SCIENCE17 - All About Rivers

I’m not really sure how I came to the subject of rivers for a blog article.  I do know that in the course of my blogging over the last five and half years, rivers kept coming up in both my history and travel articles, where I came to an appreciation of both their importance to world history and their beauty.  When I started looking into the subject of rivers, I found a lot of interesting stuff that I didn’t know, so I decided to write this article and share what I found.

 

After a short introduction, I will discuss the anatomy of a river, rivers through history, rivers of the world, rivers as dynamic systems, human intervention on rivers, and conclude with some interesting facts about rivers.

I will list my principal sources at the end.

 

Introduction

A river is natural stream of flowing water of considerable volume.  The word “river” comes from the Old English word “rievere” or “reofor,” which means “to flow.”  Rivers are found on every continent in 177 countries.  (18 countries have no rivers at all.)  Some rivers flow all year round; others flow only seasonally or during wet years.  A river may span much of a continent, or may be only a few hundred feet long.  There are an incredible 150,000 named rivers across the planet, and possibly even more that we don’t yet know about.

The longest rivers in the world are the Nile in Africa and the Amazon in South America.  For centuries, scientists have debated which river is longer.  According to Encyclopedia Britannica data, published on January 5, 2024, the Nile River is the longest river in the world at 4,132 miles.  It flows from central Africa northward through northeastern Africa to the Mediterranean Sea, and has three principal streams: the Blue Nile, the Atbara, and the White Nile. 

The course of the Nile River from central Africa to the Mediterranean Sea.

 

According to Britannica, the Amazon is second in length at “at least” 4,000 miles, flowing from tributaries in southern Peru, eastward to the Atlantic Ocean, on the northeastern coast of Brazil. 

Note:  Measuring the length of a river is difficult because it is hard to pinpoint its exact beginning and end.  Also, the length of rivers can change as they change course, are dammed, or the solid material deposits at the end of the river grow and recede.  With these factors considered, National Geographic estimates the Nile River is 3,437-4,180 miles long, while the Amazon River is 3,903-4,225 miles long. The length of the Amazon is still under intense study and debate

There is no debate, however, that the Amazon carries more water than any other river on Earth, gushing out at least 53,000 gallons into the Atlantic Ocean each second of the day.  The Amazon River discharges more water into the Atlantic Ocean than the next seven largest rivers combined. Approximately one-fifth of all the fresh water entering the oceans comes from the Amazon.  The drainage area of the Amazon covers approximately 2.7 million square miles, and is home to one-third of all known living species on Earth.  The Amazon’s mouth is so wide that one cannot see the opposite bank from the middle of the river. 

The course of the Amazon River from tributaries in southern Peru to the Atlantic Ocean.

 

The world’s shortest river is the Roe River, only 201 feet long, that runs from Giant Springs to the Missouri River, near Great Falls, Montana.

The world’s deepest river is the Congo River in Central Africa.  While its true depth remains a mystery, scientists believe the waters run at least 750 feet deep in parts.

Rivers are essential for supporting various ecosystems and human civilizations.  They provide water for drinking, agriculture, industry, and transportation.  They also shape landscapes, create fertile floodplains, and sustain a diverse range of plant and animal species.

Rivers are essential components of the Earth’s hydrological cycle, the continuous exchange of moisture between the oceans, the atmosphere, and the land.  Rivers carry large quantities of water from the land to the ocean.  There, seawater constantly evaporates.  The resulting water vapor forms clouds.  Clouds carry moisture over land and release it as freshwater precipitation.  This freshwater feeds rivers and smaller streams.  This water cycle constantly replenishes Earth’s supply of fresh water, which is essential for almost all living things.  Rivers contain only 0.003% of the water on Earth, but sustain much of her life. 

Rivers have created some of the world’s most-treasured natural heritage.  More than 30 river landscapes - from the Pantanal to the Okavango Delta - have been recognized as World Heritage Sites. 

On the fourth Sunday of every September, millions of people from more than 70 countries celebrate World Rivers Day.  This global event raises awareness of the importance of rivers and how we can best protect them for years to come.

 

Anatomy of a River

No two rivers are exactly alike.  Yet all rivers have certain features in common and go through similar stages as they age.

The beginning of a river is called its source or headwaters.  The source may be a melting glacier, melting snow, a lake with an outflowing stream, a spring, or even an underground aquifer.

