Environmental impacts of dams

From Wikipedia, the free encyclopedia

The environmental impacts of dams have come under a lot of discussion in recent years. For a long time it was thought that the only effects of dams were good, for example, inexpensive hydroelectric power. In the last few years, however, the negative aspect of dams has come into focus. Many people have said that dams are very harmful to the environment, but only recently has research shown that this is not always true. Whatever the consensus, dams do affect the environment, for good and bad; and they do this by affecting both their upstream and downstream environments.

Contents 1 Impact above dam 1.1 Flood plain creation 1.2 Breaking up the ecosystem 1.3 Water for energy 1.4 Reservoir Delta/Sediment Buildup 2 Impact below dam 2.1 Flow rate 2.2 Water content 2.3 Flood prevention 2.4 Water temperature 2.5 Coastal Impact 2.6 Channel Erosion 3 References

[edit] Impact above dam

[edit] 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.^[1] 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.^[2] However, this creates a habitat for many kinds of animals and plants that cannot live near a moving river.^[3]

[edit] 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.^[2] Also, dams do let water flow through them, though at a modified rate, so organisms in the water still eventually make their way downstream.

[edit] Water for energy

The water that is built up in a dam's floodplain is usually converted into hydroelectric energy. Although all dams release some amount of greenhouse gases,^[4] they release less than the amount of fossil fuels that are used to produce energy would,^[5] thereby lessening the amount of greenhouse gas emissions that result from production of energy.^[6]

[edit] Reservoir Delta/Sediment Buildup

This forms as the sediment being carried by a river or stream flows into the still waters of a reservoir. The bedload and suspended loads of sediment that would typically travel to the mouth of the river settles and forms a reservoir delta. The buildup of sediment here can lead to future problems with water and sediment flow should the dam later be removed as this material would not continue to be held in place by the still body of water residing in the reservoir.^[7]

[edit] Impact below dam

[edit] 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.^[2] However, downstream flooding is lessened through this process as well.

[edit] 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. Some sediment that may be detrimental to the river’s ecosystem may be retained by dams and kept from harming the habitat downstream.^[8]

[edit] 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.^[9][10] 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.

[edit] 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.^[11] 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.^[12]

[edit] Coastal Impact

As previously stated, the dams reservoir restricts how much sediment is able to reach the open waters at the mouth of the river. The reduced amount of material leads to the lack of expansion or enhancement along a beach, coastline or delta. This absence of new sedimentary deposits allows erosion and weathering to become more visible and "active," resulting in the reduction of beach and delta size.^[13]

[edit] Channel Erosion

The erosion of a river channel can be initiated or accelerated by the addition of a dam. This is due to the reservoir collecting a majority of the sedimentary bedload. As the water passes through the dam and starts moving downstream again, the river brings a reduced bedload along with it. The waters, in a search to recover the lost sediment, begins to erode the banks and bed of the channel much more extensively.^[14]

This event can lead to even more problematic issues. If people have built too close to the banks, the river may eventually begin to undercut the structures. The eroded channel could create a lower ground water level in the area, thus impacting bottomland crops such as alfalfa or corn resulting in a smaller supply. Finally as the water level is lowered, it may bring about a downcutting cycle to tributary channels and gullies that surround the river.^[15]

[edit] References