For other uses, see River (disambiguation).
"Rivers" redirects here.
For other uses, see Rivers (disambiguation).
"Riverine" redirects here.
For riverine warfare, see Brown-water navy.
In some cases a river flows into the ground and becomes dry at the end of its course without reaching another body of water.
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 glaciers).
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 by the fact that 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 borders, 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.
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.
The river channel typically contains a single stream of water, but some rivers flow as several interconnecting streams of water, producing a braided river.
Anastamosing rivers are similar to braided rivers and are quite rare.
They have multiple sinuous channels 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.
A river flowing in its channel is a source of energy which 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, meanders may form through erosion of the river banks and deposition on the inside of bends.
Rivers that carry large amounts of sediment may develop conspicuous deltas at their mouths.
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.
Most but not all rivers flow on the surface.
Because of the gradient in pressure due to the overlying weight of the glacier, such streams can even flow uphill.
Permanence of flow
Main article: Intermittent river
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.
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.
Alluvial rivers have channels and floodplains that are self-formed in unconsolidated or weakly consolidated sediments.
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 particular 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 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.
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 and lower Nile rivers.
- Rejuvenated river: A river with a gradient that is raised by tectonic uplift. Examples are the Rio Grande and Colorado River.
The ways in which a river's characteristics vary between its upper and lower course are summarized by the Bradshaw model.
Power-law relationships between channel slope, depth, and width are given as a function of discharge by "river regime".
Other systems are based on a whole eco-system approach such as developed by the New Zealand Ministry for the Environment.
In Europe, the requirements of the Water Framework Directive has led to the development of a wide range of classification methods including classifications based on fishery status A system of river zonation used in francophone communities divides rivers into three primary zones:
- The crenon is the uppermost zone at the source of the river. It is further divided into the eucrenon (spring or boil zone) and the hypocrenon (brook or headstream zone). These areas have low temperatures, reduced oxygen content and slow moving water.
- The rhithron is the upstream portion of the river that follows the crenon. It has relatively cool temperatures, high oxygen levels, and fast, turbulent, swift flow.
- The potamon is the remaining downstream stretch of river. It has warmer temperatures, lower oxygen levels, slow flow and sandier bottoms.
The International Scale of River Difficulty is used to rate the challenges of navigation—particularly those with rapids.
Class I is the easiest and Class VI is the hardest.
Stream order classification
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. , "coastal fleuve").
Rivers have been a source of food since pre-history.
They are often a rich source of fish and other edible aquatic life, and are a major source of fresh water, which can be used for drinking and irrigation.
Rivers also provide an easy means of disposing of waste water and, in much of the less developed world, other wastes.
Rivers have been used for navigation for thousands of years.
The earliest evidence of navigation is found in the Indus Valley Civilization, which existed in northwestern India around 3300 BC.
Rivers have been important in determining political boundaries and defending countries.
In some heavily forested regions such as 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.
In the 1890s the first machines to generate power from river water were established at places such as Cragside in Northumberland and in recent decades there has been a significant increase in the development of large scale power generation from water, especially in wet mountainous regions such as Norway.
In parts of the world this can generate extensive new lake habitats as gravel pits re-fill with water.
In other circumstances it can destabilise the river bed and the course of the river and cause severe damage to spawning fish populations which rely on stable gravel formations for egg laying.
Rapids are often used for recreation, such as whitewater kayaking.
Main article: River ecosystem
The organisms in the riparian zone respond to changes in river channel location and patterns of flow.
The ecosystem of rivers is generally described by the river continuum concept, which has some additions and refinements to allow for dams and waterfalls and temporary extensive flooding.
The concept describes the river as a system in which the physical parameters, the availability of food particles and the composition of the ecosystem are continuously changing along its length.
The food (energy) that remains from the upstream part is used downstream.
The general pattern is that the first order streams contain particulate matter (decaying leaves from the surrounding forests) which is processed there by shredders like Plecoptera larvae.
All changes are gradual and the distribution of each species can be described as a normal curve, with the highest density where the conditions are optimal.
In rivers succession is virtually absent and the composition of the ecosystem stays fixed .
Main article: River chemistry
The chemistry of rivers is complex and depends on inputs from the atmosphere, the geology through which it travels and the inputs from man's activities.
The chemical composition of the water has a large impact on the ecology of that water for both plants and animals and it also affects the uses that may be made of the river water.
