River

A river is a natural flowing

watercourse

, usually freshwater, flowing towards an

ocean

,

sea

,

lake

or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of water. Small rivers can be referred to using names such as

stream

, creek, brook, rivulet, and rill. There are no official definitions for the generic term river as applied to geographic features, although in some countries or communities a stream is defined by its size. Many names for small rivers are specific to geographic location; examples are "run" in some parts of the United States, "burn" in Scotland and northeast England, and "beck" in northern England. Sometimes a river is defined as being larger than a creek, but not always: the language is vague.

Rivers are part of the hydrological cycle. Water generally collects in a river from precipitation through a

drainage basin

from surface runoff and other sources such as groundwater recharge, springs, and the release of stored water in natural ice and snowpacks (e.g., from

glacier

s).

Rivers and streams are often considered major features within a landscape; however, they actually only cover around 0.1% of the land on Earth. They are made more obvious and significant to humans since many human cities and civilizations are built around the freshwater supplied by rivers and streams. Most of the major cities of the world are situated on the banks of rivers, as they are, or were, used as a

source of water

, for obtaining

food

, for transport, as

border

s, as a defensive measure, as a source of hydropower to drive machinery, for

bathing

, and as a means of disposing of

waste

.

Potamology is the scientific study of rivers, while limnology is the study of inland waters in general.

Topography

Source and drainage basin

A river begins at a source (or more often several sources) which is usually a watershed, drains all the streams in its

drainage basin

, follows a path called a rivercourse (or just ''course'') and ends at either at a

mouth

or mouths which could be a

confluence

,

river delta

, etc. The water in a river is usually confined to a channel, made up of a stream bed between banks. In larger rivers there is often also a wider floodplain shaped by

flood

-waters over-topping the channel. Floodplains may be very wide in relation to the size of the river channel. This distinction between river channel and floodplain can be blurred, especially in urban areas where the floodplain of a river channel can become greatly developed by housing and industry.

The term upriver (or upstream) refers to the direction towards the source of the river, i.e. against the direction of flow. Likewise, the term downriver (or downstream) describes the direction towards the mouth of the river, in which the current flows. The term left bank refers to the left bank in the direction of flow, right bank to the right.

River channel

Rivers can flow down mountains, through valleys (depressions) or along

plain

s, and can create

canyons

or gorges. The river channel 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 now found in only a few regions worldwide, such as the

South Island

of

New Zealand

. They also occur on

peneplain

s and some of the larger

river delta

s. Anastamosing rivers are similar to braided rivers and are quite rare. They have multiple sinuous channel - 1 carrying large volumes of sediment. There are rare cases of river bifurcation in which a river divides and the resultant flows ending in different seas. An example is the bifurcation of Nerodime River in

Kosovo

.

A river flowing in its channel is a source of energy that acts on the river channel to change its shape and form. In 1757, the German hydrologist Albert Brahms empirically observed that the submerged weight of objects that may be carried away by a river is proportional to the sixth power of the river flow speed. This formulation is also sometimes called Airy's law. Thus, if the speed of flow is doubled, the flow would dislodge objects with 64 times as much submerged weight. 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. A river valley that was created from a U-shaped glaciated valley, can often easily be identified by the V-shaped channel that it has carved. In the middle reaches where a river flows over flatter land,

meander

s may form through erosion 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. Rivers whose mouths are in saline

tidal

waters may form

estuaries

.

Throughout the course of the river, the total volume of water transported downstream will often be a combination of the free water flow together with a substantial volume 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.

Types and ratings of rivers

Rivers have been classified by many criteria including their topography, their biotic status, and their relevance to white water

rafting

or

canoe

ing activities.

Subsurface rivers: Subterranean and Subglacial

Most but not all rivers flow on the surface. Subterranean rivers flow underground in

cave

s or caverns. Such rivers are frequently found in regions with

limestone

geologic formations. Subglacial streams are the braided rivers that flow at the beds of glaciers and

ice sheet

s, permitting meltwater to be discharged at the front of the glacier. Because of the gradient in

pressure

due to the overlying weight of the glacier, such streams can even flow uphill.

