Reservoir facts for kids

Kardzhali Reservoir in Bulgaria is a giant lake in the Rhodope Mountains.

Some reservoirs, such as this one in Argos, Peloponnese, are used for fun outdoor sports.

A reservoir is a giant artificial lake built to hold and store fresh water. Most reservoirs are made by building a strong dam across a river. The dam holds back the river so water collects behind it. Reservoirs provide clean drinking water for millions of people worldwide. They also help generate clean electricity through hydroelectric power generation.

Reservoirs come in many shapes and sizes. People create them by blocking a streambed, digging deep basins, or building massive walls called levees. Along with supplying drinking water, reservoirs keep river levels safe during floods. They also store extra water so communities have plenty to use during dry weather.

Different Types of Reservoirs

Lake Vyrnwy Reservoir was the first large stone dam built in the United Kingdom.

People build different kinds of reservoirs depending on the landscape and their water needs. Engineers look at the natural shape of the land before choosing the best design.

Dammed Valley Reservoirs

The East Branch Reservoir helps supply water to New York City.

Cherokee Reservoir in Tennessee was built by the Tennessee Valley Authority.

A dammed reservoir is created when workers build a tall dam right across a river valley. This style uses the steep natural topography of the hills as giant walls. Water fills up behind the dam to create an artificial lake. An on-stream reservoir sits directly on the natural riverbed. It collects water from creeks, smaller rivers, and rainwater that runs off the hillsides. In contrast, an off-stream reservoir gets water diverted through an aqueduct or a large pipeline.

Engineers place the dam where the valley is narrow and strong. This narrow spot makes building safer, cheaper, and much faster. Building a huge reservoir can change the land completely. In some places, whole towns or historic monuments had to move to avoid the rising water. For instance, the famous ancient temples of Abu Simbel were moved before the Aswan Dam created Lake Nasser on the Nile in Egypt. In Wales, people moved from the village of Capel Celyn to make way for Llyn Celyn. A similar move happened at Borgo San Pietro of Petrella Salto before builders filled Lago del Salto.

During construction, workers direct the river away using special tunnels or side channels. In mountainous areas, builders might enlarge an existing lake instead of starting from scratch. When the new water level rises above the surrounding watershed divide, builders add extra side dams. An example of this is the Clywedog Reservoir in Mid Wales. If a valley is too steep for one big lake, engineers build several smaller ones in a row. Along the River Taff, for example, the Llwyn-onn, Cantref, and Beacons Reservoir form a connected chain of lakes.

Coastal Reservoirs

Aerial view of the Plover Cove coastal reservoir.

A coastal reservoir is built right where a river mouth meets the ocean coast. These reservoirs catch river water before it pours into the salty sea. Because land on shore can be crowded and expensive, coastal designs save valuable inland space. Many coastal reservoirs operate throughout Asia and Europe today. Famous examples include the Saemangeum project in South Korea and Marina Barrage in Singapore. Others include Qingcaosha in China and the famous Plover Cove Reservoir in Hong Kong.

Bank-Side Reservoirs

The Queen Mother Reservoir in England gets its water from the River Thames.

A bank-side reservoir is built next to a river rather than directly across it. Pumps or siphon pipes lift water from the river into the storage basin. Workers shape the basin by digging out earth and creating a tall encircling embankment. The walls and floor need a waterproof seal, often made using thick layers of rolled clay. Years ago, workers used a tough mix called puddled clay to keep water inside.

Water often rests in these giant basins for several months. While it sits, dirt settles to the bottom, which reduces muddy turbidity. Natural biological activity in the water also helps clean away impurities. If the nearby river becomes dirty or suffers from a summer drought, operators simply turn off the pumps. The giant city of London relies on bank-side storage. Water pumped from the River Thames and the River Lea fills deep basins like the Queen Mary Reservoir.

Service Reservoirs and Water Towers

Bankstown Reservoir is an elevated water storage tank in Sydney.

A service reservoir holds clean, treated water ready for people to use in their homes. These structures sit close to towns and cities so water reaches taps quickly. Many service reservoirs are built high in the air as classic water towers supported by strong pillars. Putting water high up creates natural water pressure, known to scientists as hydraulic head.

Other service reservoirs are giant water tanks, covered pools, or underground cisterns. Modern basins use tough geomembrane sheets to stop leaks and floating covers to prevent evaporation in hot climates. In London, Thames Water uses historic brick-lined underground chambers built during the 1800s. The huge Honor Oak reservoir opened in 1909 and remains one of Europe's largest covered reservoirs. It forms part of the Thames Water Ring Main, and its grassy roof is now used as a golf course.

