Rivers

A river is a natural waterway that transits water through a landscape from higher to lower elevations called divides. The divide determines which way a river will flow. 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.

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.

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.

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. The area drained by a river and its canals 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.

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. Anastomosing 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 visualise rivers, it is important to recognise 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. A river with a steep gradient that has very few tributaries and flows quickly is called a youthful river. Its channels erode deeper rather than wider. A river with a gradient that is less steep than those of youthful rivers and flows more slowly than youthful rivers is called a mature river.

A mature river is fed by many tributaries and has more discharge than a youthful river. Its channels erode wider rather than deeper. A river with a low gradient and low erosive energy is called an old river. Old rivers are characterised by flood plains. A river with a gradient that is raised by tectonic uplift is called a rejuvenated river. Most rivers flow on the surface. However, other rivers may flow underground in caves or caverns. Such rivers can be found in karst regions with limestone geologic formations. An intermittent river (or ephemeral river) only flows occasionally and can be dry for several years at a time. These rivers are found in regions with limited or highly variable rainfall, or can occur due to geologic conditions such as having a highly permeable riverbed.

Man has been using rivers for navigation since the dawn of civilisation. Rivers provide the cheapest means of transport. Rivers have been used by man since the dawn of civilisation 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. 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 regions 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 in sanitary) means of disposing of waste. The rocks and gravel generated and moved by rivers have i 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. Fast flowing rivers and waterfalls are harnessed as sources of energy, via watermills and hydroelectric plants