Dams are actually a massive barriers built across rivers and streams to confine and utilize the flow of water for human purposes such as irrigation and generation of hydroelectricity. This confinement of water creates lakes or reservoirs.
Dams are a structure also seen in nature - beavers build dams to keep the water deep enough to cover the openings to their homes, protecting them from predators. Concrete dams are expensive, however over the last 40 years of mechanization has reduced by a factor of four the number of man hours required to place a cubic meter of concrete in a mass concrete dam.
One of the first major problems with dam is the land erosion. Dams hold back the sediment load normally found in a river flow, depriving the downstream of this. In order to make up for the sediments, the downstream water erodes its channels and banks. This lowering of the riverbed threatens vegetation and river wildlife, hence lead to flood.
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It is true as one of the reasons dams are built is to prevent flooding. However, most ecosystems which experience flooding are adapted to this and many animal species depend on the floods for various lifecycle stages, such as reproduction and hatching. Annual floods also deposit nutrients and replenish wetlands.
Â Â Â Â Â The cyclic floods contribute to the extinction of salmon by flushing away their spawning gravels during the day and leaving them high and dry at night. Riverbeds become scoured, stripped of their organic materials, sediment, vegetation, and macroinvertebrates.
2) Species Extinction
As fisheries become an increasingly important source of food supply, more attention is being paid to the harmful effects of dams on many fish and marine mammel populations. The vast majority of large dams do not include proper bypass systems for these animals, interfering with their lifecycles and sometimes even forcing species to extinction as it stopping the fish migration.
This cold, clear water will be starved of nutrients and provide little or no habitat for animals. In addition, animals that once used the "muddiness" of the river's water to conceal them from predators are now overly vulnerable to predation, and may quickly go extinct. A river with dams eventually becomes little more than a dead channel of water.
Â Besides, fish passage is a concern with dams. Many fishes must move upstream and downstream to complete their lifecycles. Dams are often built without fish ladders. When fish ladders are provided, they seldom work as needed. If enough adult fishes do manage to climb above a dam, there remains the issue of their young: how will they get back downstream? Many are killed by predators while they wander, lost, in the reservoir above the dam. Many are killed in their fall downward through the dam to the river below by the high levels of nitrogen gas at the base of the dam.
There are many fishes that cannot climb dam ladders or leap over low dams. Some of these fishes swim upstream every year to breed, then let the water carry them back downstream. The eggs of pelagic spawners float downstream, too, which is why the adults must swim far upriver to breed, or else the baby fish would soon end up out to sea.
3) Removing Sediment
Â Â Â Â Â Another reason that riverbeds become scoured and armored is that dams remove all the sediment from the river. It is natural that the river, which is accustomed to carrying sediment and now will pick up the sediment from the streambed below the dam.
Dams are engineered to withstand the force of a certain number of tons of water--however large the reservoir is planned to be. However, they are not engineered to withstand the additional force of tons of wet sediment pressing on their backsides.Â Â The sediment in a dammed river reaches the slow-moving reservoir above the dam and drops out, settling behind the dam. The muddier the river, the faster this heap of sediment will build up. Either the dam bursts, killing people and destroying settlements downstream, or the reservoir's water pours over the top of the dam. In effect, a huge man-made waterfall has been constructed, and will remain there for thousands of years.
4) Starving the River
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Â Dams hold back not only sediment, but also debris. The life of organisms (including fish) downstream depends on the constant feeding of the river with debris which includes leaves, twigs, branches, and whole trees, as well as the organic remains of dead animals. Debris not only provides food, it provides hiding places for all sizes of animals and surfaces for phytoplankton and microorganisms to grow. Without flooding and without a healthy riparian zone, this debris will be scarce. The bottom level of the food web is removed. All in all, the loss of sediment and debris means the loss of both nutrients and habitat for most animals.
5) Changes to Earth's Rotation
Nasa geophysicist Dr. Benjamin Fong Chao found evidence that large dams cause changes to the earth's rotation, because of the shift of water weight from oceans to reservoirs. Because of the number of dams which have been built, the Earth's daily rotation has apparently sped up by eight-millionths of a second since the 1950s. Chao said it is the first time human activity has been shown to have a measurable effect on the Earth's motion.
The construction of large dams completely change the relationship of water and land, destroying the existing ecosystem balance which, in many cases, has taken thousands of years to create. Currently there are around 40,000 large dams which obstruct the world's rivers, completing changing their circulation systems: this is not going to occur.
6) Interrupting Natural Cycles
The effect of a dam is to alter the pattern of disturbances that the plants and animals of a river have evolved for. Many aquatic animals coordinate their reproductive cycles with annual flood seasons. Every flood is valuable in that it takes nutrients from the land and deposits them in the river, providing food for the stream's residents. Floods also provide shallow backwater areas on vegetated and shaded riversides; the young of many animals depend on these backwaters to protect them from large predators.
Vegetation, too, depends upon these regular cycles of flood. Quite often, people will decide that they can spare no water at all and no flooding will occur. Or they may have built the dams specifically to stop flooding, so they can build houses in the floodplains. When this happens, riparian vegetation, the vegetation bordering the river, changes forever. An example of this may be found in much of the Southwest United States, where enormous floodplains of cottonwood and marsh have been replaced by dry, barren areas of tamarisk and grass.
7) Changing Temperatures
Reservoirs are layered in temperature. However, rivers tend to be fairly homogenous in temperature. They are warm at the top and cold at the bottom. If water is released downstream, it is usually released from the bottom of the dam, which means the water in the river is now colder than it should be. Many macroinvertebrates depend on a regular cycle of temperatures throughout the year. As we change that, we compromise their survival. For example, a certain stonefly may feel the cold temperatures and delay its metamorphosis. This may mean that at a certain lifestage, it will be living in the depth of winter rather than in autumn as it should have been.
Generally, the way to make dams work for humans everywhere is to address the social and environmental impacts both downstream and upstream of any dam project before the structure is even built, and to evaluate the situations in river basins where dams have already been constructed.
The political and institutional consideration of dam construction should be addressed, Scudder says. Too often, a dam project is undertaken at a specific locale because of its political expedience, and this is not the best way to minimize the negative human and ecological impact. Restructuring governmental departments that oversee dams can also maximize negative environmental, agricultural, or other impacts.
Â Â Â Â Â Â Â Â Â Â Once a dam is built and its reservoir is formed, the region that is served by the dam will be developed; will be filled with cities, roads, parking lots, and houses. This, unfortunately, lowers the water table due to water extraction and urban runoff. That lowers the river even further. Eventually, the new human populace will run out of water--but they will still want to "grow" (sell land for profit). At this point, they will demand yet another dam.
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Dams are built to be beneficial, not to make our next generation to be suffered from them.