Farming and Irrigation Reservoirs

An irrigation reservoir stores water specifically for watering crops on farms. Farmers fill these ponds by pumping groundwater from deep underground or by catching local stream runoff. The water is saved until dry seasons arrive, when thirsty plants need extra watering.

Farmers and scientists sometimes discuss the best ways to share water stored in agricultural reservoirs. When planned carefully, these pools protect crops while keeping enough water in local streams for wild animals.

Ancient History of Storing Water

People have built artificial reservoirs for thousands of years to survive dry climates. Around 3000 BC, farmers in ancient Arabia turned old volcanic craters into handy water storage pools.

In ancient India, dry seasons led people to invent deep stepwells and smart water resource management systems. Workers built an impressive reservoir at Girnar around 3000 BC. In ancient Greece, builders formed artificial lakes as early as the 5th century BC. Much later, in the 11th century, builders created the huge Bhojsagar lake in Madhya Pradesh, India.

In Africa, the ancient Kingdom of Kush invented an earth-walled reservoir known as a Hafir. More than 800 hafirs were built across the ancient city of Butana. These pools captured water during the rainy season, keeping people, crops, and cattle healthy for months. Visitors can still see the Great Reservoir near the Lion Temple at Musawwarat es-Sufra.

In Sri Lanka, kings built amazing water systems to feed their communities. The famous ruler Parākramabāhu I of Sri Lanka believed that every single raindrop should help people before reaching the sea. He ordered the construction of Parakrama Samudra, a reservoir so huge people called it the "Sea of Parakrama." In ancient Yemen, the great Marib Dam was built around 800 BCE by the Sabaeans to store water for centuries.

How We Use Reservoirs Today

Gibson Reservoir is a massive concrete dam and lake in Montana.

Modern reservoirs do much more than just store water behind a wall. They are important community tools that protect homes, generate power, and provide outdoor fun.

Drinking Water Supply

Most reservoirs capture raw water from rivers and send it along to a water treatment facility. The time water spends sitting quietly in a reservoir is called its retention time. This resting period allows heavy particles like silt to drop straight to the lake floor. Tiny natural creatures like algae, harmless bacteria, and zooplankton break down organic matter naturally.

In deep lakes, the water separates into warm and cold layers through a process called Lake stratification. During the summer, elements like manganese and phosphorus gather in the chilly, airless bottom water. In autumn, the lake water mixes together again. Water managers must test deep water carefully during a drought so water treatment plants run smoothly.

Clean Hydroelectric Energy

A cross section showing how water flows through a dam to turn turbines.

About a quarter of the world's large dams help generate clean hydroelectricity. Water stored high in the reservoir carries plenty of energy. When water rushes downhill through large pipes, it spins giant water turbines that produce electricity.

Some power stations use a clever system called pumped storage. When people use less electricity, like late at night, pumps push water into an upper lake. Later, when millions of people turn on their lights, operators release that stored water downhill. As it rushes back down into the lower lake, it creates instant electrical energy. Some rivers use run-of-the-river hydro, which makes power directly from flowing water without needing a giant storage basin.

Managing River Water and Preventing Floods

The peaceful Kupferbach reservoir near Aachen in Germany.

Reservoirs allow engineers to control how water travels down a river basin:

• Drinking supplies: Dam operators release water in measured amounts so towns far downstream have steady water to drink.
• Canal shipping: Reservoirs supply water to boat systems like the Rideau Canal, filling navigation lock chambers so boats can cross high hills.
• Farming networks: Stored water travels through long canal networks to reach dry fields.
• Flood protection: Flood-control dams catch heavy storm waters and release them very slowly to prevent flooding in downstream towns.
• Helping wildlife: Dam operators release sudden bursts of cool water, called freshets, to help migrating salmon swim upstream.
• White-water sports: Scheduled water releases create lively white water rapids for people paddling a kayak.

Keeping Water Flows Balanced

Reservoirs help balance river flow across the changing seasons. When a giant storm approaches, workers open the dam's spillway gates early. Letting water out beforehand leaves plenty of empty room behind the dam to swallow the storm surge safely.

Careful forecasting helps operators avoid problems like the 2010–2011 Queensland floods. Reservoirs like the Burrendong Dam in Australia protect downstream valleys by absorbing sudden cloudbursts. In Wales, natural lakes like Llyn Tegid help manage water within the River Dee regulation system.

Fun and Outdoor Recreation

Boating across scenic Chatuge Lake in North Carolina.

Many reservoirs are wonderful places to enjoy the outdoors. People visit them to go boating, sailing, and fishing. Quiet reservoir shores are perfect for bird watching, studying natural history, and hiking. To keep the water safe and clean, many reservoirs have clear rules for visitors.

How a Reservoir Operates

Water cascades down the spillway at the Llyn Brianne dam in Wales.

