Environmental Impacts From Dam Construction Environmental Sciences Essay
The first 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.
As an example, a fish on a certain river may only reproduce during April of every year so that its offspring will have abundant food and places to hide. If the flood never comes because a dam holds the river back (because people want the water for themselves), the offspring may be produced during a time when they cannot possibly survive. If the fish can wait until the next flood, which may be in July or may be in October, its young will be born during the wrong time of year, and will have to contend with the absence of their normal food supply and temperatures for which they are not prepared.
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of this may be found in much of the Southwest United States, where enormous floodplains of cottonwood and marsh have been replaced by dry, barren areas of tamarisk and grass.
Armoring the Riverbed
If the dam is allowed to release water from its reservoir, it will often do so only once in awhile, rather than in frequent, small floods as are seen in nature. This leads to scouring and armoring of the riverbed. The higher energy of the sudden floods picks up and removes smaller sediments like silt, sand, and gravel, as well as aquatic plants and animals, leafy debris, and large woody debris. Complex sets of habitats are erased. The riverbed below the dam becomes like a pavement of cobbles and loses its value as habitat for plants, macroinvertebrates, and fish.
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 has none, will pick up the sediment from the streambed below the dam. It is almost as though the river has been "starved" of its sediment. As in everything else in nature, balance will be achieved one way or the other, often at the expense of one or more species.
What happens to the sediment in a dammed river? It reaches the slow-moving reservoir above the dam and drops out, settling behind the dam. If this seems worrisome to you, it should. Dams are engineered to withstand the force of a certain number of tons of water--however large the reservoir is planned to be. They are not engineered to withstand the additional force of tons of wet sediment pressing on their backsides. The muddier the river, the faster this heap of sediment will build up. What happens when it builds up too high? 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.
Can we not remove the sediment from behind dams? Unfortunately, the answer is "No--not yet, we can't." There appears to be no safe and economic way to do it.
What about the river downstream of the dam? Isn't its nice clear, cold water a great improvement--especially in regions like the American Southwest where rivers tend to be muddy? The answer is only "Yes" if you have decided the original ecology of the river doesn't matter. If you only want water to look at and drink, then you will be happy enough. If you want a living river ecosystem filled with fish and birds, you will be sadly disappointed. 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.
Starving the River
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. This debris 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. Adding to the problem, although debris might come from the river above the dam, it is instead trapped in the reservoir, and never appears downstream. 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.
Temperature is another problem. Rivers tend to be fairly homogenous in temperature. Reservoirs, on the other hand, are layered. 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. When we change that, we compromise their survival. For instance, 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.
Stopping Fish Migration
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. They aren't killed by the fall itself, but by the high levels of nitrogen gas at the base of the dam. In other words, like divers who go too deep, they get the "bends."
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. Otherwise, the baby fish would soon end up out to sea!
Perhaps deadliest of all to salmon and steelhead species is the typical hydropower practice of releasing large amounts of water in powerful surges during the day in order to provide electricity when demand and prices are highest, and cutting down flow during the night in order to replenish reservoirs for the next day. The cyclic floods caused by this popular practice 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.
Low flows below dams killed thousands of salmon on the Klamath in 2002
In addition to huge social impacts, large dams directly impact rivers in a variety of physical and biological ways. The most significant is the alteration of a river's flow, which affects downstream ecosystems and the landscape through which the river flows.
A dam also holds back sediments that would naturally replenish downstream ecosytems. When a river is deprived of its sediment load, it seeks to recapture it by eroding the downstream river bed and banks, undermining bridges and other riverbank structures. Riverbeds downstream of dams are typically eroded by several meters within the decade of first closing a dam; the damage can extend for tens or even hundreds of kilometers below a dam.
Riverbed deepening will also lower groundwater tables along a river, threatening vegetation and local wells in the floodplain and requiring crop irrigation in places where there was previously no need. Altering the riverbed reduces habitat for fish that spawn in river bottoms, and for invertebrates.
Large dams have led to the extinction of many fish and other aquatic species, the disappearance of birds in floodplains, huge losses of forest, wetland and farmland, erosion of coastal deltas, and many other unmitigable impacts. And contrary to the dam industry’s claims that dams offer “green” energy, most reservoirs emit greenhouse gases, some in fairly high quantities.
The most significant consequence of this myriad of complex and interconnected environmental disruptions is that they tend to fragment the riverine ecosystem, isolating populations of species living up and downstream of the dam and cutting off migrations and other species movements. Because almost all dams reduce normal flooding, they also fragment ecosystems by isolating the river from its floodplain, turning what fish biologists term a 'floodplain river' into a 'reservoir river'. The elimination of the benefits provided by natural flooding may be the single most ecologically damaging impact of a dam. This fragmentation of river ecosystems has undoubtedly resulted in a massive reduction in the number of species in the world's watersheds.
Some of the environmental effects of dams can benefit some species. For example, impounding a reservoir will create habitat for lake fish and warm water released from a reservoir can increase the abundance of species of fish which failed to thrive in the cool river. But because dams alter the conditions to which local ecosystems have adapted, the overall impact of a dam will almost without exception be to reduce species diversity.
Plants and animals which are closely adapted to valley bottom habitats can often not survive along the edge of a reservoir. Dams also tend to be built in remote areas which are the last refuge for species which have been displaced by development in other regions. No one has any idea how many species of plants and animals are now extinct because their last habitat was flooded by a dam but the number is likely far from negligible. As well as destroying habitat, reservoirs can also cut off migratory routes across the valley and along the river. Because it isolates populations, this ecosystem fragmentation also leads to the risks of inbreeding from a smaller genetic pool. It is often not just the forests within the reservoir area, around the dam site and transmission lines and in the areas slated to be converted to agriculture which are lost when a dam is built in a forested area. In many cases farmers displaced by a reservoir have had to clear forests further up the sides of the valley to grow their crops and build new homes. The access to previously remote areas allowed by new roads and reservoirs can also accelerate deforestation: every large dam which has been built in a forest area in Thailand has attracted loggers as well as developers who have built golf courses and resorts along the edges of reservoirs.
As well as flooding and fragmenting some of the world's best wildlife habitats, reservoirs have also inundated some of the world most beautiful and spectacular river scenery. Probably the greatest loss of the planet's scenic heritage to a reservoir was the inundation of the spectacular Sete Quedas waterfall at Guáira on the Brazilian-Paraguayan border, now just a rock formation at the bottom of Itaipú Reservoir. At Guáira the mighty Paraná suddenly narrowed to a width of just 60 metres — less than a tenth as wide as the Horseshoe Falls at Niagara — and then thundered over 18 separate cataracts each more than 30 metres high. More water surged and boiled through the rocks and whirlpools of Sete Quedas than any other waterfall in the world — about half as much again as over both falls at Niagara combined. 'A more imposing spectacle can scarcely be conceived,' a 19th century French traveller wrote of Sete Quedas.
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