Understanding and characterising river water chemistry requires a well designed and managed sampling and analysis.
Further information: Brackish water
Brackish waster occurs in most rivers where they meet the sea.
The extent of brackish water may extend a significant distance upstream, especially in areas with high tidal ranges.
Main article: Flood
Flooding is a natural part of a river's cycle.
The majority of the erosion of river channels and the erosion and deposition on the associated floodplains occur during the flood stage.
In many developed areas, human activity has changed the form of river channels, altering magnitudes and frequencies of flooding.
In many cases human activities in rivers and floodplains have dramatically increased the risk of flooding.
Straightening rivers allows water to flow more rapidly downstream, increasing the risk of flooding places further downstream.
Building on flood plains removes flood storage, which again exacerbates downstream flooding.
The building of levees only protects the area behind the levees and not those further downstream.
Levees and flood-banks can also increase flooding upstream because of the back-water pressure as the river flow is impeded by the narrow channel banks.
Detention basins finally also reduce the risk of flooding significantly by being able to take up some of the flood water.
Studying the flows of rivers is one aspect of hydrology.
Rivers flow downhill with their power derived from gravity.
The direction can involve all directions of the compass and can be a complex meandering path.
Rivers flowing downhill, from river source to river mouth, do not necessarily take the shortest path.
For alluvial streams, straight and braided rivers have very low sinuosity and flow directly down hill, while meandering rivers flow from side to side across a valley.
Main article: Streamflow
Volumetric flow rate, also known as discharge, volume flow rate, and rate of water flow, is the volume of water which passes through a given cross-section of the river channel per unit time.
Volumetric flow rate can be thought of as the mean velocity of the flow through a given cross-section, times that cross-sectional area.
Mean velocity can be approximated through the use of the Law of the Wall.
In general, velocity increases with the depth (or hydraulic radius) and slope of the river channel, while the cross-sectional area scales with the depth and the width: the double-counting of depth shows the importance of this variable in determining the discharge through the channel.
In its youthful stage the river causes erosion in the water-course, deepening the valley.
Hydraulic action loosens and dislodges the rock which further erodes the banks and the river bed.
Over time, this deepens the river bed and creates steeper sides which are then weathered.
The steepened nature of the banks causes the sides of the valley to move downslope causing the valley to become V-shaped.
Waterfalls also form in the youthful river valley where a band of hard rock overlays a layer of soft rock.
Differential erosion occurs as the river erodes the soft rock more readily than the hard rock, this leaves the hard rock more elevated and stands out from the river below.
A plunge pool forms at the bottom and deepens as a result of hydraulic action and abrasion.
Sediment yield is the total quantity of particulate matter (suspended or bedload) reaching the outlet of a drainage basin over a fixed time frame.
Yield is usually expressed as kilograms per square kilometre per year.
Sediment delivery processes are affected by a myriad of factors such as drainage area size, basin slope, climate, sediment type (lithology), vegetation cover, and human land use / management practices.
The theoretical concept of the 'sediment delivery ratio' (ratio between yield and total amount of sediment eroded) captures the fact that not all of the sediment is eroded within a certain catchment that reaches out to the outlet (due to, for example, deposition on floodplains).
Such storage opportunities are typically increased in catchments of larger size, thus leading to a lower yield and sediment delivery ratio.
Main article: River engineering
Rivers are often managed or controlled to make them more useful or less disruptive to human activity.
- Dams or weirs may be built to control the flow, store water, or extract energy.
- Levees, known as dikes in Europe, may be built to prevent river water from flowing on floodplains or floodways.
- Canals connect rivers to one another for water transfer or navigation.
- River courses may be modified to improve navigation, or straightened to increase the flow rate.
River management is a continuous activity as rivers tend to 'undo' the modifications made by people.
Dredged channels silt up, sluice mechanisms deteriorate with age, levees and dams may suffer seepage or catastrophic failure.
The benefits sought through managing rivers may often be offset by the social and economic costs of mitigating the bad effects of such management.
As an example, in parts of the developed world, rivers have been confined within channels to free up flat flood-plain land for development.
Floods can inundate such development at high financial cost and often with loss of life.
Rivers are increasingly managed for habitat conservation, as they are critical for many aquatic and riparian plants, resident and migratory fishes, waterfowl, birds of prey, migrating birds, and many mammals.
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/River.