At their most basic, rivers are geographical phenomena which cause fresh water to move through dry land from one place to another, usually as part of a natural drainage system. Every continent on Earth, including Antarctica, has rivers.
There are no hard and fast rules regarding what actually constitutes a river. The word ‘river’ is, in effect, a catch-all term; creeks, brooks, and streams are, in effect, different names for small rivers.
Throughout history, rivers have played a large part in human development and day to day life – and they continue to do so today. Many of the ancient civilizations including those founded in Egypt, China, India, and Rome grew up along large rivers, which provided what seemed to be an inexhaustible source of fresh drinking water and, later, a convenient means of transportation between various destinations.
Today, hydroelectric generating plants built on rivers which have been dammed account for about 25% of the world’s available electricity; rivers also account for around 30% of the planet’s available fresh water. In the United States alone, inland waterways made up of a complex system of connected rivers are used to transport around 650 million tons of goods worth an estimated $75 billion each year. In the last few years, multi-million dollar industries have grown up around river cruises and whitewater river rafting.
The Basic Anatomy of a River
While individual rivers have their own unique characteristics, all rivers have a couple of things in common. All rivers flow downhill due to gravity (although some very fast flowing rivers will go uphill for very short distances, in much the same way as a fast rolling ball will be able to travel over short inclines due to its speed). All rivers also have a source where it begins (elevated, but sometimes not by much) and a ‘mouth’ into which it drains, such as a sea, ocean, lake, another larger river, or even a desert.
The difference between the elevation of a river’s source and mouth will often be a determining factor in a river’s speed and size. For example, the Amazon River is the largest river in the world (it discharges over 55 million gallons of water per second) and is considered by many to be the fastest-flowing; its source is high in the Andres Mountains of Peru and it flows into the Atlantic Ocean, at sea-level, off the coast of Brazil about 4,000 miles away.
Rivers can have a number of sources including lakes, run-off from melting ice and snow (particularly in mountainous and highland regions), smaller streams and brooks, and glaciers. Many large rivers have their source where two smaller rivers converge: for example, the source of the Ohio River is where the Monongahela and Allegheny rivers meet. Rivers can be short or long, wide or narrow, fast or slow.
There are 165 ‘major’ rivers in the world, and literally tens of thousands of smaller ones. The longest river in the world is the Nile River in the Middle East at just over 4,100 miles, while the shortest is the Roe River in central Montana at just over 200 feet.
A common misconception is that all rivers run north to south (or south to north below the equator); however, this is a myth. Many of the world’s great rivers flow east to west or west to east.
So what are some of the different types of rivers in the world today?
When the average person thinks of a river, they are most probably thinking of a perennial – also sometimes also referred to as a permanent – river. This type of river was perhaps best described by lyricist Oscar Hammerstein II in the song Ol’ Man River: “Long ol’ river forever keeps rollin’ on.”
Simply put, a perennial river is a river that normally never goes dry and continues to flow throughout the year. Although the height and flow-rate can be affected by heavy rains or lengthy periods of drought, in most cases a perennial river has a stable source or flows through areas where the rainfall exceeds the evaporation rate, which ensures its continuous flow.
Many perennial rivers are quite old – predating human beings by tens of millions of years – and over the millennia have cut paths for themselves which have resulted in sometimes radical geological alterations. Perhaps the most familiar and graphic example of this is the Grand Canyon which was carved by the flow of the Colorado River to a depth of over 6,000 feet and a width of about 18 miles in some places over the course of an estimated 70 million years.
While not as dramatic as the Grand Canyon, all perennial rivers have well-established river beds (which are below the water line) and banks (which are above) through which they flow. Unless interfered with by man, perennial rivers are continually deepening and widening (albeit sometimes imperceptibly to the naked eye) as their beds and banks are eroded slowly over time by the continual flow of water.