From its source, a river flows downhill.  Precipitation and tributaries add to the river’s flow.  For instance, the Amazon River receives water from more than 1,000 tributaries.  Together, a river and its tributaries make up a river system, called a drainage basin or watershed.

When two or more rivers meet, it is called a confluence.

The end of a river is its mouth, where the river empties into another body of water - a larger river, a lake, or the ocean.

The flowing water of a river has great power to carve and shape the landscape.  Many landforms, like the Grand Canyon in Arizona, were sculpted by rivers over time through weathering or erosion.

The energy of flowing river water comes from the force of gravity, which pulls the water downward. The steeper the slope of a river, the faster the river moves and the more energy it has.

A river’s current is usually strongest near the river’s source.  Heavy rainfall or snowmelt can increase the volume and speed of a river’s current, while dry periods can lead to slower, shallower currents.  Major storm events, like hurricanes, can drastically alter river flow patterns.

A swift current can move even large boulders. These break apart, and the pieces that are carried in the moving water scrape and dig into the river bottom, or bed.  Little by little, a river tears away rocks and soil along its bed, and carries them downstream, carving a narrow, V-shaped valley.  Rapids and waterfalls are common to rivers, particularly near their sources.

Eventually, the river flows to lower land.  As the slope of its course flattens, the river cuts less deeply into its bed, and begins to wind from side to side in looping bends called meanders, widening the river valley.

Amazon River “meandering” through Peru.

 

At the same time, the river begins to leave behind sediment or silt - some of the rocks, sand, and other solid material it collected upstream.  Once the sediment is deposited, it is called alluvium.  Alluvium may contain a great deal of eroded topsoil from upstream and from the banks of its meanders.  Because of this, a river deposits very fertile soil on its floodplain, the area next to the river that is subject to flooding.

The deepest part of a river bed is called a channel, usually located in the middle of a river, where the current us usually strongest.

Near the end of its journey, the river slows and may appear to move sluggishly.  It has less energy to cut into the land, and it can no longer carry a heavy load of sediment.  Where the river meets the ocean or a lake, it may deposit so much sediment that new land, a delta, is formed. 

The delta of the Mississippi River, where the river meets the Gulf of Mexico, is a 4,700 square mile area of land.

 

Not all rivers have deltas. The Amazon does not have a true delta, for instance. The strength of the tides and currents of the Atlantic Ocean prevent the build-up of sediment.  Deltas almost always have fertile soil.  

 

Rivers Through History

Rivers have always been important to people.  In prehistoric times, people settled along the banks of rivers, where they found fish to eat and water for drinking, cooking, and bathing.

Later, people learned that the fertile soil along rivers was good for growing crops.  By depositing nutritious silt on floodplains and deltas, rivers have created our most fertile agricultural lands, from the Mekong Delta to California’s Central Valley.

The world’s first great civilizations arose in the fertile flood plains of the Nile in Egypt, the Indus in southern Asia, the Tigris and the Euphrates in the Middle East, and the Huang (Yellow) in China.

The Tigris and Euphrates River Valley, the Cradle of Civilization.

 

Centuries later, rivers provided routes for trade, exploration, and settlement.  The Volga River in Eastern Europe allowed Scandinavian and Russian cultures, near the source of the river, to trade goods and ideas with Persian cultures, near the mouth of the Volga in southern Europe.  The Hudson River in the U.S. state of New York is named after English explorer Henry Hudson, who used the river to explore what was then the New World.

When towns and industries developed, the rushing water of rivers supplied power to operate machinery. Hundreds of factories operated mills powered by the Thames in England, the Mississippi in the United States, and the Ruhr in Germany.

Rivers remain important today.  If you look at a world map, you will see that many well-known cities are on rivers.  Great river cities include New York City, New York; Buenos Aires, Argentina; London, England; Cairo, Egypt; Kolkata, India; and Shanghai, China.  Rivers continue to provide transportation routes, water for drinking and for irrigating farmland, and power for homes and industries.  Rivers sustain fish populations offshore.  Because of the nutrients they carry to the sea, 80% of the world’s fish catch comes from continental shelves, the edges of a continent lying beneath the ocean.

 

Rivers of the World

The map below shows the location of the world’s ten longest rivers.  (You may have to increase the magnification to read the details.)

The table below lists some of the world’s most important rivers by continent.

