River

A river is a natural stream of fresh water that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. Rivers are a key part of the water cycle, which is how water moves around the Earth. Water gets into rivers from precipitation like rain, from runoff flowing down slopes, from melting glaciers or snow, and even from water seeping up from underground aquifers.

Rivers shape the land around them. They can overflow their banks and flood nearby areas, spreading helpful nutrients. The sediment, or alluvium, that rivers carry helps form deltas and islands. Rivers often twist and turn instead of flowing in straight lines, and their paths can change over time. They create canyons and valleys through erosion.

Rivers have been very important for both humans and animals for thousands of years. They provide food, transportation, and drinking water. People have built dams to control rivers, irrigate crops, and even create hydroelectricity. Rivers face threats from water pollution, climate change, and human activities like building dams and canals. Efforts to reduce pollution and restore river habitats are helping protect these important natural resources.

Topography

Definition

A river is a natural flow of freshwater that moves across land or through caves toward a lower place, like a lake, an ocean, or another river. Streams are smaller bodies of water that flow in natural channels, often called watercourses. The study of how water moves on Earth is called hydrology, and how it shapes the land is studied in geomorphology.

Source and drainage basin

Rivers are part of the water cycle, the way water moves around Earth. All the water in rivers comes from precipitation, like rain or snow. The land around rivers is usually higher, so water flows downhill into the river. The beginning of a river, called its headwaters, includes small streams that feed into the river. These small streams often start in mountains. The area of land that feeds a river is called its drainage basin or watershed. A ridge, a higher area of land, separates one drainage basin from another.

Some water soaks into the ground and can still reach rivers through underground layers. This helps rivers flow even during dry times. In warmer months, melting snow and ice add more water to rivers, keeping them full.

Flow

Rivers flow downhill because of gravity. They do not always flow from north to south. As rivers move, they join together to become larger. Smaller rivers that flow into larger ones are called tributaries. Rivers usually run through valleys between hills or mountains.

Rivers can also change their shape by bending or meandering. When something blocks a river, it can change direction. Over time, this creates bends in the river. In flat areas, rivers may flood during times of heavy rain, leaving rich soil that helps plants and animals grow.

Non-perennial rivers

Many rivers do not flow all year. This can happen in dry places where there is not enough rain, or in cold areas where rivers freeze in winter. These rivers still support life and play important roles in nature.

Subterranean rivers

Some rivers flow underground through caves, especially in areas where rock dissolves to form spaces. These underground rivers support many tiny living things and are studied by scientists.

Terminus

Rivers usually end by flowing into the ocean, but sometimes they may dry up before reaching it. Where a river meets the ocean, it can form a delta, a triangle-shaped area of land made from soil carried by the river.

Classification

Main article: Stream order

In the study of water, a stream order is a number that shows how rivers connect in an area where water flows together. The smallest parts of a river are called 1st order. When two 1st order rivers join, they become a 2nd order river. The order only goes up when two rivers of the same order join. This number helps us learn about rivers, like how big the area is that the river flows through.

Ecology

The ecosystem of a river includes many living things in the water, on the banks, and nearby land. Creatures in a river have different jobs. Some eat leaves and other organic material. Others eat algae on rocks. Some eat dead organisms, and predators eat living things.

Rivers change along their length. Shady areas with deciduous trees often have lots of leaves. In wider, sunnier parts, invertebrates and fish thrive. Downstream, predators like fish are more common.

Rivers can also flood, creating habitats like lakes and marshes. Plants along riverbanks help keep the soil in place and provide homes for animals. Smaller rivers hold smaller fish, while larger rivers can hold many kinds of fish.

Rivers help move things, too. Some fish, like salmons, swim upstream to lay eggs. Spores from fungal creatures also travel with the river's flow.

Human uses

Infrastructure

Modern river engineering includes many structures to control floods, help boats travel, and manage ecosystems. These projects often make floods smaller and more predictable and open more areas for boats. However, they can also reduce the amount of sediment large rivers carry. For example, the Mississippi River used to carry 400 million tons of sediment each year, but this has dropped by 60% due to dams, reservoirs, and other structures.

Basic river projects start with removing obstacles like fallen trees. This can grow into dredging, where sediment is dug up to make deeper channels for boats. These activities need regular care because riverbanks move, floods bring new objects, and sediment keeps building up. Artificial channels are sometimes made to cut through winding parts of a river, making travel shorter and more direct. This shortening is called channelization and has made traveling the Missouri River much easier.

Dikes are built perpendicular to rivers to help them flow straighter by speeding up water in the middle. Levees are like dams on river sides, built to stop flooding. They are often made by piling up soil or clay. Some levees also have floodways to redirect floodwater away from farms and towns.

Dams block water flow and can be used to help boats by keeping water levels higher upstream. They can also produce hydroelectricity, which is a clean energy source. Dams turn parts of rivers into lakes or reservoirs, giving cities a steady supply of drinking water. There are over 75,000 dams taller than 6 feet in the U.S., and globally, dams create reservoirs covering a large area. Dam building peaked in the 1970s but has since slowed, with new dams mostly in China, India, and other parts of Asia.

History

Pre-industrial era

The first civilizations appeared between 5,500 and 3,500 years ago near rivers. Rivers provided fresh water, fertile soil, and ways to travel, helping societies grow. Three early civilizations were the Sumerians between the Tigris–Euphrates river system, the Ancient Egyptian civilization on the Nile, and the Indus Valley Civilization on the Indus River. Deserts around these areas made rivers even more important, leading people to live near them and form the first cities. These civilizations also began farming in dry lands, which helped people specialize in different jobs and organize better.

