A river is a natural waterway that transits water through a landscape from higher to lower elevations. It is an integral component of the water cycle. The water within a river is generally collected from precipitation through surface runoff, groundwater recharge as seen at baseflow conditions during periods of lack of precipitation and release of stored water in natural reservoirs, such as a glacier.The area drained by a river and its tributaries is called catchment, catchment basin, drainage basin or watershed. The term watershed is also used to mean a boundary between catchments, which is also called a water divide, or in some cases, continental divide.Excessive abstraction of water for use in industry, irrigation, etc., can also cause a river to dry before reaching its natural terminus.The mouth, or lower end, of a river is known by hydrologists as its base level.
A river may have its source in a spring, lake, from damp, boggy landscapes where the soil is waterlogged, from glacial melt, or from surface runoff of precipitation. Almost all rivers are joined by other rivers and streams termed tributaries the highest of which are known as headwaters. Water may also originate from groundwater sources. Throughout the course of the river, the total volume transported downstream will often be a combination of the free water flow together with a substantial contribution flowing through sub-surface rocks and gravels that underlie the river and its floodplain called the hyporheic zone. For many rivers in large valleys, this unseen component of flow may greatly exceed the visible flow.From their source, all rivers flow downhill, typically terminating in a sea or in a lake, through a confluence. In arid areas rivers sometimes end by losing water to evaporation. River water may also infiltrate into the soil or pervious rock, where it becomes groundwater.
The water in a river is usually confined to a channel, made up of a stream bed between banks. In larger rivers there is also a wider flood-plain shaped by flood-waters over-topping the channel. Flood plains may be very wide in relation to the size of the river channel. This distinction between river channel and flood-plain can be blurred especially in urban areas where the flood-plain of a river channel can become greatly developed by housing and industry. The river channel itself typically contains a single stream of water but some rivers flow as several interconnecting streams of water, producing a braided river. Extensive braided rivers are found in only a few regions worldwide, such as the South Island of New Zealand. They also occur on peneplains and some of the larger river deltas. Anastamosing rivers are similar to braided rivers. They have multiple sinuous channels carrying large volumes of sediment. Due to the dynamics of this type of system, they are also quite rare.
A river flowing in its channel is a source of considerable energy which acts on the river channel to change its shape and form. In mountainous torrential zones this can be seen as erosion channels through hard rocks and the creation of sands and gravels from the destruction of larger rocks. In U shaped glaciated valleys, the subsequent river valley can often easily be identified by the V shaped channel that it has carved. In the middle reaches where the river may flow over flatter land, loops meanders may form through eroding of the river banks and deposition on the inside of bends. Sometimes the river will cut off a loop, shortening the channel and forming an oxbow lake or billabong. Rivers that carry large amounts of sediment may develop conspicuous deltas at their mouths, if conditions permit. Rivers, whose mouths are in saline tidal waters, may form estuaries. River mouths may also be fjords or rias.Although the following classes are a useful simplified way to visualize rivers, it is important to recognize there are other factors at work here. Gradient is controlled largely by tectonics, but discharge is controlled largely by climate and sediment load is controlled by various factors including climate, geology in the headwaters, and the stream gradient.
Youthful river a river with a steep gradient that has very few tributaries and flows quickly. Its channels erode deeper rather than wider. Examples Brazos River, Trinity River, Ebro River.Mature river a river with a gradient that is less steep than those of youthful rivers and flows more slowly than youthful rivers. A mature river is fed by many tributaries and has more discharge than a youthful river. Its channels erode wider rather than deeper. Examples Mississippi River, Ohio River, River Thames. Old river a river with a low gradient and low erosive energy. Old rivers are characterized by flood plains. Examples Ganges River, Tigris, Euphrates River, Indus River, Nile River Rejuvenated river a river with a gradient that is raised by tectonic uplift.It has been noted that on average, the air distance euclidean distance from the beginning to the end of most rivers is about one third their actual length. For rivers that flow in plain areas, this number is very close to Pi Einstein wrote an explanation of why this is so.
Man has been using rivers for navigation since the dawn of civilization. Rivers provide the cheapest means of transport. In this figure a country boat is seen plying over Hooghly River at Kolkata, India. Rivers have been used by man since the dawn of civilization as a source of water, for food, for transport, as a defensive barrier, as a source of power to drive machinery, and as a means of disposing of waste. For thousands of years rivers have been used for navigation The earliest evidence of navigation is found in the Indus Valley Civilization, which existed in north-western India around 3300 BC. Riverine navigation provides the cheapest means of transport and is still used extensively on major rivers of the world like the Ganges, the Nile, the Mississippi, and the Indus.In some highly-forested countries like Scandinavia and Canada, lumberjacks use the river to float felled trees downstream to lumber camps for further processing, saving much effort and cost by transporting the huge heavy logs by natural means. Rivers have been a source of food since pre-history. Apart from being a rich source of fish, rivers indirectly aid cultivation by supplying water for the crops. Rivers sustain their own food chain. They are a major source of fresh water, hence, it is no surprise to find most of the major cities of the world situated on the banks of rivers. Rivers also provide an easy if unsanitary means of disposing of waste.
Most riverbanks in Japan are used as places for playing, recreation and parties.The rocks and gravel generated and moved by rivers have been greatly used in construction. In more recent generations, the beauty of rivers and their wider habitats has contributed greatly to tourist income from areas well endowed with attractive riverine scenery.In upland rivers, rapids with whitewater or even waterfalls occur. Rapids are often used for recreational purposes see whitewater kayaking. Fast flowing rivers and waterfalls are harnessed as sources of energy, via watermills and hydroelectric plants.Rivers have been important historically in determining political boundaries and defending countries. For example, the Danube was a longstanding border of the Roman Empire, and today forms most of the border between Bulgaria and Romania. The Mississippi in North America, and the Rhine in Europe, are major east-west boundaries in those continents. The Orange and Limpopo Rivers in Southern Africa form the boundaries between various provinces and countries along their routes.
The flora and fauna of rivers have developed to utilise the very wide range of aquatic habitats available from torrential waterfalls through to lowland mires. Although many organisms are restricted to the fresh-water in rivers, some, such as Salmon and Hilsa have adapted to be able to survive both in rivers and in the sea. Flooding is a natural part of a river's cycles. The majority of the erosion of river channels and the erosion and deposition on the associated floodplains occur during flood stage. Human activity, however, has upset the natural way flooding occurs by walling off rivers, straightening their courses and by draining of natural wetlands. Flooding can be mitigated.
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