A river is a natural flowing
watercourse, usually
freshwater, flowing towards an
ocean
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
,
sea,
lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
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
creek,
brook, rivulet, and
rill
In hillslope geomorphology, a rill is a shallow channel (no more than a few inches/ decimeters deep) cut into soil by the erosive action of flowing surface water. Similar but smaller incised channels are known as microrills; larger incised c ...
. 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
A burn is an injury to skin, or other tissues, caused by heat, cold, electricity, chemicals, friction, or ultraviolet radiation (like sunburn). Most burns are due to heat from hot liquids (called scalding), solids, or fire. Burns occur ma ...
" 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 ]water cycle
The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly cons ...
. Water generally collects in a river from precipitation through a drainage basin
A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, ...
from surface runoff and other sources such as groundwater recharge
Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs ...
, springs, and the release of stored water in natural ice and snowpacks.
Rivers 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
Transport (in British English), or transportation (in American English), is the intentional movement of humans, animals, and goods from one location to another. Modes of transport include air, land ( rail and road), water, cable, pipelin ...
, as border
Borders are usually defined as geographical boundaries, imposed either by features such as oceans and terrain, or by political entities such as governments, sovereign states, federated states, and other subnational entities. Political borders c ...
s, as a defensive measure, as a source of hydropower
Hydropower (from el, ὕδωρ, "water"), also known as water power, is the use of falling or fast-running water to produce electricity or to power machines. This is achieved by converting the gravitational potential or kinetic energy of a w ...
to drive machinery, for bathing
Bathing is the act of washing the body, usually with water, or the immersion of the body in water. It may be practiced for personal hygiene, religious ritual or therapy, therapeutic purposes. By analogy, especially as a recreational activity, the ...
, and as a means of disposing of waste
Waste (or wastes) are unwanted or unusable materials. Waste is any substance discarded after primary use, or is worthless, defective and of no use. A by-product, by contrast is a joint product of relatively minor economic value. A waste prod ...
. In the pre-industrial era, larger rivers were a major obstruction to the movement of people, goods, and armies across them. Towns often developed at the few locations they could be crossed. Many major cities such as London
London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
are located at the lowest point at which a river could be bridged.
Potamology is the scientific study of rivers, while limnology
Limnology ( ; from Greek λίμνη, ''limne'', "lake" and λόγος, ''logos'', "knowledge") is the study of inland aquatic ecosystems.
The study of limnology includes aspects of the biological, chemical, physical, and geological characteris ...
is the study of inland waters in general.
Topography
Source and drainage basin
A river begins at a source
Source may refer to:
Research
* Historical document
* Historical source
* Source (intelligence) or sub source, typically a confidential provider of non open-source intelligence
* Source (journalism), a person, publication, publishing institute o ...
(or more often several sources) which is usually a watershed, drains all the streams in its drainage basin
A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, ...
, follows a watercourse, and ends either at a mouth or mouths which could be a confluence, river delta
A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more ra ...
, etc. The water in a river is usually confined to a channel, made up of a stream bed between banks
A bank is a financial institution that accepts deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital markets.
Becaus ...
. In larger rivers there is often also a wider floodplain
A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...
shaped by flood
A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
-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
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
flows. The term left bank refers to the left bank in the direction of flow, right bank to the right in the direction of flow.
Channels
Rivers can flow down mountains, through valley
A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams ove ...
s or along plain
In geography, a plain is a flat expanse of land that generally does not change much in elevation, and is primarily treeless. Plains occur as lowlands along valleys or at the base of mountains, as coastal plains, and as plateaus or uplands ...
s, and can create canyons or gorges. The river channel typically contains a single stream, but some rivers flow as several interconnecting streams, producing a braided river. Braided rivers occur on peneplain
390px, Sketch of a hypothetical peneplain formation after an orogeny.
In geomorphology and geology, a peneplain is a low-relief plain formed by protracted erosion. This is the definition in the broadest of terms, albeit with frequency the usage ...
s and some of the larger river delta
A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more ra ...
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 end in different seas. An example is the Nerodime River in Kosovo
Kosovo ( sq, Kosova or ; sr-Cyrl, Косово ), officially the Republic of Kosovo ( sq, Republika e Kosovës, links=no; sr, Република Косово, Republika Kosovo, links=no), is a partially recognised state in Southeast Euro ...
.
