A river is a flowing stream that leads to the sea, a lake, or a river. The water is usually fresh; rivers begin as small streams that get larger the further they flow.
Some rivers flow all year round, whereas others only flow in certain seasons or if there’s been a lot of rain. Rivers can be thousands of miles long and form geological features of a landscape, like valleys, gorges, and canyons. The Grand Canyon is perhaps the most incredible example of rivers’ impact on a landscape.
While rivers are major features in a landscape, they only cover around 0.1% of land on Earth. Nevertheless, they’re essential to life for many animals, and throughout history, human civilizations have been built around the freshwater they provide. Most major cities in the world are situated on the banks of rivers. More than just a source of water, rivers have historically helped people obtain food, travel and transport goods, defend territory, and more.
The Geography of a River
How are rivers formed?
No matter how big or small, all rivers have a starting point where the water begins its flow. This is called a headwater.
Rivers generally begin in upland areas. When rain water and melting snow collect on high ground, it begins to form little streams, which flow downhill because of gravity. These small streams, brooks, or creeks join together, becoming larger and larger until they start rivers.
The water shapes the landscape on its way down, eroding rock and carving out networks of valleys. When it reaches the lower ground, rivers widen and take a more winding route, with most rivers emptying into the sea.
What are the three stages of a river?
The Upper Course
The source of a river is often a spring found on a hill, mountain, or glacier. The part of a river near the head is called a young river. Rain falling in highland areas flows downwards and collects in channels, forming a stream. As the stream continues to run downhill, it is joined by other streams and increases in size and speed. The point where two rivers join is called a confluence.
The Middle Course
Fast-flowing water causes erosion as a river reaches its middle course, which makes it deeper and wider. The river erodes left and right, forming horse-shoe-like loops called meanders.
The Lower Course
In the lower course, a river is in flatland and flows slowly. The force of the water is lower than in the other stages, so the river deposits all the bits of eroded land it has been carrying with it.
What are the main parts of a river?
In the upper course of a river, you’re likely to find:
- Source — Have you ever wondered what the start of a river is called? Well, wonder no more! The beginning of a river is called the source. The front of a river can also be called the headwaters! What’s interesting about the beginning of a river is that, even if the water gets fast and powerful in the later stages, the source is usually quite calm. Often, the source of a river is a spring that comes from underground or marshy areas fed into by snow. The source of a river can also range drastically in terms of size. For instance, it can be huge, with many smaller water sources flowing together. On the other hand, the source can also be tiny, with just a small amount of water coming in from a lake or pond. The start of a river is very important and dictates the river’s health as a whole because anything that happens there affects everything upstream.
- Potholes — Potholes are small holes found in the bedrock of a river bed. These holes are formed from sediment and other materials carried by a river and scouring the river bed.
- V-shaped Valleys — These valleys are formed by erosion from a river over time.
- Interlocking Spurs — Interlocking spurs are projecting ridges that extend alternately from the opposite sides of the wall of a young, V-shaped valley that a river flows down.
- Waterfalls — A waterfall is when a body of water makes a steep fall off a rocky ledge into a plunge pool below. This fall is from a great height. Another name for a waterfall is a cascade. Waterfalls are formed through the process of erosion.
- Rapids — The rapids are sections of a river where the water bed has a fairly steep gradient which causes the water to move very fast, especially over rocks.
- Gorges — A gorge is a narrow valley with steep, rocky walls between hills or mountains.
In the middle course of a river, you’re likely to find:
- Floodplains — Floodplains are flat, low-lying ground formed mainly of river sediments. These areas are typically found next to rivers, lakes, and coastal waters that periodically flood when the water level is high. Floodplains are key in sustaining the plant and animal populations around them. They also benefit nearby humans, as floodplains absorb the excess water that would otherwise travel downstream and flood the areas where people live.
- Meanders — The meanders of a river are the name given to the bends that the river takes. These bends are caused by the water chipping away at the soil outside of a river bend and placing it on the inside. This process happens slowly over time.
- Oxbow lakes are U-shaped lakes created when a wide meander of a river is cut off and makes a freestanding body of water.
And in the lower course of a river, you’re likely to find:
- Large Floodplains — Large floodplains are larger areas of low-lying ground formed mainly of river sediments.
