are crescent- or horseshoe-shaped lakes situated at the side of a flowing river. They are formed from river meanders and are the result of lateral erosion cutting into the bends of the river’s course where the river is flowing at its fastest. This eventually leads to the two bends joining together and altering the river’s course. Deposition also plays a role as sediment builds up on the outside of the bend where the river flow is much slower. As the river breaks through and the bends join, the sediment builds up to cut off the meander and an oxbow lake is formed.
Largest river in the world – Amazon – Location: Brazil. Containing 20 per cent of the planet’s fresh water, the Amazon is the largest river in the world based upon the volume of water it carries.
Holiest river in the world – Ganges – Location: India. Hindus make pilgrimage to India’s largest river south of the Himalayas to bathe in its water, which is believed to wash away sins.
Shortest river in the world – D River and Roe River – Location: USA. A long controversy resulted in two 200-foot rivers being awarded the title of shortest river in the world.