User:Fleetflame/art1

Any time a dam is built across a river, it will cause changes to the surrounding environment, some of which will be good and others bad. A dam impacts the area around itself in two locales: above and below it.

Flood plain creation
When a dam is created across a river, it blocks the flow, causing water to back up into what is called a flood plain. An estimated 400,000 square kilometers of the earth has been covered with water due to damming. The newly created reservoir has more surface area than the river would have had, and therefore more evaporation occurs than is normal. This can lead to a loss of up to 2.1 meters of water per year. However, this creates a habitat for many kinds of animals and plants that cannot live near a moving river.

Breaking up the ecosystem
A dam also makes a break in the river’s natural ecosystem, blocking migratory paths of some fish (such as the Chinook Salmon) and preventing the free flow of organisms in the water. However, efforts such as building “staircases” that salmon are able to climb have to a great extent alleviated this problem. Also, dams do let water flow through them, though at a modified rate, so organisms in the water still eventually make their way downstream.

Flow rate
The amount of water that flows through a river is greatly affected by a dam built across it. Dams break up the natural flow rate and change the way water moves in a river. This can lead to more erosion downstream as the water tries to “replenish” the sediment load that it lost while passing through the dam. However, downstream flooding is lessened through this process as well.

Water content
As previously stated, water passing through a dam will lose its sediment load. This means that water downstream cannot deposit sediment in the riverbed or on the banks, and that it will actually erode the banks of the river as it tries to replenish its load. However, it is sometimes thought that the water with no load follows the path of the river better.

Flood prevention
A free flowing river will flood if the amount of water exceeds the capacity of the riverbed. This can cause land on both sides of the river to be inundated with water for extended periods of time. A dam breaks up the flow of water to the extent that even flood levels may be trapped behind it, preventing flooding downstream. Some people, however, claim that this leads to a loss of vital soil nutrients left by the floods that eventually makes the area less fertile.

Water temperature
When a dam traps water behind it, the reservoir created is deeper than the river would be, causing the water on the bottom to become colder because sunlight cannot reach it. This can possibly lead to ecological changes downstream, as certain fish and plants cannot survive in the colder water. Yet, recent studies have shown that tests to this effect may be inaccurate.