- Deltas — The delta is the end of a river. In these large, silty areas, the water in the river slows down and splits off into different channels. Deltas usually occur when a river meets an ocean, lake, or wetland.
- Estuaries — The estuary is the wide part of a river that meets the sea.
- Levees — The levee is a barrier designed to prevent flooding.
What is the start of a river called?
The start of a river is called the source. This is the place where the river begins its journey toward the sea. Rivers can have more than one source and tributaries, where different strands of rivers and streams join together to form one river.
The start of a river is also sometimes called a headwater.
What is the bottom of a river called?
Rivers flow in channels. The bottom of a river is called the river bed, and the sides of the track are called the banks.
What’s the end of a river called?
The end of the river is called the mouth. There’s often a river delta at the mouth, a large, silty area where the river splits into many slow-flowing channels. These channels often have muddy banks. The landform of a river delta is created by the deposition of sediment that the river has carried as the flow leaves the mouth and enters either an ocean, sea, estuary, lake, reservoir, or (less often) another river that can’t carry away the sediment.
Where is the river fastest?
A river normally fasts in the upper reaches.
Can a river have a beach?
Beaches can be found alongside rivers and formed from sediment or eroded materials.
What is the valley of a river?
Valleys are low-lying land areas between hills or mountains that rivers run through. River valleys are often V-shaped near the river’s source but become U-shaped and wider as the river gets closer to sea level.
The Ecology of Rivers
The ecology of rivers is all about the living organisms within the river, specifically their relationships with one another and the ecosystem. The term ‘ecosystem’ refers to the environment and is made up of the many interactions between the plants, animals, and microorganisms in said environment. In addition, the interactions between living organisms and non-living physical and chemical components make up the ecosystem.
There are several key components of every river ecosystem:
- Flowing water that is largely unidirectional
- A state of continuous physical change
- Different microhabitats that are frequently changing
- Variability in terms of the flow rates of water
- A selection of plants and animals that have adapted to live within water flow conditions
Let’s break down some of the major factors and aspects of a river ecosystem:
Water flow
Water flow is the main thing that sets river ecosystems apart from other water ecosystems. There is no one set type of water flow for all river ecosystems, as the strength and speed of the flow vary greatly depending on the kind of river you’re dealing with. In addition, a range of factors can impact a river’s water flow, such as input from snowmelt, groundwater, and rain. Another reason why water flow is such a huge aspect of river ecosystems is because it can massively affect and change the shape of riverbeds through erosion and sediment.
Substrate
The substrate refers to the surface on which the organisms in a river live. The substrate can vary. For instance, the substrate can be inorganic and composed of geological material from the surrounding area, such as pebbles, gravel, sand, or boulders. Alternatively, it can be organic, in which case it would be composed of fine particles, leaves, wood, moss, and plants. The substrate is not a permanent fixture of a river ecosystem; it can change greatly during floods, etc.
Light
Light is a vital part of the river ecosystem, as it supplies the energy for photosynthesis. Photosynthesis is the river’s primary food source so it would die without light. Light is also extremely useful for prey species living in the river, as they can hide from predators in the shadows it casts. The amount of light that a river receives completely depends on the location of the river. For example, if it is in an exposed area with little to no overhanging trees or shrubbery, the river will receive a lot of light, as the sun can shine directly onto the surface. On the flip side, if the river is in a shaded area, such as a forest, the river will receive much less light as there is limited sunlight access.
Temperature
The water temperature in rivers varies greatly between different environments. The water temperature is altered through radiation at the surface of the water and conduction to and from the air and surrounding substrate. Additionally, in deep, slow-moving rivers, the water temperature can vary significantly within that one river ecosystem, depending on the whereabouts of the water you are in. Many factors come into play when determining the temperature of the water, such as the climate, shading, and elevation. Not all animals are suited to live in these environments, as they need a set temperature. The organisms that can live in this type of water are known as poikilotherms. These organisms can live here as they can alter their internal temperatures to suit their environment.
Water Chemistry
Like many other aspects of a river ecosystem, the water chemistry varies between different environments. The water chemistry is largely determined by inputs from the surrounding area and can also be impacted by rainfall and pollution from human sources.