A reservoir catches water from upstream rain, melting snow, and bubbling underground groundwater. When the reservoir gets too full, excess water pours safely over a concrete spillway. Water is pulled from the reservoir using gravity so it can travel down pipes without extra pumping.

Engineers build tall concrete towers inside the reservoir called a draw-off tower. These towers have gates at different depths. This allows operators to draw clear water from the sunny surface or cool water from the bottom. Dam managers regularly send a steady stream of "compensation water" downstream to protect fish and plants.

Important Reservoir Terms

A water level marker shows how much water is inside a reservoir.

Scientists use special words to describe how much water a reservoir can safely hold:

• Inactive or dead storage: The water lying deep at the lake bottom that cannot flow out through the dam's gravity pipes.
• Active or live storage: The working water that operators can release for drinking supplies, electricity, or boat navigability.
• Flood capacity: The extra empty space kept ready at the top to trap sudden floodwaters.
• Spillway crest: The top edge of the concrete overflow wall.
• Surcharge capacity: The temporary water that piles up higher than the spillway during a torrential rainstorm.

Modern water managers use computer programs like WAFLEX and Water Evaluation and Planning to balance water supplies across whole regions.

Keeping Reservoirs Safe

Storing billions of gallons of water requires excellent engineering and constant inspections. If a dam cracks, a sudden wave of water can flood communities downstream. In 1925, a dam failure at Llyn Eigiau in Wales showed the world why strict dam safety laws are so important. Today, engineers monitor dams every day using sensors and cameras to ensure they stay rock-solid.

History also shows that dams have played roles in major conflicts. During World War II, British pilots from No. 617 Squadron took part in Operation Chastise, also known as the Dambusters raid. Their mission targeted big dams along the Ruhr and Eder rivers to stop factory production. Today, international laws strictly protect dams so they remain safe for nearby communities.

Environmental Effects of Reservoirs

Brushes Clough Reservoir near Shaw and Crompton, England.

Building a reservoir creates a brand-new body of water, but it also alters the natural world around it. Engineers and scientists study these changes carefully before starting any project.

Greenhouse Gases and Climate

When a new reservoir fills with water, grasses, trees, and soil end up deep underwater. As this plant matter slowly breaks down in an anaerobic (oxygen-free) space, it produces methane and carbon dioxide. Methane is a very strong greenhouse gas that traps heat in Earth's atmosphere.

Hydroelectric dams produce clean electricity without burning coal or producing smokestack pollution like sulfur dioxide, nitric oxide, or carbon monoxide. However, reservoirs in warm tropical regions can release more greenhouse gases than those in cold northern regions because warm plants decay much faster. For instance, the Balbina Dam in Brazil covered a huge, flat jungle and released high amounts of gases. In contrast, Brazil's Tucuruí Dam and reservoirs in Canada produce far fewer emissions than fossil fuel power plants.

Fish and Wildlife

A great cormorant hunting for fish at Farmoor Reservoir in Oxfordshire.

Dams can block fish that need to swim upriver to lay their eggs. To solve this, engineers often build special fish ladders or water elevators alongside the dam. At the same time, new reservoirs create great wetland homes for water birds like ducks, herons, and the great cormorant.

Flooding soil can also change how natural minerals behave. It can turn harmless mercury in dirt into a harmful chemical called methylmercury. This substance can enter the food chain through zooplankton and small fish. Sometimes, changing a flowing river into a still lake allows an invasive species to move in and crowd out native animals.

Local Weather and Geology

A giant lake can gently change the local climate of the surrounding valley. The water adds extra moisture to the breeze and keeps temperatures from getting too hot or too cold. In some dry areas, such as the valleys of South Australia, this mild weather helps farmers grow high-quality fruits.

Very large reservoirs can even cause tiny underground tremors, a phenomenon called reservoir-triggered seismicity. The massive weight of water pressing down on deep rock cracks called a stratum can trigger small, harmless shakes. Historic examples include Greece's Marathon Dam and Kremasta Dam, America's famous Hoover Dam, India's Koyna Dam, Zambia's Kariba Dam, and China's Xinfengjiang Dam.

Liptovská Mara is a large reservoir that changed the local climate in Slovakia.

The World's Largest Reservoirs

Lake Volta in Ghana photographed from space.

Reservoirs can be measured by the flat surface area of the water or by the total volume of water they hold.

Largest Reservoirs by Surface Area

The largest reservoir by surface area is Lake Volta in Ghana, covering thousands of square kilometers.

Largest Reservoirs by Water Volume

Lake Kariba holds more water than any other artificial reservoir on Earth.

When measured by how much water they can hold, Lake Kariba along the border of Zambia and Zimbabwe is the biggest in the world.

See Also

In Spanish: Embalse para niños