Perennial rivers are often dammed to restrict their flow for the purpose of creating reservoirs, aiding in irrigation, improving navigation, and building hydroelectric plants. Perennial rivers have been dammed for centuries, with the first known dam dating back to ancient Mesopotamia, having been built around 3,000 BC.
Sadly, the flow-rate and depths of some of the world’s perennial rivers have been decreasing in the last couple of decades due to increasing manipulation of the river by humans for the purposes of consumption and irrigation, and climate change factors.
Periodic, also often referred to as ephemeral or intermittent, rivers differ from perennial rivers in that they do not flow throughout the year. Generally speaking, periodic rivers will only flow above the surface anywhere from one quarter to three-quarters of the year, and some will only flow for a few days at a time.
Regardless of the frequency of above-ground flow, most periodic rivers will have a well-established bed and bank, and will often have water beneath them. The river beds remain dry until significant rain or snowmelt alters the amount of water either at the source or in the level of the groundwater beneath them. Most periodic rivers have predictable seasons of flow which remain more or less constant year after year.
Periodic rivers are most often found in arid areas where there is minimal rainfall. While not flowing consistently throughout the year, it is not uncommon for a periodic river with water beneath it to have pools within its banks even when the river itself is not flowing.
Periodic rivers are often quite short, but can also be relatively long. For example, the Ugab River in Southern Africa is slightly over 310 miles long. Periodic rivers have been increasing in number over the last fifty years or so due to environmental conditions, climate change, human manipulation of perennial rivers, and other factors.
An episodic river is a river that only flows following a particular event (or episode) such as heavy rainfall, early snowmelt, or swollen runoff channels from other rivers. They differ from perennial and periodic rivers in that they usually have no stable source, usually have no groundwater beneath them, and are almost completely dependent on climatic conditions for their water.
Some episodic rivers are thought to be quite old, have well-defined beds and banks, and were once probably perennial or periodic rivers. Most, but not all, episodic rivers do not have predictable times of flow.
Although many episodic rivers will flow for very short periods of time every year or two, it is not uncommon for them to remain completely dry for years and sometimes decades. For example, the 450-mile long Nossob River in the Kalahari region of Southern Africa has not had a significant flow since the late 1980s.
Not named for its vacation or cruise appeal, an exotic river is one that flows through a dry environment in which not much other freshwater exists. Exotic rivers usually have their sources in humid or mountainous areas, and travel through extremely arid environments, such as a desert. Most of the better known exotic rivers are perennial, although some significant examples are periodic.
Exotic rivers have played a crucial part in the formation of many of the world’s most important civilizations both by providing drinking water to large numbers of people and irrigating otherwise barren land to provide food for previously nomadic humans and livestock. Ancient civilizations that grew along the banks of exotic rivers include Egypt (the Nile), Mesopotamia (the Tigris and Euphrates) and China (Yellow or Huang, and Wei).
Today, many of these same rivers (and other exotic rivers) still provide drinking and irrigation water to deserts and areas with little rainfall, as well as providing transportation and electric power to tens of millions of people via hydroelectric generating stations.
Most of the major deserts in the world (except those located in Australia) have at least one exotic river running through it. In the United States, the Colorado River is considered to be an exotic river as it flows though many barren and arid areas of Arizona, Utah, and Nevada.
A tributary (also referred to as an affluent) river is any river that does not flow into an ocean or sea, but rather has its mouth at a lake or another river. Although the name might convey the idea that tributary rivers are small, in reality, many of the major rivers in the world are tributaries of other rivers.
A prime example of this is the Missouri River. The longest river in North America at just over 2,340 miles and one of the most powerful, Missouri flows west to east from the Rocky Mountains in Montana to St. Louis, Missouri, where it empties into the Mississippi River, making it a tributary of the Mississippi. Along with its course, Missouri has hundreds of tributaries of its own.
In some cases, the confluence (or meeting) of two separate tributary rivers will create a new river. For example, the Ohio River is formed in Pittsburg, Pennsylvania by the confluence of the Monongahela and Allegheny rivers – at which point the two tributaries end and the new river starts. The Ohio then, in turn, becomes a tributary of the Mississippi River about one thousand miles later in southern Illinois.