The most important component of water chemistry in every river ecosystem is oxygen. This is because the majority of living organisms need it to survive. For the most part, oxygen enters the river at its surface, but its solubility lessens as the temperature of the water increases. This means that rivers with an overall lower water temperature, which tend to be fast-flowing rivers, generally contain more oxygen. Conversely, rivers with a higher water temperature, which tend to be slower-flowing rivers, have a lower oxygen content. Several factors can limit the water’s ability to absorb oxygen, such as:
- Poor water circulation
- High levels of animal activity
- Large amounts of organic decay in the waterway
Bacteria
While they may seem insignificant, bacteria play a key role in keeping the river ecosystem running smoothly. Each river ecosystem contains a vast amount of bacteria, which is essential for energy recycling. The part of bacteria is to decompose organic material into inorganic compounds, which plants and other microbes can use for energy.
Plants
Plants, like bacteria, play a key role in sustaining life in a river’s ecosystem. This is because, when plants photosynthesize, they convert light energy into chemical energy that can then be used as fuel for other organisms within the river.
Algae is a type of plant that is found in the majority of river ecosystems. It is also the most significant source of primary food in rivers. For the most part, algae can be found floating freely. This means that algae get swept away quickly in fast-flowing waters and, therefore, cannot sustain large populations of organisms. In slow-moving waters, however, algae can stick around in the same place for longer and tend to build themselves up to large numbers. To avoid being swept away, some species of algae, such as moss, fix themselves to various objects in the river.
Invertebrates
Invertebrates are animals with no backbone. The invertebrates in river ecosystems include snails, mussels, clams, and limpets. Invertebrates can live in almost every habitat available in the river, including the water’s surface, below the substrate, under rocks, floating in the current, and more. To avoid fast-flowing currents, some invertebrates live in the substrate area or on the sheltered downstream side of rocks. However, invertebrates cannot prevent the current altogether, as they depend on it to supply food and oxygen.
Invertebrates are both consumers and prey in river ecosystems.
Fish
Different species of fish exist in different kinds of habitats in the river, depending on their genetic makeup. For instance, most fish avoid the current by living near the bottom of rivers, on the banks, or behind obstacles like rocks. These fish only venture into the current to get food or change location. On the other hand, some species of fish never go into the current. A variety of fish also requires different lotic systems, such as springs, streams, wetlands, and oceans, for various life cycle stages.
Like invertebrates, fish are both consumers and prey species in river ecosystems.
Birds
When we think of the different wildlife found in rivers, we often forget birds, but numerous birds live in river ecosystems. However, birds are not restricted to only living in the river, unlike fish and other wildlife. For this reason, birds tend to split their time between the river and different terrestrial habitats.
Uses of Rivers
Rivers have a wide range of uses for animals, humans, and the environment. Some of these uses include:
Construction material
Rivers are home to many coarse sediments, sand, and gravel, vital construction materials. In some circumstances, using these materials in construction can give way to new lake habitats as gravel pits fill back up with water. Alternatively, in other cases, using river materials for construction can harm and destabilize the riverbed and course. It can also damage spawning fish populations that rely on stable gravel formations to lay their eggs.
Energy production
Fast-flowing rivers are a popular energy source through structures like watermills and hydroelectric plants. While hydroelectric plants are a much more recent invention, there is evidence to suggest that watermills have been used for hundreds of years. Before steam power was invented, watermills were widely used to grind cereal and process wool and other textures. However, with the increased need for sustainable energy sources in recent years, there has been a ride in the development of large-scale power generation from water.
Food source
Rivers are a rich source of food for a wide range of species, and they have existed since before recorded history. Rivers contain edible aquatic life, such as fish, mussels, clams, etc. Moreover, rivers are a huge source of fresh water widely used for drinking and irrigation. Rivers are also extremely useful for determining how urban areas like cities are structured. They are often the focal point for urban renewal projects, such as river walks and greenways. Moreover, rivers are used as a method of disposing of wastewater. In the past, they have also been used to dispose of other waste (many countries in the developing world still use this method).
River Facts (KS2)
- Rivers flood when there’s heavy rainfall. Floods can cover surrounding areas with water that rises above people’s homes.
- When rivers flood, they deposit nutrients in the land, making it fertile for farming.
- Some rivers flow through underground caves. These form when rock cracks above allow rainfall to fall through and collect.
- At 300-340 million years old, the Finke River is the oldest in the world. It runs through central Australia, and Aboriginal legend says it was formed when the Rainbow Serpent shot out from Lake Eyre.