Continent	River	Location	Length  (miles)	Importance
Europe	Volga     Thames      Rhine	Across Russia     Southern England     Alps to North Sea	2290     210     760	Transports timber from northern forests, grain from farms along its valley, and manufactured goods.   Supports London’s 1000-year history as England’s main port and trade center. Historic heartland of rowing in the UK.   Carries barges laden with farm products, coal, iron ore, and a variety of manufactured goods.
Asia	Yangtze       Ganges    Tigris /Euphrates	Across China       India, Bangladesh   Western Asia	3915       1569    1200 /1700	Highway for trade through the world’s most populous country.  Agricultural river.  3rd longest river. Home of world’s largest hydroelectric power plant.   Geographers, historians, and mythologists alike regard Ganges as the heart of Indian culture, tradition and living.   In fertile crescent, “cradle of civilization,” where Mesopotamians first emerged.
North America	St. Lawrence   Mississippi    Colorado	Southeastern Canada   Mid United States   Western United States	310   2340     1450	French first explored in 1500s.  Connects Great Lakes to Atlantic Ocean. Major waterway today.   Spanish/French first explored in 1500s/1600s.  Major inland freight transportation route today.   Formed Grand Canyon.  Opened up southwestern U.S. for mining/settlement in 19th century.
South America	Amazon     Parana	Brazil, Columbia, Peru   Brazil, Paraguay, Argentina	4000+     3030	Co-longest river; max water flow.  Largest drainage basin. Crosses world’s largest tropical rain forest.   Itaipú Dam provides major hydroelectric power.  Watershed contains large cities:  São Paulo, Buenos Aires, Rosario, Asunción, Brasília, and La Plata.
Africa	Nile     Congo	Northeastern Africa   Across Mid Africa	4132     2920	Co-longest river.  Ancient Egyptian civilization arose along river 5000 years ago. Small flow for major river.   Second in water flow; deepest river in world. Flows through huge equatorial rain forest.  Transport highway.
Australia	Murray     Darling	Southeastern Australia   Southeastern Australia	1610     1702	Greatest indigenous population along river before arrival of Europeans in 1600s.   Along with Murray River, region is chief supplier of wool, beef, wheat, and oranges.

Dynamic Rivers

Rivers are not static and they can also be destructive forces.  They routinely flood, and on rare occasions, they can abruptly shift pathways.  They can even reverse direction - either permanently or temporarily.

Changing Pathways.  These “channel-jumping” events, which are called avulsions, can wipe out entire cities along the largest waterways, yet they also create the fertile deltas that have nurtured many societies

Avulsions occur because of sediment deposition.  Over time, rivers deposit sediment at the avulsion site, choking up the river with sediment.  Water always flows downhill, so as its current course becomes increasingly blocked, it eventually jumps to a new location.

Much like earthquakes, river avulsions happen periodically in the same places.  They disperse sediment and water across the rivers’ flood plains.

Avulsions have caused some of the deadliest floods in human history.  Avulsions on China’s Yellow River killed over 6 million people in the late 19^th and early 20^th centuries.  Similar events have been linked to the decline of Mesopotamian civilization along the Tigris and Euphrates rivers in what is now Iraq, Kuwait, Turkey, and Syria.

Deadly flood of China’s Yellow River in 2000.

 

In 2008, the Kosi River in India shifted its course by over 60 miles in a matter of days, displacing over 3 million people.

In the U.S., the Mississippi River has changed course many times over the past 7,000 years. 

A river may not change course over many decades, or even centuries.  Scientists’ understanding of where these events occur is poor, and rests largely on a handful of detailed observations on large deltas, plus laboratory and computer models.

Reversed Direction.  Rivers can sometimes reverse their directions due to natural or man-made factors, including geologic eruptions, erosion, natural disasters, and city planning for a variety of reasons.

The Amazon River was born millions of years ago.  Prior to the massive growth of the Andes, water flowed in a westerly direction towards a group of lakes that steered the flow north to be deposited into the Caribbean Ocean.  Tectonic movement eventually evened out the lakes and created a large wetland area that eventually disappeared because the Andes continued to grow upwards and outwards.  The changes to the Earth’s crust and tectonic movements eventually reversed the flow of many of the westerly flowing rivers to an easterly direction towards the Atlantic Ocean, as opposed to the Pacific Ocean or Caribbean Sea.

Erosion also played a major part in the directional change of the Amazon.  The growth of the Andes increased the chances of rainfall, which produced more erosion coming off the mountain range, prompting the buildup of sediment faster than the river could wash it away. This was a major contributing factor to the course of the Amazon River flowing “backwards.”

The Wisconsin River, also permanently reversed direction naturally during the Pleistocene Period (2.58 million to 11,700 years ago) due to glacial action.

