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Activity of the rivers

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the Rivers participate in landscape formation. They wash off soil, destroy stones and transfer sand, a pebble and cobble-stones downstream. These waterways can even change a direction, cutting off bends in the usual channel and forming fresh lakes.

Any river has the beginning (source). It is a spring - a place More often, whence it makes the way a thin stream from under stones. This streamlet replenishes with the rain water which is flowing down from nearby territory - pool, - and overcoming many thousand kilometres, the deep river runs into the sea.

The Current of the rivers depends on a landscape. On different sites of a way from a source to a mouth (to a confluence place, for example, in the sea) speed of a current is not identical. activity of the rivers Many rivers originate in mountain district, whence a fast stream flow down downwards. In Norway and lengthways andskih coastal territories of the South America of the river short and fast. Young mountain streams so rough what to adapt for them it is possible only to few water plants and fishes. Widespread enough phenomenon here are falls. Water there always pure and cold, and the river bed is covered by a pebble. On this site the river usually flows on a bottom of abrupt V-shaped gorges and valleys, forming zhivopisnejshie landscapes.

In an average part the river slowly flows on flat district, sheltering many kinds of plants and fishes. Its bottom is covered by small gravel and ooze. It becomes wider and is deeper, as bears in itself waters of neighbouring streams and water of inflows.


Flooding and erosion

In the bottom current the river even more slowly. Its coast become flat and if owing to a torrential rain or snow thawing the water level considerably rises, it overflows banks and zataplivaet the narrow coastal zone named pojmoj. On the way to the river sea destroy a stone and cut valleys. They move a ground, stones and other breeds. Flowing water possesses considerable force, but bearing ability at it insignificant. Stones and small splinters which are transferred by water, strengthen its abrasive (destructive) effect. Force of flowing water lifts everything that lies on a river bed and its coast. Stones in water chaotically hit about other stones and about coast. In a fast and chaotic current the big stones crumble on small parts. Even the small material, such as sand or silt, possesses abrasive properties, like a cleaning powder used in house conditions. Under their influence sharp parts of stones grind off, turning in the course of time to a smooth pebble.


Capacity of the river

Capacity of the river substantially depends on volume of water and from a bias. For example, the small mountain streamlet sometimes turns to the prompt stream, capable to move huge boulders when owing to thawing of snow or a storm the water volume quickly increases.

The Slow flat rivers sometimes become fast when owing to tectonic movements earth level raises, increasing a stream bias. Such rivers form new deep valleys. Probably, vivid example of activity of the rivers is the Grand Canyon in the southwest USA. This huge gorge in a rock was stretched on 450 km, and the maximum depth of the canyon which has been cut out by waters of Colorado, makes 1,6 km.

For millions years surface level on which the river flows, has raised. As the river of Colorado laid itself a way through rocks, the earth rose, and the channel all became deeper and deeper. Scientists consider that for all this time the river had to cut almost 3 km of rock on the way, and 1,4 km of its top layer have been completely washed off from a plateau.


Carrying over of materials

The Size of pieces of the breed transferred by water, depends on speed of a current. At speed in 30 km/h the river can move even huge boulders which are rolled on a bottom. The water flowing with speed of 10 km/h, moves a pebble. At speed the river can transfer to 0,5 km/h only sand and silt. Liquid, i.e. Dissolved in water, materials also a feather carrying a watercourse. Water is capable to dissolve breed, especially such soft and pliable, as limestone.

When the river reaches flat district, force of a stream not always suffices for the further transportation of the collected breed. Therefore it gradually postpones the "эю°ѕ" on a bottom. The big stones settle faster, small lay down on a bottom later.


Branching

Adjournment of the flat river obrazovyvajut the shallows in turn forming a network of small, moving sleeves. This process is called as branching. Many the branching rivers around Great Plains in the North America.

One more form of adjournment is observed, when the river from mountain district flows down on plain. It can spread and arrange deposits in the form of a fan. Such adjournment are called as carrying out cones.

Zataplivaja the neighbouring earths, the river usually layers layers near to the coast. The coast exceeding level of plain are as a result built. Such coast name natural priruslovymi shaft. Very often level of the river proceeding between such shaft, is considerably above plain level.


Silt and delta

Hundred years ago, before building of a high-rise Aswan dam in Egypt, Nile annually zataplival inundated arable lands along the coast, leaving a layer of fertile silt. Knowingly ancient Egyptians worshipped to Nile as to a deity-supporter.

The silt Brought by Nile reached sea coast. There it collected, creating delta - flat district where the river spread on several channels. From the form of delta of Nile reminding the Greek capital letter (delta), also the name of such formation has gone. Today the most part namytogo Nile of silt accumulates at the bottom of lake Naser behind the Aswan dam. As a result of it the coastal line of delta of Nile gradually recedes.


Types of deltas

The Rivers can create three kinds of deltas depending on relative density of their water and density of water of the sea into which they run. If because of cargo of deposits river water is more dense sea the delta is extended. If river water of approximately identical density with sea, is formed the triangular delta similar nilskoj. If density of river water more low the delta is formed mnogorukavnaja. The delta Mississipi in Louisiana (USA) has such outlines, for example.

On aerial photographs of the big rivers, such as Amazon and Mississipi, the cone of the decoloured water stretched in ocean is visible. Decolouration is caused by the weighed particles of a ground. Annually Mississipi brings to Gulf of Mexico about 700 tons of materials. For millions years these ocean deposits which have been washed up from old rocks, are condensed and harden, forming new sedimentary breed.

The Most part of deposits is washed away from a surface of continents. Annually the world rivers bring in the sea about 8000 million tons of materials, 77 tons of soil on 1 км2 are thus lost. For example, the rivers USA move enough material to lower a country landscape at least on 6 sm everyone 1000.


Meanders

If erosion lasted with the same speed during any 14 million the years, all territory of the USA would appear flush with the sea. Fortunately, landshaftoobrazie occurs under the influence of many geological processes, therefore it is unlikely that continents have disappeared under ocean water as a result of erosion. Meeting a pas the way a barrier, for example mountain, water changes the direction bypassing an obstacle. The flat river can do smooth, the correct form turns - meanders. A word "ьхрэфЁ" for the first time used by ancient Greeks, has gone from the name of the river the Meander in Troy (sovr. Big Menderes in Turkey) which has fancifully twisting form.


Staritsy

From an outer side of a meander the river washes away coast, and on internal namyvaet sandy and gravelly deposits. The material on an external wall moves downstream, and in due course meanders move downwards - wander. They never are at a stop, and sometimes completely come off the basic channel of the river.

During high waters the water which has overflowed banks, sometimes connects nearby bends, the channel is straightened, and one of bends is isolated and turns to inundated lake - staritsu. At tectonic lifting of district the river deepens a channel, and meanders become vrezannymi.


the Water cycle

The Rivers make the important part of process which geographers name a water cycle in the nature. It is the process beginning with evaporation of sea water under the influence of solar heat and formation of clouds. Then they return the moisture to the earth in the form of deposits. Their big part drops out in the sea. The others get on a land, but, flowing down from heights or turning to a source, they come back in due course in the sea through river system.