- Different names, such as creeks, streams, and brooks, refer to small rivers.
- In the USA alone, there are around 3.5 million miles of rivers.
- Seventy-six rivers worldwide are over 1000 miles (ca. 1,609 km) long.
- When people build dams to restrict a river’s flow, the lake that forms is called a reservoir.
- At 220 miles (ca. 354 km), the River Severn is the longest river in Britain.
- The Congo River is the deepest. It is thought to be around 230 m deep in parts — enough to submerge Big Ben 2.5 times!
- Rivers form beautiful geographic features as they travel from their sources, such as valleys, canyons, lakes, and waterfalls.
- Despite being essential to life on the planet, our rivers are polluted with chemicals, sewage, and waste. It is thought that up to 400 million tonnes of garbage pollute rivers yearly.
- Lots of places are named after rivers or river crossings. For example, Oxford got its name because there was a place on the River Thames where people could cross the river with their oxen.
- The Welsh word for a river mouth is ‘aber,’ so many towns in Wales have ‘aber’ in their names to describe the rivers they are on.
- We celebrate World Rivers Day on the fourth Sunday of September each year. The event highlights the beauty and importance of rivers and strives to increase public awareness around the improved stewardship of rivers.
Features of a River
The helpful diagram below highlights the various features of a river.
Rivers Around the World
The Amazon River
The Amazon is the second-longest river in the world and has the largest capacity. At its widest point, the river is six miles wide.
The Amazon flows through Peru, Brazil, Ecuador, Colombia, and Venezuela before flowing into the Atlantic Ocean.
The Ganges River
This 2525 km river begins in the Himalayas. It flows through Nepal, India, and Bangladesh and empties into the Bay of Bengal.
Hindus believe its waters are purifying and that bathing in them will forgive sins and cure illness.
The Mississippi River
The Mississippi is the longest river in North America at 3779 km. Its source is Lake Itasca in Northern Minnesota, and it travels through ten states before emptying in the Gulf of Mexico.
Amazing facts about the Mississippi River
Impress your children with these amazing facts about the Mississippi River!
- It takes 90 days for a drop of water to travel the entire Mississippi River.
- From its source in northern Minnesota to the Gulf of Mexico, the Mississippi River drops 1,475 feet (0.45 kilometers).
- It’s home to 360 species of fish, 326 species of birds, 145 species of amphibians, and 50 species of mammals.
- The Mississippi River is the third-largest watershed in the world.
- The deepest place on the Mississippi River is 200 feet deep and is located near Algiers Point in New Orleans.
- At its skinniest point, the river is between 20 and 30 feet (ca. 9 m) wide, but at its largest, it is more than 11 miles (ca. 18 km) wide!
- The name Mississippi comes from the Anishinabe people, who called the river ‘Messipi,’ which means Big River or Father of the Waters.
- The river runs through 10 states, including Louisiana, Mississippi, Arkansas, Tennessee, Kentucky, Missouri, Illinois, Iowa, Wisconsin, and Minnesota. This includes a massive 110 counties.
- Every day, the river distributes water to over 18 million people.
- It inspired many of Mark Twain’s famous stories, including the well-known ‘The Adventures of Huckleberry Finn and ‘The Adventures of Tom Sawyer.’
- A Slovenian swimmer named Martin Strel swam the entire length of the ‘Great River Road’ (2,414 miles (ca. 3,885 km) / 3,885 kilometers) in 68 days in 2002.
The Murray River
With its source located high in the Australian Alps, the Murray River is 2575 km long and flows into the Indian Ocean. It’s home to a diversity of wildlife that can only be found in Australian waters, such as golden perch, Murray cod, and the platypus.
The River Nile
The Nile is thought to be the longest river in the world at 6650 km. It’s located in North-Eastern Africa and runs through Sudan, Burundi, Rwanda, DR Congo, Tanzania, Kenya, Ethiopia, and Egypt.
Most of the cultural and historical sites of Ancient Egypt are found along the riverbanks of the Nile.
The Yangtze River
The Yangtze is the longest river in Asia at 6418 km, and it flows from the glaciers on the Tibetan Plateau to the East China Sea at Shanghai.
It’s heavily polluted by local industry and is home to endangered species like the Chinese alligator, Chinese Paddlefish, and the river pig.