Effectively the opposite of a tributary, a distributary river branches off from another river to form a brand new river. The point at which the distributary branches off from the main river is generally referred to as a fork. The distributary river itself is sometimes referred to as a channel or arm.
Generally speaking, a distributary river will have a steeper grade than the river that feeds it; the force of gravity will cause a percentage of the water in the main river to flow down into the distributary. In some cases, a distributary river will re-join the main river from which it was created further along its route, while in other cases the distributary will have its mouth somewhere else, sometimes traveling all the way to the sea.
Because of their steeper grades, distributary rivers will often grow at a faster rate than the main river that feeds it, drawing off more and more water over time. This can sometimes have disastrous effects when it comes to modern man’s manipulation of a river.
An example of this is the Atchafalaya River in Louisiana, a distributary of the Mississippi. As it has grown, the Atchafalaya has drawn off an increasing amount of the Mississippi’s flow causing experts to worry that eventually the decreased flow of the Mississippi would endanger the ports at New Orleans and Baton Rouge. A dam, called the Old River Control Structure, was completed in the mid-1960s to control the amount of water flowing into the Atchafalaya at the point where the two rivers fork.
An underground (or subterranean) river is, as the name suggests, a river that has its banks and bed beneath the surface of the earth. Naturally occurring underground rivers are usually part of an otherwise above ground river that travels for a distance beneath the earth via caves, or disappearing into sinkholes (also called cenotes) and continuing on underground before re-appearing above ground further on.
Most underground rivers are relatively short, although there are exceptions. For example, the longest underground river in the world is generally considered to be the Sistema Sac Actum, which flows for 95 miles through the Sac Actum cave system in Quinta Roo, Mexico. Perhaps the best known (and certainly most exploited for tourism purposes) is the Puerto Princesa Subterranean River in the Philippines. Named a UNESCO World Heritage Site in 1999, this river is over 5 miles long, much of which is navigable by boat.
Some underground rivers have actually been created by man building cities above a flowing river, effectively making them artificially subterranean. A well-known example of this is the UK’s River Fleet, which travels about four miles under London’s streets before emptying into the River Thames.
An aqueduct (not to be confused with the bridge of the same name) is a manmade river, or watercourse, which conveys water from a source – usually a lake, natural river, or reservoir – to where it is needed. They are most frequently constructed to irrigate large tracts of farmland, or supply cities and their suburbs with drinking water
Aqueducts date back thousands of years when they were most commonly used for irrigation. Examples of ancient aqueducts can be found in Egypt, Rome, India, China, and South America dating back as far as 600 BC. Ancient aqueducts were often simple short ditches, sometimes reinforced with rocks, into which their sources could flow.
Modern aqueduct systems are often quite complex feats of engineering that travel long distances, sometimes across deserts, over valleys, and through mountains. They often utilize reinforced concrete or metal ‘ditches’, above ground pipes, and bridges to carry water over and through various types of terrain.
Modern aqueducts are often massive projects and can be found in many countries the world over. Notable aqueducts in North America include the Colorado River Aqueduct, which travels nearly 250 miles to supply Los Angeles with drinking water, and the Catskill Aqueduct, which supplies New York City with around 400 million gallons of water per day.
Though not very common (and not to be confused with the pipes that supply water to taps in cities and other municipalities) manmade pipeline ‘rivers’ that convey fresh water considerable distances do exist, and so are mentioned here.
The best, and largest, example is the Great Man-Made River in Libya. Beginning at the Nubian Sandstone Aquifer and running underground for over 1,700 miles through the Saraha Desert, this manmade river supplies over 1,500 wells along its route and is reportedly built to transport over 1.5 billion gallons of water a day (although it currently handles far less than that) for irrigation and drinking. It is the main water supply for millions of Libyans in the cities of Tripoli, Sirte, Benghazi, and the surrounding areas, and for dozens of smaller towns and villages in the Saraha.