Permanence of flow: Perennial and Ephemeral

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 because of geologic conditions such as a highly permeable river bed. Some ephemeral rivers flow during the summer months but not in the winter. Such rivers are typically fed from chalk aquifers which recharge from winter rainfall. In England these rivers are called ''bournes'' and give their name to places such as

Bournemouth

and

Eastbourne

. Even in humid regions, the location where flow begins in the smallest tributary streams generally moves upstream in response to precipitation and downstream in its absence or when active summer vegetation diverts water for

evapotranspiration

. Normally dry rivers in arid zones are often identified as arroyos or other regional names.

The meltwater from large hailstorms can create a slurry of water, hail and sand or soil, forming temporary rivers.

Stream order classification of rivers: Fleuve and Rivière

The Strahler Stream Order ranks rivers based on the connectivity and hierarchy of contributing tributaries. Headwaters are first order while the

Amazon River

is twelfth order. Approximately 80% of the rivers and streams in the world are of the first and second order.

In certain languages, distinctions are made among rivers based on their stream order. In French, for example, rivers that run to the sea are called ''fleuve'', while other rivers are called ''rivière''. For example, in

Canada

, the Churchill River in

Manitoba

is called ''la rivière Churchill'' as it runs to Hudson Bay, but the Churchill River in

Labrador

is called ''le fleuve Churchill'' as it runs to the

Atlantic Ocean

. As most rivers in France are known by their names only without the word ''rivière'' or ''fleuve'' (e.g. ''la

Seine

'', not ''le fleuve Seine'', even though the Seine is classed as a ''fleuve''), one of the most prominent rivers in the

Francophonie

commonly known as ''fleuve'' is ''le fleuve Saint-Laurent'' (the Saint Lawrence River).

Since many ''fleuves'' are large and prominent, receiving many tributaries, the word is sometimes used to refer to certain large rivers that flow into other ''fleuves''; however, even small streams that run to the sea are called ''fleuve'' (e.g. '' fleuve côtier'', "coastal ''fleuve''").

Topographical classification: Bedrock and Alluvial rivers

Rivers can generally be classified as either alluvial, bedrock, or some mix of the two. Alluvial rivers have channels and floodplains that are self-formed in unconsolidated or weakly consolidated sediments. They

erode

their banks and deposit material on bars and their floodplains.

Bedrock rivers

Bedrock rivers form when the river downcuts through the modern sediments and into the underlying bedrock. This occurs in regions that have experienced some kind of uplift (thereby steepening river gradients) or in which a particularly hard lithology causes a river to have a steepened reach that has not been covered in modern alluvium. Bedrock rivers very often contain alluvium on their beds; this material is important in eroding and sculpting the channel. Rivers that go through patches of bedrock and patches of deep alluvial cover are classified as mixed bedrock-alluvial.

Alluvial rivers sub-types: Youthful, Mature, Old and Rejuvenated

Alluvial rivers can be further classified by their channel pattern as meandering, braided, wandering, anastomose, or straight. The morphology of an alluvial river reach is controlled by a combination of sediment supply, substrate composition, discharge, vegetation, and bed

aggradation

.

At the start of the 20th century

William Morris Davis

devised the "cycle of erosion" method of classifying rivers based on their "age". Although Davis's system is still found in many books today, after the 1950s and 1960s it became increasingly criticized and rejected by geomorphologists. His scheme did not produce testable hypotheses and was therefore deemed non-scientific. Examples of Davis's river "ages" include:
* Youthful river: A river with a steep gradient that has very few tributaries and flows quickly. Its channels erode deeper rather than wider. Examples are the Brazos, Trinity and

Ebro

rivers.
* Mature river: A river with a gradient that is less steep than those of youthful rivers and flows more slowly. A mature river is fed by many tributaries and has more discharge than a youthful river. Its channels erode wider rather than deeper. Examples are the

Mississippi

, Saint Lawrence,

Danube

,

Ohio

, Thames and

Paraná

rivers.
* Old river: A river with a low gradient and low erosive energy. Old rivers are characterized by flood plains. Examples are the Yellow, lower

Ganges

,

Tigris

, Euphrates,

Indus

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