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, 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
A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...
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
A meander is one of a series of regular sinuous curves in the channel of a river or other watercourse. It is produced as a watercourse erodes the sediments of an outer, concave bank ( cut bank) and deposits sediments on an inner, convex ba ...
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
Billabong ( ) is an Australian term for an oxbow lake, an isolated pond left behind after a river changes course. Billabongs are usually formed when the path of a creek or river changes, leaving the former branch with a dead end. As a result ...
. Rivers that carry large amounts of sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
may develop conspicuous deltas at their mouths. Rivers whose mouths are in saline tidal
Tidal is the adjectival form of tide.
Tidal may also refer to:
* ''Tidal'' (album), a 1996 album by Fiona Apple
* Tidal (king), a king involved in the Battle of the Vale of Siddim
* TidalCycles, a live coding environment for music
* Tidal (servic ...
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
The hyporheic zone is the region of sediment and porous space beneath and alongside a stream bed, where there is mixing of shallow groundwater and surface water. The flow dynamics and behavior in this zone (termed hyporheic flow or underflow) is re ...
). For many rivers in large valleys, this unseen component of flow may greatly exceed the visible flow.
Types and ratings
Rivers have been classified by many criteria including their topography
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary sc ...
, their biotic status, and their relevance to white water rafting
Rafting and whitewater rafting are recreational outdoor activities which use an inflatable raft to navigate a river or other body of water. This is often done on whitewater or different degrees of rough water. Dealing with risk is often a ...
or canoeing activities.
Subsurface rivers: subterranean and subglacial
Most but not all rivers flow on the surface. Subterranean river
A subterranean river is a river that runs wholly or partly beneath the ground surface – one where the riverbed does not represent the surface of the Earth. It is distinct from an aquifer, which may flow like a river but is contained within a per ...
s flow underground in cave
A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...
s. Such rivers are frequently found in regions with limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
geologic formations
A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock expo ...
. Subglacial streams are the braided rivers that flow at the beds of glaciers and ice sheet
In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at La ...
s, permitting meltwater to be discharged at the front of the glacier. Because of the gradient in pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
from the overlying weight of the glacier, such streams can even flow uphill.
Permanence of flow: perennial and ephemeral
An intermittent river
Intermittent, temporary or seasonal rivers or streams cease to flow every year or at least twice every five years.(Tzoraki et al., 2007) Such rivers drain large arid and semi-arid areas, covering approximately a third of the earth's surface. ...
(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
Permeability, permeable, and semipermeable may refer to:
Chemistry
*Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion
*Vascular permeability, the movement of fluids and molecules betwe ...
river bed. Some ephemeral rivers flow during the summer months but not in the winter. Such rivers are typically fed from chalk
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Ch ...
aquifers which recharge from winter rainfall. In England these rivers are called ''bournes'' and give their name to places such as Bournemouth and Eastbourne
Eastbourne () is a town and seaside resort in East Sussex, on the south coast of England, east of Brighton and south of London. Eastbourne is immediately east of Beachy Head, the highest chalk sea cliff in Great Britain and part of the la ...
. Even in humid regions, the location where flow begins in the smallest tributary
A tributary, or affluent, is a stream or river that flows into a larger stream or main stem (or parent) river or a lake. A tributary does not flow directly into a sea or ocean. Tributaries and the main stem river drain the surrounding drai ...
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
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 in the world are of the first and second order.
The ways in which a river's characteristics vary between its upper and lower course are summarized by the Bradshaw model
The Bradshaw Model is an idealised geographical model which suggests how a river's characteristics vary between the '' upper course'' and '' lower course'' of a river. It indicates how discharge, occupied channel width, channel depth, and average ...
. Power-law relationships between channel slope, depth, and width are given as a function of discharge by "river regime The river regime generally describes the character of the typical fluctuations of flow of a river, but can also refer to the mathematical relationship between the river discharge and its width, depth and slope. Thus, "river regime" can describe one ...
".
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
, image_map = Manitoba in Canada 2.svg
, map_alt = Map showing Manitoba's location in the centre of Southern Canada
, Label_map = yes
, coordinates =
, capital = Winn ...
is called ''la rivière Churchill'' as it runs to Hudson Bay, but the Churchill River in Labrador
, nickname = "The Big Land"
, etymology =
, subdivision_type = Country
, subdivision_name = Canada
, subdivision_type1 = Province
, subdivision_name1 ...
