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The source of every river is its spring. This is generally found in the higher grounds of amountain range, which receive relatively large amounts of precipitation. The source of the river may also be sometimes the melting water of a glacier.
When the ground is saturated with water, the water starts running down in tiny rivulets. In glacial regions, water usually collects inlakes and then flows down to the valley. Little channels and rivulets, first formed by the water, ultimately join into one water stream. This stream often uses troughs formed by glaciers or tectonic movements.
This is how a riverbed is formed. As the speed of the water increases, the water carves deeper into the rock and soil. The riverbed is literally carved out by sand and gravel washed out by water. This process is called abrasion. Valleys with more or less sloping walls form alongside the trough or channel. Even the hardestrocks in the vicinity of the riverbed may be eroded by the constant movement of washed out stones.
The channel of the stream may run straight, but may also be intricate. In general, a river in the valley floor moves in irregular direction. It often divides into various channels. A riverbed may be situated in the middle of a valley meadow or alongside its edge. In addition to straight river flow we have a meandering and braided river flows.
Meander (in Greek, Maiandros, a river which in the antiquity was known by its many curves), is formed by a river channel winding in its valley meadow as a result of deep erosion caused by the river in the rocky bed. Lateral movements are caused by erosion and sediments.
On the outside of the bend, called contact slope, thecurrent is the strongest. This is where erosion occurs. On the other side, so-called slide slope, is where sediments are deposited. With lateral movements and downward movement, the river washes the sediments on its outer bend and forms a sandy and muddy area, creating a natural dyke.
Braided rivers form, when the river stream divides into several channels. They are found in a completely different areas, such as plateaus or regions rich in glacial sand. They are generally found in areas of steep slopes, and in the presence of fast-changing volume of water running through and loose sediments. A typical form represents a wide, shallow riverbed with many fast-shifting sediments of sand. When the river ultimately reaches the ocean or a lake, the speed of its stream decreases. The alluvium, or sediment, brought by the stream of water, creates large deltas, and fills lakes and ocean bays.
An average river is divided into three parts: upper river, middle river, and lower river (or upstream, midstream, and downstream). Upstream, the major force is a deep erosion. Here, the water forms a valley or a gorge with the help of coarse rock. In the middle, erosion and accumulation are approximately balanced. In the lower part, or downstream, where the water flow is gradually slower, is the place where lateral erosion and especially accumulation occur.
Rivers not only take excess water to the sea, they also change the shape of the earth by transporting and accumulating sediments. By the grinding action of the water current, the river may produce deep furrows in the surface of the earth. Fragmented, loose stones are carried by the current and are deposited elsewhere. This process is called accumulation.
The rate of accumulation depends on the slope of the water flow and the volume of water. The faster the current, the higher is the transporting force of the current. The current transports sediments in three ways. First, by traction, that is to say, by dragging or moving them on the bed of the river stream, with occasional saltation. The second one is by suspension, when particles circulating in the turbulent running water are transported by the stream. and third, in the form of solution, when water transports material resulting from the chemical corrosion of rock, together with atmospheric particles, found inrainwater.
During their transport, pieces of rock are ground, they get smaller, sometimes they are dissolved. Some 20 kilometres later, even hard pieces of quartz turn into round pebbles. In the lower part of the river, the traction type of transport disappears, only suspended particles and dissolved materials are still carried on.
In the last few decades, the humans had a major impact on the volume and quality of the material transported by rivers. Material carried by the water is captured by dykes, which decreases the amount of sediments. The increase in the transported material is caused, for example, by clearing of forests, tilling of fields, or by construction activity, and all these are linked tosoil erosion. The quality of the accumulated sediments changes due to the industrial removal of waste, use of chemical fertilizers, as well as the increase of dissolved salts. As a consequence of human activity, millions of tons of these materials reach the oceans every year.
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