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People and animals rely on water to survive, and some of the world’s greatest cities, including London, Paris, Bangkok, and Rome, are all situated along rivers. However, dams are becoming more prevalent than ever, with over 60,000 large dams worldwide.

The effect of dams on nature is widespread, with animals of all types, including fish, mammals, amphibians, reptiles, and insects losing their homes. The interrupted flow of water causes less sediment in the water, which means that new plants will not get the nutrients they need to grow.

A dam is a barrier put along a river to hold back the flow of water. The water is held at the top of the dam in a reservoir and can be used for electricity production or water supply. The water flow can often be adjusted, letting it through to the river below.

It is estimated that two-thirds of the longest rivers do not flow unimpeded by at least one dam. This has a significant effect on the wildlife that needs the water and the surrounding trees and plants.

Clean rivers provide food, drinking water, a place to swim and bathe and a way of travelling for people and animals. Although dams provide water to areas that otherwise have none, their damage to the wildlife and surrounding nature cannot be understated.

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Dams are needed

In the United States, large cities such as Denver, Phoenix, and Arizona wouldn’t exist without dams or canals, but this is similar worldwide. With over half of the world classed as arid or semi-arid, dams bring water to people that wouldn’t usually be able to get it. This allows farmers to work all year round rather than waiting for the annual monsoon, with the rains being caught in reservoirs rather than returning to the sea.

Although dams and reservoirs allow us to turn the water in reservoirs into electricity through turbines, the destruction that dams and reservoirs do to the surrounding area is monumental.

Effect on rivers

The river’s speed can make a massive difference in the wildlife that it contains. The strategies that they have used to survive can suddenly change, and survival is not guaranteed. An unimpeded river that falls from the mountains has a different force from one that is let out from a dam.

As the water falls, it pours over rocks and pushes the sediment down to areas where the river is calmer. Here, new plants will bed themselves into the nutrient-rich sediment, allowing fish to feed on them and bringing birds to the area.

If a dam impedes the river, then the sediment may not get over the rocky parts of the mountains. The plants will not grow in these rocky parts, so the fish will not be able to feed on them, and the birds will not come to feed on the fish.

The effect of one dam can be catastrophic, but with so many barriers being built, the damage is tenfold. There are at least 60 dams on the Missouri River and 25 on the Tennessee River. These can’t even be called rivers at this point but regulated streams. This isn’t just happening in North America but also in Asia. In India alone, they have over 5,000 dams.

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Effect on wildlife

As the reservoir at the top of a dam fills, the river underneath is changed forever. Floodwaters that rise due to the dam can mean death for many animals living in the valleys. The numbers rescued are often small, although many companies will do this for good publicity while not telling us about the hundreds of thousands that may have died.

The companies often rescue large animals for a more positive spin, while small mammals, reptiles, amphibians, insects, and plants are left to drown.

Waterways can become clogged with plants such as water hyacinth. These quickly spread over the water’s surface, causing huge problems to the water.

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Fish that live in the reservoirs are also impacted. Lake-dwelling fish supersede fish that were initially suited to rivers. They first tend to thrive on the release of nutrients from the soil, but as the vegetation in the reservoir rots down, it uses up the oxygen in the water. The water can become stagnant, killing all the fish.

Fisheries that are downstream of a dam can also see a considerable change. Without the nutrients brought down by the free-flowing river, fisheries can die off quickly.

Migratory fish such as salmon and sturgeon are usually the most affected as they can’t get to their spawning grounds. Many dams do have fish passage facilities such as fish ladders or turbines. However, these usually are only designed for one type of fish with others not able to get through.

Reservoirs also cause another problem, and if you have been near a reservoir, you may know what it is. Mosquitos love still water, and reservoirs are a breeding ground for them. Mosquitos carry malaria, and small invertebrates such as snails have tropical diseases that can cause problems to humans. Reservoirs being built in some tropical areas can cause everyone in the neighbourhood to get ill.

Silt is needed to fertilise the lands of farmers, and reservoirs hold back the nutrient-rich silt. In some areas where they wait for the annual rains, farmers beneath the dam miss out on the water and the silt that the fast-flowing rivers bring. This silt can often lead to problems for the barriers, as they clog it up, putting it out of action.

The flooding around dams also kills trees and plant life. This causes further problems as it decomposes, releasing vast amounts of carbon.

Artificial dams can cause many problems, but one animal, the beaver, also makes a dam. Beavers can make dams so large they can be seen on satellite images. These dams can cause their problems for the surrounding environment. These problems are similar to those made by man. However, beaver dams do create new wetlands where new ecosystems can thrive.