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 Francophone commonly known as ''fleuve'' is ''le fleuve Saint-Laurent'' (the St. 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
Rivers can generally be classified as either alluvial
Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. All ...
, 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
Erode () is a city in the Indian state of Tamil Nadu. Erode is the seventh largest urban agglomeration in the state, after Chennai, Coimbatore, Madurai, Tiruchirapalli, Tiruppur and Salem. It is also the administrative headquarters of the ...
their banks and deposit material on bars and their floodplains.
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
Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. ...
. 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
Aggradation (or alluviation) is the term used in geology for the increase in land elevation, typically in a river system, due to the deposition of sediment. Aggradation occurs in areas in which the supply of sediment is greater than the amount of ...
.
Biotic classification
There are several systems of classification based on ecological conditions typically assigning classes from the most oligotroph
An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates o ...
ic or unpolluted through to the most eutrophic
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytoplan ...
or polluted. 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.
Navigability
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.
Streamflow
Studying the flows of rivers is one aspect of hydrology.
Characteristics
Direction
Rivers flow downhill with their power derived from gravity. A common misconception holds that all or most rivers flow from north to south, but this is not so: rivers flow in all directions of the compass and often have complex meandering paths.
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. Bedrock rivers typically flow in either a fractal pattern, or a pattern that is determined by weaknesses in the bedrock, such as faults, fractures, or more erodible layers.
Rate
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. It is typically measured in cubic metres per second (cumec) or cubic feet per second (cfs).
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 fluid dynamics, the law of the wall (also known as the logarithmic law of the wall) states that the average velocity of a turbulent flow at a certain point is proportional to the logarithm of the distance from that point to the "wall", or the b ...
. In general, velocity increases with the depth (or hydraulic radius The Manning formula or Manning's equation is an empirical formula estimating the average velocity of a liquid flowing in a conduit that does not completely enclose the liquid, i.e., open channel flow. However, this equation is also used for calculat ...
) 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.
Effects
Fluvial erosion
In its youthful stage, a river causes erosion in the watercourse, deepening the valley. Hydraulic action
Hydraulic action, most generally, is the ability of moving water (flowing or waves) to dislodge and transport rock particles. This includes a number of specific erosional processes, including abrasion, at facilitated erosion, such as ''static eros ...
loosens and dislodges aggregate 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.
Waterfall
A waterfall is a point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf.
Waterfalls can be formed in severa ...
s also form in the youthful river valley where a band of hard rock overlays a layer of soft rock. Differential erosion
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement), ...
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
A plunge pool (or plunge basin or waterfall lake) is a deep depression in a stream bed at the base of a waterfall or shut-in. It is created by the erosional forces of cascading water on the rocks at formation's base where the water impacts.Mars ...
forms at the bottom and deepens as a result of hydraulic action and abrasion.
Flooding
Flood
A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
ing 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 Flood stage is the water level or stage at which the surface of a body of water has risen to a sufficient level to cause sufficient inundation of areas that are not normally covered by water, causing an inconvenience or a threat to life and property ...
. In many developed areas, human activity has changed the form of river channels, altering magnitudes and frequencies of flooding. Some examples of this are the building of levees, the straightening of channels, and the draining of natural wetlands.
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 basin
A detention basin or retarding basin is an excavated area installed on, or adjacent to, tributaries of rivers, streams, lakes or bays to protect against flooding and, in some cases, downstream erosion by storing water for a limited period of time. ...
s finally also reduce the risk of flooding significantly by being able to take up some of the flood water.
Sediment yield
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) indicates that not all of the sediment is eroded within a certain catchment that reaches out to the outlet (e.g., deposition on floodplains). Such storage opportunities are typically increased in catchments of larger size, thus leading to a lower yield and sediment delivery ratio.
Brackish water
Brackish water
Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...
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.
Ecosystem
River biota
The organisms in the riparian zone
A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the terrestrial biomes of the Earth. Plant habitats and communities along the river margins and banks a ...
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. The products of these shredders are used by collectors, such as Hydropsychidae, and further downstream algae that create the primary production
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...
become the main food source of the organisms. 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.
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 characterizing river water chemistry requires a well designed and managed sampling and analysis.
Uses
Construction material
The coarse sediments, gravel, and sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...