River Flooding
Rivers burst their banks and flood for various reasons, determined by factors in the surrounding landscape. As a result, floods can be disastrous and have a positive long-term impact.
What causes a river to flood?
Floods occur when a river bursts its banks, causing water to spill onto the floodplain. The main cause of the flooding is heavy rain; the quicker the rainwater reaches the river channel, the higher the chances of a flood.
Several factors of the surrounding landscape will affect how quickly rainwater reaches the channel. For instance:
- Steep-sided channels — A river channel enveloped by steep slopes leads to faster surface run-off.
- A lack of woodland or vegetation — Trees and plants catch or drink water, so a lack of either in the drainage basin means the surface run-off will be high.
- A drainage basin that consists primarily of impermeable rock — Water can’t percolate through the rock later, leading it to run quickly over the surface.
- Drainage basins in urban areas — These are made up primarily of impermeable concrete, prompting overland flows. In addition, drains and sewers divert water to the river channel, while homes with sloping roofs increase the run-off.
What are some flood management techniques?
River Engineering
River engineering is a branch of civil engineering that studies human intervention in the course, characteristics, and flow of a river to achieve some benefit. It’s an ancient practice; before recorded history, people intervened in the system and behavior of rivers for various reasons, such as management of water resources, protection against flooding, or facilitating passing along or across rivers. Methods of flood management or prevention are often based on improving the river’s flow.
Afforestation
Afforestation is the deliberate planting of trees in a drainage basin to increase interception and storage of water while limiting surface run-off. Afforestation reduces a river’s discharge, so it’s less likely to flood. It also prevents mass wasting, which lowers the amount of soil entering the river and keeps the capacity of the river high. The method is particularly effective at reducing the risk of flooding when combined with floodplain zoning.
As well as offering protection against flooding, afforestation creates new habitats for animals and filters pollutants out of rainwater, thus improving water quality. The disadvantages of the method are that it requires a lot of space to be effective, while it can be unpopular among farmers since the trees can sap nutrients from the soil.
Floodplain Zoning
Floodplain zoning organizes flood defenses to ensure that land near the river that often floods isn’t built on. Instead, the land could be used for pastoral farming, playing fields, and the like, areas that rarely get flooded and are used for houses, transport, and industry.
Wetland Restoration
Wetland restoration involves creating conditions that facilitate the creation of wetlands (swamps or marshes). Wetlands can store large volumes of water, helping reduce the discharge in a river. Wetlands don’t reduce flooding where they’re located, but rather further downstream of the river. Like afforestation, wetland restoration creates new habitats for animals and increases biodiversity. However, they limit the land available for farming, which means they can be unpopular with farmers.
River Restoration
River restoration is the restoration of a river that’s undergone hard engineering to its original course. It ranges from un-straightening a channel to removing artificial levees or diversion spillways. This may seem paradoxical, as restoring the river to its original method will undo the engineering that aimed to prevent flooding in the first place. However, restoration can divert flooding to less valuable land, meaning the risk of downstream flooding falls.
The method can prevent and revert any potential environmental or ecological damage introduced by engineering projects while it is very cheap. Issues mainly arise when it’s served in areas still in use. The local environment agency decides whether the land is valuable, and their decision may not necessarily represent the opinions of the people who use it.
What are the effects of flooding?
Floods can wreck people’s homes, ruin people’s possessions, and dramatically disrupt communities and communications. Still, flooding can have some positive impacts on an area. For example, when a river floods, it deposits fine silt (alluvium) onto the floodplain, making it fertile and excellent for agriculture. Regular flooding may be necessary for those living on or near floodplains to support their farming and provide them with food.
In general, floods have a greater impact on less economically developed countries (LEDCs) than on more economically developed countries (MEDCs). This is due to many factors; for instance, LEDCs tend to have more farms, and farming communities are often situated near fertile flood plains. Another important factor is that LEDCs don’t always have the resources to prevent flooding or manage its aftermath.
How are white water rapids formed?
White water rapids are sections of turbulent water. They usually occur in the upper course of a river, and layers of hard and soft rock form them. The layers of soft rock erode faster than the layers of hard rock, so the river’s bed is uneven. Rapids are popular with tourists, adventurers, and thrill-seekers looking to raft down them.
Rivers in Europe
- The Volga is the longest river in Europe, at 2,290 miles (ca. 3,685 km) in length. It travels across Russia and empties into the Caspian Sea and has been used to transport resources for centuries.
