Water energy has downsides too. Dams cost a lot to build. They flood land and force people to move. They change rivers and harm fish. They can release methane from rotting plants. And they do not work everywhere. Understanding these downsides helps us make better choices about when and where to build dams.
Building a dam costs a huge amount of money. The concrete, steel, and machinery are expensive. The engineering and planning take years. Workers must build roads, tunnels, and power plants.
The Three Gorges Dam in China cost over 30 billion dollars. The Hoover Dam cost 49 million dollars in the 1930s. That is over 800 million dollars in today’s money.
Not every country can afford these costs. Many places that need electricity the most cannot pay for a dam. They have to choose cheaper options like solar or gas power.
Dams flood large areas of land. That land often has towns, farms, and villages. People who live there must move away.
The Three Gorges Dam displaced over 1.3 million people. That is more people than live in the city of Dallas, Texas. They lost their homes, their land, and their way of life.
The World Commission on Dams estimates that 40 to 80 million people have been displaced by dams worldwide. This is a serious human cost. It is one of the hardest things about building large dams.
Dams change rivers completely. A free flowing river becomes a still lake. The ecosystem that depended on the river’s flow is gone.
Animals lose their habitats. Plants are flooded. The water temperature changes. The amount of oxygen in the water changes. The whole river system is transformed.
Forests are often flooded when a dam is built. All the trees and plants underwater start to decay. This releases methane, a strong greenhouse gas. Some researchers say large reservoirs can release as much methane as a small coal plant.
Many fish species migrate up and down rivers. Salmon are the most famous example. They hatch in freshwater rivers. They swim to the ocean to grow. Then they swim back up the same river to lay their eggs.
A dam blocks this journey. The fish cannot get past the dam wall. Even with fish ladders, many fish do not make it. Some species have declined by over 90 percent in rivers with dams.
The turbines can also kill fish. Fish that go through the turbine blades rarely survive. New turbine designs are safer, but they are not perfect.
Rivers carry sediment. This is sand, silt, and clay that erodes from the land. When a river reaches a reservoir, the water slows down. The sediment settles to the bottom.
Over time, the reservoir fills with sediment. This reduces the amount of water the reservoir can hold. Less water means less power. It also means less water for irrigation and drinking.
Some reservoirs lose 0.5 to 1 percent of their storage each year. After 50 years, that is a big loss. Eventually the reservoir can become completely filled with mud. The dam becomes useless.
Not every place can use water energy. You need a river with a good flow and a steep drop. Flat rivers do not work. Dry areas do not work.
Most of the best hydro locations in developed countries are already used. New dams must be built in harder places. Or they must be built on rivers where the environmental cost is higher.
Some countries have lots of hydro potential but cannot use it. The Democratic Republic of Congo has huge rivers. But it does not have the money or infrastructure to build dams.
Hydropower depends on water. In dry years, there is less water. Less water means less power.
California experienced a severe drought from 2012 to 2016. Hydro production fell by over 60 percent. The state had to use more natural gas and coal to make up the difference.
Climate change is making droughts worse in many regions. A dam built in a wet period might not work as well in a drier future. This is a risk for hydro power.
Think about building a big wall across a creek. The water piles up behind the wall. That is a dam.
Now think about what happens to the creek. The water does not flow the same way. Fish cannot swim past. The area behind the wall gets flooded. The creek changes.
Building a dam is a big decision. It gives us clean electricity. But it changes the place where we build it. We get power. But the river is not the same.
It is like choosing between two things. Both are good in some ways. But you can only pick one. That is the hard choice about dams.
The ratio of benefits to costs varies widely for dams. Some dams provide cheap power for a century. Others struggle with sedimentation from day one. The geology, hydrology, and ecology of each site are unique.
Methane emissions from reservoirs are a growing concern. Tropical reservoirs in warm climates produce more methane than cool climate reservoirs. The methane can offset some of the climate benefits of hydropower.
Decommissioning old dams is another challenge. Dams do not last forever. Removing a dam is expensive and complex. The sediment built up behind it can be contaminated. Releasing it downstream can harm the river.
Belo Monte Dam, Brazil. Built in the Amazon rainforest. It flooded a large area of forest. It displaced indigenous communities. Environmental groups have strongly criticized it.
Aswan High Dam, Egypt. Built in the 1960s. It stopped the annual flood of the Nile River. The flood once brought nutrient rich silt to farmlands. Without that silt, farmers must use chemical fertilizers.
Three Gorges Dam, China. Displaced 1.3 million people. It also changed the ecosystem of the Yangtze River. The dam traps sediment that used to flow to the coast.
Discussion questions:
Vocabulary:
Last updated: June 15, 2026
What happens to people living where a dam is built?
What is sediment buildup in a reservoir called?
Why do some reservoirs release methane?
How does a dam affect fish migration?
What happens when a reservoir fills with sediment?
Answers: B: They must move to new homes, B: Sedimentation, B: Decaying plants release it, B: It blocks their path upstream, B: The reservoir holds less water
What is the biggest disadvantage of water energy?
The biggest disadvantage is the environmental impact. Dams flood large areas of land. They change river ecosystems. They can block fish migration.
How much does it cost to build a dam?
Large dams cost billions of dollars. The Three Gorges Dam cost over 30 billion dollars. Not every country can afford that kind of investment.
Do hydro dams hurt fish?
Yes, they can. Dams block fish from swimming upstream to reproduce. Fish ladders can help, but not all fish use them. Some fish populations have declined because of dams.
Can a dam cause earthquakes?
In rare cases, yes. The weight of the water in a large reservoir can put pressure on the ground. This can trigger small earthquakes in areas that are already geologically active.
Do hydro plants release greenhouse gases?
They do not burn fuel. But reservoirs can release methane. When plants decay underwater, they produce methane. This is a strong greenhouse gas.