, generated and moved by rivers are extensively used in construction. In parts of the world this can generate extensive new lake habitats as gravel pit
A gravel pit is an open-pit mine for the extraction of gravel. Gravel pits often lie in river valleys where the water table is high, so they may naturally fill with water to form ponds or lakes. Old, abandoned gravel pits are normally used either ...
s fill with water. In other circumstances it can destabilize the river bed, and the course of the river and cause severe damage to spawning
Spawn is the eggs and sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of releasing the eggs and sperm, and the act of both sexes is called spawning. Most aquatic animals, except for aquat ...
fish populations which rely on stable gravel formations for egg laying. In upland rivers, rapids with whitewater or even waterfalls occur. Rapids are often used for recreation, such as whitewater kayaking.
Energy production
Fast flowing rivers and waterfalls are widely used as sources of energy, via watermill
A watermill or water mill is a mill that uses hydropower. It is a structure that uses a water wheel or water turbine to drive a mechanical process such as milling (grinding), rolling, or hammering. Such processes are needed in the production of ...
s and hydroelectric plants. Evidence of watermills shows them in use for many hundreds of years, for instance in Orkney at Dounby Click Mill. Prior to the invention of steam power, watermills for grinding cereals and for processing wool and other textiles were common across Europe. In the 1890s the first machines to generate power from river water were established at places such as Cragside in Northumberland
Northumberland () is a county in Northern England, one of two counties in England which border with Scotland. Notable landmarks in the county include Alnwick Castle, Bamburgh Castle, Hadrian's Wall and Hexham Abbey.
It is bordered by land ...
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.
Food source
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 help to determine the urban form of cities and neighborhoods, and their corridors often present opportunities for urban renewal
Urban renewal (also called urban regeneration in the United Kingdom and urban redevelopment in the United States) is a program of land redevelopment often used to address urban decay in cities. Urban renewal involves the clearing out of blighte ...
through the development of foreshoreways such as river walks. Rivers also provide an easy means of disposing of wastewater
Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of domestic, industrial ...
and, in much of the less developed world, other wastes.
Navigation and transport
Rivers have been used for navigation
Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land 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. Riverine navigation provides a cheap means of transport and is still used extensively on most major rivers of the world like the Amazon, the Ganges
The Ganges ( ) (in India: Ganga ( ); in Bangladesh: Padma ( )). "The Ganges Basin, known in India as the Ganga and in Bangladesh as the Padma, is an international river to which India, Bangladesh, Nepal and China are the riparian states." is ...
, the Nile
The Nile, , Bohairic , lg, Kiira , Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered the longest ...
, the Mississippi
Mississippi () is a state in the Southeastern region of the United States, bordered to the north by Tennessee; to the east by Alabama; to the south by the Gulf of Mexico; to the southwest by Louisiana; and to the northwest by Arkansas. Miss ...
, and the Indus.
In some heavily forested regions such as Scandinavia
Scandinavia; Sámi languages: /. ( ) is a subregion in Northern Europe, with strong historical, cultural, and linguistic ties between its constituent peoples. In English usage, ''Scandinavia'' most commonly refers to Denmark, Norway, and Swe ...
and Canada, lumberjacks use rivers 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.
Political borders
Rivers have been important in determining political boundaries and defending countries. For example, the Danube
The Danube ( ; ) is a river that was once a long-standing frontier of the Roman Empire and today connects 10 European countries, running through their territories or being a border. Originating in Germany, the Danube flows southeast for , p ...
was a long-standing border of the Roman Empire
The Roman Empire ( la, Imperium Romanum ; grc-gre, Βασιλεία τῶν Ῥωμαίων, Basileía tôn Rhōmaíōn) was the post- Republican period of ancient Rome. As a polity, it included large territorial holdings around the Mediter ...
, and today it forms most of the border between Bulgaria
Bulgaria (; bg, България, Bǎlgariya), officially the Republic of Bulgaria,, ) is a country in Southeast Europe. It is situated on the eastern flank of the Balkans, and is bordered by Romania to the north, Serbia and North Macedo ...
and Romania
Romania ( ; ro, România ) is a country located at the crossroads of Central, Eastern, and Southeastern Europe. It borders Bulgaria to the south, Ukraine to the north, Hungary to the west, Serbia to the southwest, Moldova to the east, and ...