The Mississippi River temporarily reversed flow after Hurricane Isaac in 2012.  The amount of water that was dumped on the Mississippi River during the hurricane was enough to increase the flow of water into the mouth of the Mississippi. The water that was pushed into the Gulf of Mexico from the hurricane surged into the Mississippi River delta faster than the water coming out the Mississippi could hold it back.  Although this backwards flow wasn’t permanent - the situation only lasted for about 24 hours - the river showed an increase of water to the extent that it was flowing faster backwards than it does forwards on average.

The Mississippi River also reversed during Hurricane Katrina in 2005.  Rivers flowing backwards is a common occurrence during hurricanes alongside coastal routes around the world.

Tidal rivers, whose flow and level are by causes by tides, reverse their directions about twice a day.  Notable examples include the Hudson River, with its outlet in New York City; and the Saint John River (reversing falls) and the Salmon River, both with outlets in Canada’s Bay of Fundy.

At least four rivers temporarily reverse their flow annually due to flooding:  the Krupa, in Europe; the Petexbatun in Central America; the Qiantang, in Asia; and the Tonle Sap, also in Asia.

The Chicago River is one instance of a purposeful man-made river reversal. The river first flowed into Lake Michigan in the late 1800s, but was reversed to flow into the Mississippi River, when contaminated drinking water threatened the populations of people who relied on that source for clean drinking water and sanitation purposes.  A series of locks keep the river from reversing back to the original direction.

Working to reverse the Chicago River with a series of locks.

 

Human Intervention

Over-exploitation of rivers by humans through dam building and pollution threatens the water security of almost 5 billion people around the world.

Dams.  Humans have built dams for thousands of years to prevent flooding, or to allow people to develop or “reclaim” land previously submerged by a river.  Other dams are used to change a river’s course for development or agriculture.  Still others provide water supplies for nearby rural or urban areas.  Many dams are used to provide electricity to local communities.

In 1882, the world’s first hydroelectric power plant was built on the Fox River in the U.S. city of Appleton, Wisconsin.  Since then, thousands of hydroelectric plants have been built on rivers all over the world to harness the energy of flowing water to produce electricity.

Hydroelectric power is renewable because water is constantly replenished through precipitation.  Because hydroelectric plants do not burn fossil fuels, they do not emit pollution or greenhouse gases.

Today, hydropower accounts for 16% of the world’s electricity - but often at great cost to river ecosystems and communities.  Dams and hydroelectric plants change the flow and temperature of rivers.  These changes to the ecosystem can harm fish and other wildlife that live in or near the river.  And although hydroelectric plants do not release greenhouse gases, rotting vegetation trapped in the dams’ reservoirs can produce them.  Decaying plant material emits carbon dioxide, a major greenhouse gas, into the atmosphere.

Dams also affect people living near the rivers.  For example, more than 1.3 million people had to move from their homes to make way for China’s Three Gorges Dam and its reservoir on the Yangtze River.   In addition, dams can affect fish populations and the fertility of flood plains.  Fish may not be able to migrate and spawn.  Farmers who depended on the fertile flooding may be cut off from the river by a dam.  This can harm the livelihood of fishermen and farmers who live along the river, as well as consumers who must pay higher prices for food.

China’s massive Three Gorges Dam on the Yangtze River is the largest in the world.

 

Because of over-exploitation, once mighty rivers like the Colorado, Indus, Nile, Rio Grande, and Yellow are struggling to reach the sea.

Dams have already fragmented two thirds of the world’s great rivers.  They store about 1,680 cubic miles of water, or one sixth of the water flowing in rivers.  Currently, another 3,700 hydropower dams are under construction or in the pipeline.  Many of them are located in river basins with high ecological values such as the Amazon and Mekong.

Pollution.  Extensive use of rivers by humans has contributed to their pollution from directly dumping garbage and sewage, disposal of toxic wastes from factories, and agricultural runoff containing fertilizers and pesticides.

By the 1960s, many of the world’s rivers were so polluted that fish and other wildlife could no longer survive in them.  Their waters became unsafe for drinking, swimming, and other uses.

Stricter laws have restricted the substances factories can dump into rivers, limited the amount of agricultural runoff, banned toxic pesticides such as DDT, and required treatment of sewage.

Although the situation in some parts of the world has improved, serious problems remain.  An estimated 300-400 million tons of waste is thought to pollute our rivers and seas every year!

