Dams
An honors project by Daniel Krohm
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For thousands of years, beavers have physically changed the North American landscape more than any other creature on earth (besides humans) with their dam building. Humans have controlled rivers for hundreds of years, but in the past fifty years, humans have decided to copy the beaver on a much larger scale by building dams around the world to control and harness the power of the river ways. In the United States alone there are over 100,000 dams that have been built. Of all the rivers over 600 miles that flow in the United States, only the Yellowstone still flows freely. Around the world there are over 38,000 dams that are over 50 feet high, about 33,000 of these dams have been built in the last 35 years.
The main purpose of dams is to store water. The earth's man-made dams collect and store about 35% of the earth's renewable fresh water supply. According to Judith Paterson the shift of water weight caused by water being stored in reservoirs has offset the rise in global sea levels by 1.2 inches over the last 40 years. Dr. Benjamin Fong Chao of the Goddard Space Flight Center has calculated that the water weight being stored in the mid-latitude regions away from the equator has slowed the earth's rotation by 0.2 millionths of a second per day for the last 40 years.
One purpose for dam water storage is to keep the river levels constant. Water flowing in a river can be stored behind a dam and released at an adjustable rate to keep the water level of the river below the dam constant. This can be helpful for flood control and navigation. Keith C. Peterson writes about the motivation for constructing dams on the Lower Snake River, Early proponents dreamed of an open river to the sea. Dams would eliminate dangerous rapids and create a transportation artery for moving grain to distant markets. In China, proposed construction of the Three Gorges Dam on the Yangtze River will allow 10,000 ton ships to reach the industrial city of Chongqing and also protect the people from floods that occur about once every ten years.
Water stored behind dams is also a major source of water for many farmers and metropolitan areas. The Western United States is especially reliant on reservoir water for irrigation and household water needs because the river systems run at higher levels during the winter and early spring, but their water levels drop significantly during the time the water is most needed for irrigation during the summer. Dams allow water to be trapped so the water can be utilized all year round. Ten dams on the Colorado River provide water to 2,000,000 acres of farmland and 21,000,000 households. Without reservoir water many current agricultural centers would not be able to exist in the Western United States. In Third World country's, dams are seen as the best way to advance agriculture through better irrigation. R. S. Odingo says this about dam building in developing countries, Dams are constructed for irrigation, which is the commonest, though one of the most expensive ways of increasing agricultural output.
Water not taken from the reservoir can be passed through turbines to produce power. In the United States, dams produce 300 billion kilowatt hours annually. That is the energy equivalent to 520 million barrels of oil or 129 million tons of coal. The amount of water passed through the turbines, which can greatly influence the water level of the river below the dam, is regulated by the demand for power at any particular time.
Although dams are utilized for numerous purposes, the advantages gained from dam building do not come without their costs. Dam building influences many people's lives and affects the environment in a variety of ways. In building a dam, a large area above the dam site has to be cleared for water storage. The loss of the land to water often impacts the area's culture, agricultural production, and environment. Erling Hoh in the Magazine Natural History said this about the Three Gorges Dam in China, The reservoir will inundate many sites of historical and archeological significance, and its environmental impact will be considerable, what with the loss of scarce farmland and the resettlement of 1.13 million people. In Portugal, a proposed dam project reservoir would put ancient rock carvings underwater. In Malaysian Borneo, a new dam would create a reservoir the size of Singapore and necessitate the clearing of 170,000 acres of jungle. A significant problem caused by new dams in North America is mercury poisoning. New flooding above a dam removes mercury from the ground that is now underwater. This mercury is deposited on the bottom of the new reservoir. Microorganisms through the process of methylation convert mercury into methyl mercury which is soluble in water. The mercury then can pass through the food chain and eventually reach humans through consumption of fish from the reservoir waters. A place where this process has been observed is in the Smallwood Reservoir of the Churchhill Falls Hydroelectric Project in Labrador, Canada. The initial impact of a dam is often not the most damaging to the environment. Dams permanently alter rivers in numerous ways and these alterations can affect the ecosystem of the river negatively both above and below the dam. Sandra Postel said this about many dams projects, "Many rivers now resemble elaborate plumbing works, with the timing and amount of flow completely controlled by planners and engineers so as to maximize their benefits to human activity.... Rivers are central to the planet's ecology, and they can not be turned on and off at will without damaging other parts of the system.