. The Mississippi in North America and the Rhine
), Surselva, Graubünden, Switzerland
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, source1_elevation =
, source2 = Rein Posteriur/Hinterrhein
, source2_location = Paradies Glacier, Graubünden, Switzerland
, source2_coordinates=
, so ...
in Europe are major east–west boundaries in those continents. The Orange
Orange most often refers to:
*Orange (fruit), the fruit of the tree species '' Citrus'' × ''sinensis''
** Orange blossom, its fragrant flower
*Orange (colour), from the color of an orange, occurs between red and yellow in the visible spectrum
* ...
and Limpopo Rivers in southern Africa
Southern Africa is the southernmost subregion of the African continent, south of the Congo and Tanzania. The physical location is the large part of Africa to the south of the extensive Congo River basin. Southern Africa is home to a number o ...
form the boundaries between provinces and countries along their routes.
Sacred rivers
Sacred rivers and their reverence is a phenomenon found in several religions, especially religions in which nature is revered. For example, the Indian-origin religions of Buddhism
Buddhism ( , ), also known as Buddha Dharma and Dharmavinaya (), is an Indian religions, Indian religion or Indian philosophy#Buddhist philosophy, philosophical tradition based on Pre-sectarian Buddhism, teachings attributed to the Buddha. ...
, Hinduism
Hinduism () is an Indian religion or '' dharma'', a religious and universal order or way of life by which followers abide. As a religion, it is the world's third-largest, with over 1.2–1.35 billion followers, or 15–16% of the global p ...
, Jainism
Jainism ( ), also known as Jain Dharma, is an Indian religion. Jainism traces its spiritual ideas and history through the succession of twenty-four tirthankaras (supreme preachers of ''Dharma''), with the first in the current time cycle bein ...
, and Sikhism
Sikhism (), also known as Sikhi ( pa, ਸਿੱਖੀ ', , from pa, ਸਿੱਖ, lit=disciple', 'seeker', or 'learner, translit=Sikh, label=none),''Sikhism'' (commonly known as ''Sikhī'') originated from the word ''Sikh'', which comes fro ...
revere and preserve Sacred groves of India, groves, forests, List of Banyan trees in India, trees, Sacred mountains#India, mountains and rivers as sacred. Among the most sacred rivers in Hinduism are the Ganges, Yamuna, and Sarasvati River, Sarasvati rivers. Other sacred rivers for Indian religions include the Rigvedic rivers, the Narmada River, Narmada, the Godavari River, Godavari, and the Kaveri rivers. The Vedas and Gita, the most sacred of Hindu texts, were written on the banks of the Sarasvati river.
Management
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 Aqueduct (water supply), 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. Dredging, Dredged channels silt up, sluice, 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 plant, aquatic and riparian plants, freshwater fish, resident and fish migration, migratory fishes, waterfowl, bird of prey, birds of prey, bird migration, migrating birds, and many mammals.
Concerns
Man-made causes, such as the Exploitation of natural resources, over-exploitation and pollution, are the biggest threats and concerns which are making rivers Dead zone (ecology), ecologically dead and drying up the rivers.
Plastic pollution imposes threats on aquatic life and river ecosystems because of plastic's durability in the natural environment. Plastic debris may result in entanglement and ingestion by aquatic life such as turtles, birds, and fish, causing severe injury and death. Human livelihoods around rivers are also impacted by plastic pollution through direct damage to shipping and transport vessels, effects on tourism or real estate value, and the clogging of drains and other hydraulic infrastructure leading to increased flood risk.
See also
;Arts, entertainment, and media
* "Ol' Man River, Old Man River"
* ''The Riverkeepers'' (book)
;General
* Drought
* Fluvial
* Reach (geography)
* Salt tide
* Water conflict
;Crossings
* Bridge
* Ferry
* Ford (crossing)
* Tunnel
;Habitats
* Exposed riverine sediments
* Riparian zone
;Lists
* Lists of rivers
* List of international border rivers
* List of rivers by continent
* List of rivers by discharge
* List of rivers by length
* List of waterways
;Transport
* Barge
* Raft
* River transport
* Riverboat
* Sailing
* Steamboat
* Towpath
* Yacht
References
Further reading
*
* — a non-technical primer on the geomorphology and hydraulics of water.
*
{{Authority control
Rivers,
Bodies of water
Fluvial landforms
Geomorphology
Sedimentology
Water streams