The Citarum River in Indonesia, for instance, is often cited as the most polluted river in the world. Textile factories near the Citarum dump toxic wastes into the river.  The garbage floating on top of the river is so thick that water is invisible.

The Citarum River in Indonesia is the world’s most polluted river.

 

Even after communities have limited river pollution, toxic chemicals may remain.  Many pollutants take years to dissolve, and build up in the river’s wildlife.  Toxic chemicals may cling to algae, which are eaten by insects or fish, which are then eaten by larger fish or people.  

In parts of North America and Europe, there is also the severe problem of acid rain that develops when emissions from factories and vehicles mix with moisture in the air.  The acid that forms can be toxic for many living things.   Acid rain falls as rain and snow.  It builds up in glaciers, streams, and lakes, polluting water and killing wildlife.

Environmentalists, governments, and communities are trying to understand and solve these pollution problems.  To provide safe drinking water and habitats, where fish and other wildlife can thrive, rivers must be kept clean.

River Management.  River management by balancing the needs of many stakeholders, or communities that depend on rivers, is critical for our future.   We must provide natural habitats for fish, birds, and other wildlife.  We must support industries for transporting goods and people across continents, and providing affordable power for millions of homes and businesses.  We must support farmers and agribusinesses who often rely on rivers for transportation and to supply water for irrigation.  And we must provide recreation areas and sporting opportunities such as fishing and kayaking.

 

Interesting Facts about Rivers

Hindus have always believed that the water of India’s Ganges River has purifying powers.  Although millions of people bathe in the river regularly, it does not usually spread cholera, typhoid, or other water-borne diseases.  Scientists have found that unique bacteriophages - viruses that destroy bacteria - kill germs in the water of the Ganges.  In addition, the Ganges holds up to 25 times more dissolved oxygen than any other river in the world.  The oxygen helps prevent putrefaction (rotting) of organic matter in the river.  Scientists do not yet know why the river retains so much oxygen.

Some record-breaking waterfalls formed by our planet’s rivers include:

·         Angel Falls, Venezuela - the world’s highest waterfall at a staggering 3,212 feet tall.

·         Khone Falls, Laos - the world’ widest waterfall, measuring an incredible 6.7 miles wide.

·        Inga Falls, Democratic Republic of the Congo - the world’s largest waterfall in terms of the amount of water.  Every second, 909,988 cubic feet plunges down the falls - that’s more than ten Olympic swimming pools!

Angel Falls, on a fork of the Río Kerepacupai Merú River, in Venezuela.

  

Rivers can be all kinds of colors - not just blue, clear, or muddy brown as you might expect.  In backwater rivers, found in swamps and wetlands, the waters look like strong black tea.  And in Colombia, the aquatic plants of Caño Cristales - known as the “River of Five Colors” - make the waters flow with bright blue, red, black, yellow, and green.

The River of Five Colors in Columbia.

Not all rivers flow overland.  London’s “lost rivers” are underground streams of the River Thames and River Lea that were built over as the city grew (now flow through culverts, with some of them integral parts of London's sewerage system and diverted accordingly).  A subterranean river also exists in the Philippines for example, where the Puerto Princesa Underground River flows beneath a mountain for five miles, before finally emptying into the South China Sea.

Some of the most fascinating river creatures include Amazon River dolphins (which have pink skin), electric eels (which stun prey and predators with powerful electric shocks), freshwater stingrays (which can grow to an enormous 16 feet) and freshwater turtles (which have been around for 200 million years).

Rivers are important geographically.  At least 17 countries - from India to Nigeria - are named after rivers.  Rivers make up 23% of international borders, 17% of the world’s state and provincial borders, and 12% of all county-level local borders.  Some 276 rivers flow across more than one country, and their basins cover almost half the Earth’s land surface.  The Danube River in Europe passes through more countries (10) than any other river in the world. 

 

A river is water is its loveliest form; rivers have life and sound and movement and infinity of variation; rivers are veins of the earth through which the lifeblood returns to the heart. - Roderick Haig-Brown

  

Principal Sources

My principal sources include: “Understanding Rivers,” education.nationalfeographic.org; “What are the 5 Longest Rivers in the World?” thecolector.com; “Rivers that Flow Backwards,” geographyrealm.com; “List of rivers that have reversed direction,” en.wikipedia.org; “Fascinating Facts About Rivers,” natgeokids.com; “30 Things You Didn’t Know About Rivers,” earthisland.org; plus, numerous other online sources.