In societies before factories, rivers were used for travel and resources. Shipping goods, especially floating wood on rivers, was very important. Rivers also gave drinking water and fish for food. Some rivers had fish but weren’t good for farming, like those in the Pacific Northwest. Animals like frogs, mussels, and beavers provided food and fur.

People have built things to use rivers for thousands of years. An ancient dam near Cairo, Egypt, on the Nile, was built 4,500 years ago. The Ancient Roman civilization used aqueducts to bring water to cities. Spanish Muslims used water wheels from the seventh century. Larger dams were built in Japan, Afghanistan, and India between 130 and 1492. Canals were dug in Egypt as early as 3000 BC, and tools like the shadoof were used to lift water. Big floods sometimes destroyed these early projects, but changes in climate also affected river flow.

Water wheels were used at least 2,000 years ago to get power from rivers. They turned an axle to move water into aqueducts, work metal with a trip hammer, and grind grains with a millstone. In the Middle Ages, water mills automated many jobs and spread quickly. By 1300, England alone had at least 10,000 mills. A medieval watermill could do the work of 30–60 people. Water mills often worked with dams to focus and speed up water. Water wheels were used through the Industrial Revolution to power textile mills and factories but were later replaced by steam power.

Industrial era

Rivers became more used with growing technology and population. As fish and water could come from elsewhere and goods could be moved by railways, people relied less on rivers. River engineering started projects for industrial hydropower, canals to move goods more efficiently, and flood prevention.

River travel has long been cheaper and faster than land travel. Rivers helped cities grow as goods like grain and fuel floated down to supply them. River travel is also important for the lumber industry, where logs are shipped by river. Countries with many forests and rivers, like Sweden, benefited most. The rise of highways and the automobile made this less common.

One of the first big canals was the Canal du Midi, linking rivers in France from the Atlantic Ocean to the Mediterranean Sea. The 1800s saw more canal building, with the U.S. building 4,400 miles of canals by 1830. Rivers were used more by cargo ships, and canals helped with river projects like dredging and straightening to move goods better. The Mississippi River is a major example, with its basin covering 40% of the contiguous United States. It carried crops from the American Midwest and cotton from the American South to other places and the Atlantic Ocean.

Urban rivers changed from being centers of trade and food to less important roles today. But they still matter to the cultural identity of cities and nations. Examples include the River Thames in London, the Seine in Paris, and the Hudson River in New York City. Modern efforts have improved water quality and added recreation. For instance, swimming was banned in the Seine for over 100 years because of pollution, but cleanup allowed it for the 2024 Summer Olympics. The Isar in Munich was restored from a canalized channel to a wider, natural area, helping wildlife and giving people more places to enjoy the river.

Politics

Rivers often serve as borders between countries, cities, and other territories. For example, the Lamari River in New Guinea separates the Angu and the Fore people. Up to 60% of fresh water used by countries comes from rivers that cross borders, which can cause disputes between upstream and downstream countries over water use and pollution.

Religion and mythology

Rivers have been important in human history, linked to life and fertility, but also to death and destruction through floods. This has made them central to religion, ritual, and mythology.

In Greek mythology, the underworld had several rivers. The River Styx was crossed by the dead on a boat led by Charon. Good souls went to Elysium to drink from the River Lethe to forget their past lives. Rivers also appear in stories of paradise in Abrahamic religions, like in the story of Genesis where a river from the Garden of Eden splits into four rivers, including the Tigris and Euphrates.

The book of Genesis includes a story of a great flood, and similar flood myths exist in the Epic of Gilgamesh and other cultures. In Genesis, the flood cleaned the Earth of human wrongdoing. This idea of water cleansing is similar to the Christian ritual of baptism, such as the Baptism of Jesus in the Jordan River. Floods also appear in Norse mythology, where the world comes from rivers flowing into a void. Aboriginal Australian and Mesoamerican myths also include floods, sometimes with no survivors.

Religions also hold some rivers as sacred. The Ancient Celtic religion saw rivers as goddesses. The Nile had many gods, with the goddess Isis said to cause its floods through her tears, represented by the goddess Hapi. Many African religions see certain rivers as life origins. In Yoruba religion, Yemọja rules the Ogun River in Nigeria and creates all children and fish. Some sacred rivers have rules, like not drinking from them or boating on parts of them. In Yoruba religion, the river is a living being to respect.

Rivers are very sacred in Hinduism. Evidence shows ritual bathing in rivers at least 5,000 years ago in the Indus river valley. The Ganges is the most sacred river, central to Hindu myths and believed to heal and free souls from the mortal world when cremated remains are released into it.

Threats

Freshwater fish are important, but many have disappeared because of things people do. Building dams and changing rivers can stop fish from moving and hurt their homes. Rivers that flow freely help keep water clean and support many plants and animals.

Pollution from factories and cities can also hurt river water. Chemicals and farm fertilizers can cause harmful algae to grow, which takes away oxygen and light that plants and animals need to live. In cities, rivers often flow through hard surfaces like stone and asphalt, which can cause flooding.

Climate change can make floods bigger and droughts longer, affecting rivers and the animals that live there. Efforts to protect rivers include saving wetland areas, limiting dam building, and controlling pollution to help keep rivers healthy for all living things.

Extraterrestrial rivers

Mars, the red planet, doesn’t have liquid water now. But scientists think rivers once flowed there a very long time ago. One big crater called Hellas Planitia has rocks and land shapes that look like they were made by rivers. These signs show that water moved and shaped the land billions of years ago.

On Saturn’s moon Titan, there are channels that might carry liquid methane. These rivers flow with a different kind of liquid than the water rivers we know. Scientists want to study these rivers to learn how land changes.