River and lake landscape

As a consequence of rainfalls, the waters that runs on the soil surface forms a rivulet that, joining other rivulets, becomes a stream. While the stream runs towards the valley, it receives water from other rivers (tributaries) and runs into the track it has formed, which is called bed or riverbed.
The flow of a river has different gradients: the river is flatter as it gets towards its mouth. Suddenly, the gradient can increase if the riverbed is made of more compact and non-erodible rocks. In these areas rapids form and if the bed has a vertical gradient, waterfalls form.
Abandoning the steepest part of its flow, and entering the flat area, the river current becomes slower and a part of the transported materials deposit: in this way a floodplain is formed. In these wider areas the river can become more meandering: it can form bends or serpentines, which are called meanders. The river flows into a lake or into the sea.
River erosion
Water, while running, manages to shape the landscape, since it erodes the rocky surface it runs on, it incorporates the eroded fragments, transports and deposits them when it gets to the valley. The erosive capacity of water depends on its speed, and it is higher when the riverbed is more inclined and during floods. The river water and current transport a mixture of organic and inorganic substances, as well as salt. They also transport clay, slime and sand particles, while big bits of sand, gravel and stones roll on the riverbed.
This form of erosion is called abrasion and creates typical shapes:

  • potholes: when the current is more powerful, the river manages to dig some cracks that gradually become deeper (see image “Potholes”);
  • ravines, narrow and deep incisions that are created by the river on a compact rock, that keep vertical walls stable. Those incisions that become wider towards to top part are called gorges. Some examples are: the gorges of Alcantara, at the bottom of Etna mountain, or the fascinating Grand Canyon in the United States (See image “Ravines and Gorges);
  • river valley: by constantly carving its own bed, a water stream digs a deeper and deeper crack until it shapes a valley. This represents a wide and deep depression of the Earth’s surface, and is limited by two mountainsides.

The shape of rivers
Watercourses are classified according to the shape of their flow:

  • plaited: they are formed of numerous and small canals that separate and re-join once they arrive at the valley. In these cases the river transports big quantities of sand and gravel. This is typical of dry, semi-dry areas and piedmont areas. In Friuli region, in the northern Veneto plain, watercourses are named grave (like Tagliamento, Medusa and Cellina when they arrive at the plain);
  • straight: watercourses with this shape are very rare and are normally located close to faults and where different types of rocks are located;
  • with meanders: the water flow is narrow and with frequent curves on flat land. The movement of meanders and the slime and sand that deposit during floods form floodplains, like the Po plain.

The river flows into the sea
When a river gets to the sea, sometimes it manages to scatter the transported material. In this case an estuary is formed, like the river Thames.
When the river deposits the transported material on its mouth, a river delta is formed.
The Nile, that flows into the Mediterranean, and the Mississippi river, that reaches the gulf of Mexico, shape the coast forming a delta with several river branches. The delta of Tevere river, that flows into the Mediterranean sea, is modelled by the waves and sea currents and has a pointed shape. Instead the Seine, that reaches the English Channel, is shaped by sea tides and forms an estuary.
The delta of large rivers can extend for several thousands of square kilometres, according to the quantity of debris that are transported by the river and deposited close to the sea. A delta landscape (a river delta) is characterized by canals, lagoons, islands and isolated water basins.
Lakes
Lakes fill the depressions of the Earth’s surface and have a limited duration in time. They can be classified as:

  • river lakes, when a river plain is flooded or a river branch completely separates from the river;
  • barrier lakes, when a landslide or a lava flow interrupts a river flow. They can also originate from the deposit of rocky materials that are transported by a glacier;
  • tectonic lakes form on depressions that are created after movements of the Earth’s crust. Examples of them are the Dead sea (the most salty in the earth), the Bajkal lake (the deepest lake, 1741 metres), the lakes that occupy the Rift Valley in Africa, and the Caspian sea (an old sea that has been left isolated);
  • crateral lakes form inside extinct or exploded volcanoes like: the lake of Bolsena, Vico, Bracciano, Albano and Nemi;
  • karstic lakes when, above carbonate rocks, there is a layer of clay that makes the rocks waterproof like the Lake of Scuatari in Albania;
  • artificial lakes are built by men to collect irrigation water or to produce energy.

The evolution of a lake and the marshland
Lakes do not have a long life because they tend to be filled with sediments and be invaded by the vegetation. The first transformation is the creation of a pond, which is quite shallow. Later, a marsh is formed. The marsh is a land that is covered by a thin layer of water. These waters can be rich in natural substances that favour vegetation growth. Algae, canes and floating plants are typical of this kind of landscape. They decorate the whole water surface.

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