- The Thames is one of Europe’s most historic rivers and made London an important Roman trading post from as early as 100 CE.
- The busiest river in Europe is the Rhine. It runs from the Swiss Alps through Germany and the Netherlands before it empties into the North Sea. The river is used to transport a variety of goods and resources.
The Volga River
Rivers in Asia
- The Yangtze River in China is the longest in Asia, acting as a trade highway through the world’s most populous country.
- In the Indian subcontinent, the Ganges is considered sacred to Hindus, who believe its water purifies the soul and heals the body.
- The Tigris and Euphrates rivers flow from Turkey, through Syria and Iraq, and into the Persian Gulf. The earliest evidence of civilization and agriculture was found near the rivers.
The Euphrates River
Rivers in North America
- French explorers traveled the St. Lawrence river in Canada in the 1500s, discovering fish, wildlife, and Native American tribes.
- The chief river of North America is the Mississippi, and its importance as a trade route has increased over time.
- The Colorado River helped the Grand Canyon, cutting through rock layers over millions of years.
The Colorado River
Rivers in South America
- The Amazon is stronger than any other river in the world; more water flows through the Amazon than is carried by the Nile, the Yangtze, and the Mississippi combined.
- The Paraná River on the Brazil-Paraguay border provides energy to lots of communities.
- The Río de la Plata is the widest river in the world, with a maximum width of 220 km, and forms part of the border between Argentina and Uruguay.
The Paraná River
Rivers in Africa
- The two largest rivers in Africa are the Nile and the Congo. The Lower Nile was home to one of the earliest civilizations; around 5,000 years ago, the ancient Egyptians benefitted from the Nile’s annual flooding, which made the land fertile.
- The Congo river flows through a huge equatorial rainforest and is the deepest river in the world. It’s a highway for transporting goods, and vast urban areas are situated on the river banks.
The Congo River
Rivers in Australia
- Despite Australia’s arid climate, there are still rivers running through it. The Murray River flows 2,590 km from the Snowy Mountains to a lagoon on the Indian Ocean. Indigenous Australians revered the Murray River, and the Murray Valley had the highest population density on the content before Europeans arrived in the 17th century.
- The Darling River joins the Murray River near the town of Wentworth and flows 2,739 miles (ca. 4,408 km) from the highlands of the eastern coast.
The Darling River
River Terms
- Bank — The riverbank is the land at the side of the river.
- Basin — The land water must cross to reach a river. It collects all available water from its area’s tributaries, creeks, and streams.
- Bed — The bed is the bottom of a river and can be made of sand, rocks, or mud.
- Canal — An artificial waterway that is used so that boats can transport goods across the country.
- Confluence — Where two rivers join together.
- Current — The strength and speed of the river. Water always flows downhill; the steeper the ground, the stronger the current.
- Delta — A wide muddy or sandy area where some rivers meet the sea. Here, the river slows down and drops all the sediment it’s carrying.
- Downstream — The direction that the water flows, downhill from its source towards the sea.
- Drainage basin — The area of land drained by a river.
- Erosion — When a fast-flowing river damages the riverbanks and washes bits of them downstream, making the river wider.
- Estuary — Where a river reaches the ocean and the river and ocean mix. Estuaries are normally wide and flat.
- Floodplain — The flat area around a river often gets flooded when the water level is high.
- Freshwater — Water with low concentrations of dissolved salts and other total dissolved solids. River water is often fresh water instead of seawater or brackish water.
- Meander — A river that follows a winding course.
- Mouth — The point at which a river enters the sea.
- River channel — The area a river flows in, with banks on each side.
- Saltwater — The type of water in seas and oceans.
- Silt — Small bits of dirt or sand that are carried along by a river.
- Source — The point at which a river starts its journey. This could be a spring on a hillside, a lake, a bog, or a marsh. A river may have more than one source.
- Stream — A small river.
- Tidal river — At the end of a river, near the ocean, water from the sea flows up the river when the tide comes in. This part of the river is called ‘tidal.’
- Tributary — A small river leads into a bigger river.
- Upstream — The opposite direction to how the river’s water flows.
- Watershed — Water flows down the side of hills into rivers, but the water that lands on opposite sides of the same hill might flow into different rivers. The watershed is the boundary between two river basins.
