Rivers notes

Rivers photos

Rivers quiz

Rivers keywords

Rivers are part of the HYDROLOGICAL CYCLE which moves water from the oceans, through the atmosphere and back across the land to the oceans.Most individual rivers are part of a complex system of rivers which form a network. The area covered by each network of rivers is called a DRAINAGE BASIN. The boundary line drawn around the edge of the drainage basin is called the WATERSHED. It is formed by a ridge of higher land. The SOURCE is where the main river channel begins, usually high in the mountains. It may start in a marshy hollow, a lake that overflows, or as a spring with water bubbling out of the ground. As water flows downhill along the main river channel towards the sea, it will be joined at CONFLUENCES by water from many smaller TRIBUTARIES. A drainage basin with many tributaries is said to have a high DENSITY of streams and may be more likely to flood than one with a low drainage density. The course of a river is rarely straight and most rivers have bends or MEANDERS which become more common as the river nears the sea. At the river MOUTH, the river meets the sea and the fresh water and the sea water then mix in the ESTUARY where tides will alternately reveal mudflats or salt marsh and then cover them over again.

In the UPPER COURSE of a river, the valley cross-section is narrow and V-shaped and the river occupies the whole valley. Most erosion at this stage is downwards (VERTICAL EROSION). Further downstream, in the MIDDLE COURSE, the river valley becomes wider as LATERAL EROSION begins to erode back the valley sides to form steep RIVER BLUFFS. In times of flood, water will overflow the channel and spread across the flat valley floor to form a FLOOD PLAIN covered in fine mud and silt called ALLUVIUM. In the LOWER COURSE, the river valley is very wide and very flat. The river winds its way across the flood plain in huge meanders, some of which may be cut-off in times of flood to form OX-BOW LAKES. These sections of abandoned river channel will eventually dry up and disappear.

If you study the LONG PROFILE of a river which shows a cross-section from source to mouth, you will see that the GRADIENT of the river channel starts off very steep but gradually becomes more gentle towards the sea. Although there is a natural tendency for water to flow fast down the steeper gradients, it is prevented from doing so in the upper course by the fact that the channel is very rough and FRICTION uses up about 95% of the ENERGY of the river. In the lower course, in spite of the gentler gradient, the river speeds up as the main channel is wide and deep with few natural obstructions. Rivers erode the land down to the lowest possible level which is sea level or BASE-LEVEL. If sea level falls, the river is able to cut downwards even more and often cuts into its own flood plain to form RIVER TERRACES. If sea level rises, the lower course of the river will be flooded by the sea to form drowned valleys called RIAS.

Rivers ERODE the land, TRANSPORT the eroded material and eventually DEPOSIT it in lakes or the sea. A river is able to erode by:

• HYDRAULIC ACTION resulting from the weight of the water hitting the banks
• ABRASION resulting from the rocks scraping along the river channel
• CORROSION resulting from the dissolving action of the slightly acidic water
• ATTRITION resulting from the rocks hitting against each other and becoming smaller and more rounded as they move downstream

Rivers transport material by:

• TRACTION where large rocks roll or are pushed short distances along the stream bed
• SALTATION where pebbles bounce along the bed of the stream often dislodging more pebbles
• SUSPENSION where tiny particles of mud are carried by the strong current for long distances
• SOLUTION where material is dissolved in the water and moves invisibly along

A river will deposit material when it no longer has enough energy to carry on transporting it. Energy is reduced when water goes around the inside of a meander bend, when it goes into a lake or the sea. The coarsest material is deposited first and the finest material may settle out only when the water is virtually still. If the sea currents do not remove the material, a DELTA may form in the mouth of the river.