Rapids (also often called whitewater rapids, or just whitewater) are sections of a river which flow at a much faster speed than other sections due to the steepness (or downward grade) of the river’s bed. The whitewater effect is formed by rapidly flowing water splashing over rocks in the river’s bed; this causes the formation of bubbles, which turns the surface of the river white.
Rapids are classified by the American Whitewater Association using their six-class International Scale of River Difficulty. Class 1 is a relatively mild rapid, which can be swum by strong swimmers, while Class 6 is considered potentially deadly and usually only attempted with the use of rafts by experts. The severity of rapids is determined using a number of factors including water speed, the structure of the banks, and the severity of the drop.
Rapids of varying severity are found in rivers all over the world, and will often run either continuously or intermittently for many miles. It is not uncommon for a single river to have dozens of stretches of rapids which vary in severity and classification.
In the last 60 years or so, a multi-million dollar per year industry has grown up around whitewater rafting and canoeing. Most of these businesses offer trips down Class 2, 3, and some Class 4 rapids. More difficult trips down Class 4 and Class 5 rapids are often billed as extreme whitewater rafting adventures.
Winding (also called meandering or bending) sections of rivers are almost the exact opposite of rapids; they are usually slow-moving parts of a river with a very minor downward grade and are most often found in plains or lowlands. In most cases, winding sections of a river will occur closer to the river’s mouth than to its source (at which point the flow is often heaviest) where the land levels off as it approaches sea level.
The bending of a river from it’s otherwise more or less straight path can have many causes including rocks, minor differences in the downward gradient of the land, and the make-up of the soil and terrain through which the river is flowing.
As the old adage says, “Water seeks the path of least resistance,” and when something obstructs its flow, a river that doesn’t have the power to overcome the obstacle will simple alter its course to an easier route. Over the millennia, the river will cut its way down to bedrock for its bed, establish banks, and wind its way along.
Human management and manipulation of rivers is also sometimes used to bend a river. This is often done by the use dams, sluices, levees and diversion channels in densely populated, low-lying areas to prevent flooding, or for business and agricultural purposes.
Creeks, Brooks and Streams
As previously stated, since there are no hard and fast standards defining what it takes to be a river, creeks, streams, and brooks can also be called rivers. These entities can be perennial, periodic, or episodic; tributary or distributary; and will often have well-defined beds and banks. In some cases, particularly in mountainous regions, the confluence of many creeks, brooks, and streams will serve as the source of a major river.
Although usually quite small, some creeks, brooks, and streams can be quite long and are often very important sources of fresh water. Australia’s Billabong Creek (believed by many to be the longest creek in the world) is around 350 miles long and is a major tributary of the Edward River. In the United States, Lodgepole Creek, at over 270 miles, is considered the longest creek in the country and runs through Wyoming and Nebraska before it empties into the South Platt River in Colorado.
The term inland waterway refers to a river, portion of a river or the combination of several rivers (in which case it is called an inland waterway system) used to transport cargo and people by boat, barge, or other floating vessels. Inland waterways have been used extensively throughout history, and remain important today on most continents.
Inland waterways must be navigable, meaning that they must be sufficiently wide and deep to accommodate the hulls of the vessels used on them, as well as being relatively slow moving. Inland waterways must also not have rapids or waterfalls, although these are sometimes compensated for through the use of locks.
In the United States, the inland waterway system is comprised of over 25,000 miles of navigable rivers and lakes, almost all of which are located in the central and eastern parts of the country. The largest inland waterway in the US is the Mississippi River System which is used extensively for the transportation of about half a billion tons of oil and gas, grain, coal, and other cargo every year.
Lisa has a Bachelor’s of Science in Communication Arts. She is an experienced blogger who enjoys researching interesting facts, ideas, products, and other compelling concepts. In addition to writing, she likes photography and Photoshop.