Rivers are streams of water, usually formed by the union of smaller streams, which flow from higher levels to lower levels, usually emptying into the sea or a lake, more rarely draining into dry porous soils such as many desert rivers.
If a river is in a region of impervious rock, the rainfall which does not evaporate forms surface run-off whch drains into rivers making them immediately dependent on rainfall for their water supply, as is the case in some barren, mountainous areas.
Usually, rainfall soaks into porous soils causing streams and rivers to increase and continue to flow for considerable periods after rainfall stops. It is possible that as much of half of the water of many rivers may be derived from springs flowing from higher levels underground and often from areas quite distant from the spring. Snow and ice also form reservoirs of water feeding streams with meltwater. Many rivers thus flow permanently, supplied by subterannean water seasonally replenished by rainfall and meltwater.
The course of a river is usually divided into three quite distinct sections; the torrential track or young track, with fast-running slopes as high as 50 feet per mile; the gentler valley track with gradients rarely exceeding 10 feet per mile; and the slow-flowing plain track where gradients may only be a few inches per mile. One or even two of these stages of the river may be absent.
In the torrential or young track of the river, the energy of the fast-moving water tumbling down steep gradients is suffiecient to carry large boulders as well as finer material and this has a scouring action which deepens the track of the river into a ravine and carries the material removed to lower sections of the river.
Because there is little soil in the area of the young part of the river to retain rainfall, torrents are very responsive to rainfall or its absence.
In torrential section of a river its action is destructive and contributes only to the erosion of the landscape. In the valley section, the river both erodes but also deposits coarser material carried down from above.
As the slope of the river lessens, the speed of the water reduces and with it, its energy. The loss of energy causes gravels and sands to be deposited, chiefly along the banks as terraces of alluvium, leaving the finer material in suspension to be carried further downstream as the river continues to erode the valley floor. The coarse material is depositted in a graded manner as the slope diminishes, pebbles deposited first, then gravel and, further downstream, sand.
The low gradients along the plain track or old age section of the river cause the water to move sluggishly with little energy. This section of a river is thus primarily a section of deposition as all but the finest material carried from upstream is depositted.
Any check to the movement of the water causes it to slow further depositting even more material as sand or mud banks. Where the deposition occurs at the river bank, this is built up into the stream which is deflected towards the opposite bank which is undermined, collapses and is carried away. The river's course along the plain thus becomes a winding one.
Because the deposition of material on a river bank deflects the stream towards the opposite bank causing its erosion, the tendency to form loops perpetuates itself. As the loops tighten, a point in time is reached when adjacent loops are separated by only a narrow strip of land which is eventually breached to form and island. The ends of the old loop become silted up in time, cutting them off from the main coarse of the river to form crescent-shaped lakes called "oxbow lakes".
The broad plain formed in this section of the river is frequently called a "flood plain" because the river floods the area seasonally or periodically and is subject to deposition of large amounts of material during these floods. In the case of large and sluggish rivers such as the Mississippi in North America or the Hwang Ho in Chna the river bed may become raised above the plain on an embankment with the river flowing between mud walls known as "levees". Because the flat and fertile soils of the flood plain attract human settlement and farming, the levees are often artificially strengthened to prevent the river from bursting its banks at times of flood.
When the sluggish fresh water of a river meets and mixes the salt-laden water of the sea, its capacity for carrying sediments is further reduced causing banks to be deposited across the mouth of the river, wide shallow estuaries or sandbanks. Where sea currents are weak or absent and do not wash away the deposited material, broad "deltas" are formed with the help of vegetation ("delta" is the fourth letter of the Greek alphabet which is triangular in shape - thus the adoption of the name). Typical examples of rivers forming deltas are the Nile in Egypt and the Mississippi in North America.
The form of a river valley depends to a large extent on the nature of the rocks which traverses.
In arid regions where the rock is bedded horizontally, a deep and narrow gorge is cut by the river. The finest and best-know example of such a gorge is the "Grand Canyon" of the Rio Colorado. Four hundred miles in length, the gorge is 6,000 feet deep in places.
In areas of high rainfall, the number of tributaries produces a river valley with gentler sides.
Where beds of hard rock overly softer beds in a river valley, the softer rocks are eroded away faster leaving the harder rock as a projecting ledge over which the the water pours as a waterfall. The harder rock is undercut by the erosion of the soft material beneath it causing the ledge to collapse and the waterfall to move upstream over time.
Perhaps most famous are the waterfalls of Niagara on the USA/Canadian border where hard beds of limestone overly softer shales. The erosion of the shales causes collapse of the limestone ledges causing to fals to gradually move towards Lake Erie.
As a river cuts through the harder deposits which create waterfalls, rapids are frequently created. In time these disappear as the bed of the river adjusts in relation to the surrounding land.
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The Frome and the Piddle are two chalk streams draining west Dorset into Poole Harbour.
Although it extends into Somerset and Wiltshire, the catchment area of the Dorset Stour is about twice that of the Piddle]] and Frome combined. Passing over more varied terrain, it is a very different river.
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