Humans have so utilized the water in many river systems that they have left nothing at all for the natural environment. Many rivers have so much of their water used along the path of their flow that nothing reaches the river's destination. An example of this is the Colorado River, where except in years of unusually high rains, no water reaches its mouth. Another example of this is the Ococee River in the Southeastern United States. A dam run by the TVA allows water to run through a stretch of the river on certain days, but on other days the TVA diverts the water from that stretch of the river to a flume where it is used for power production leaving a large stretch of the river dry.
Rivers that no longer discharge the same amount of water at their mouth that they had prior to dam construction can drastically affect the body of water they flow into. In the former Soviet Union, two major rivers that once discharged into the Aral Sea are now almost dry. This has caused the Aral Sea level to fall drastically. The volume of water in the Aral Sea basin today has dropped by 75%, and the basin's area has dropped by 50%. The reduced volume of the sea has caused its salinity level to triple. The lower sea level has caused numerous environmental problems many of which have direct consequences on the people living near the Sea. One of the major concerns of these people is the wind erosion that picks up anywhere from 40,000,000 to 150,000,000 metric tons of salt dust from the now dry sea bed. A large portion of this salt dust is deposited on area farmland causing severe damage to the crops grown on that land. Also, a once vibrate commercial fishing industry on the Aral has been destroyed, as all but 4 of the 24 natural fish species in the sea have been destroyed.
The increased salinity levels in the Aral is just one of numerous ways that fish species have been affected by dams around the world. An intended positive use of dams was to control floods. The lack of flooding along many rivers has decimated their fish populations. Many fish species have evolved to rely on the natural variations of river levels throughout the year for breeding. Along the Mississippi, many fish rely on flooding to reach their spawning grounds on the flood plain. With floods now controlled, fish populations have dropped drastically throughout the Mississippi. A sign of just how important floods can be for spawning fish was the 1993 floods along the Mississippi. In this year, water flowed back onto the flood plains along the Mississippi, and 1993 was also a record spawning year for native fish species.
Another way that fish are affected by dams is the explosion of native or introduced organism populations that are better suited to live in the modified river environment. These new organisms can threaten the native fish populations. An example of this is the columaris bacteria that has severely damaged salmon populations on the Columbia River. The bacteria levels in the Columbia have risen in the newly created reservoirs along the river which hold warmer water creating a better environment in which the bacteria can grow.
Dams also affect fish because they inhibit fish migration in a river. Some fish species like the striped bass have been able to spawn and grow to substantial size even though the path that they have normally taken to the ocean has been cut off, but other fish species have not been so lucky. Many fish that try to swim up or down a river become disoriented by the warm waters and slower current of the reservoirs. They often get lost and can never work their way through these large bodies of water to the dams. Of those that reach the dams, 30% of fish that try to pass directly through a dam's turbines are killed. Also, fish have to contend with nitrogen enriched water downstream of a dam. Nitrogen saturation is caused by air being mixed with water as it passes through a dam. As the water plunges to lower depths in the river, the higher pressure allows the nitrogen to dissolve in the water. The higher levels of dissolved nitrogen in the water can kill many fish.
One of the most famous migratory fish influenced by dams is the salmon. Salmon levels have plummeted in the Pacific Northwest and Atlantic Northeast, as the rivers that they have used for spawning have been dammed. In Nova Scotia, salmon spawning habitat has been cut off on the Indian, Mersey, Sissiboo, and Meteghan Rivers. Along the Snake River, in the Pacific Northwest, adult salmon migration up the river has fallen from 100,000-200,000 prior to dam development to 150-175 now.
Many projects have been commissioned to assist salmon and other migratory fish in passing the dams along their migratory path. One of these projects involves capturing the salmon, moving them over the dams, and then releasing them back into the river. This process has been utilized along the Columbia river. Many scientists believe that this process kills more salmon than it saves. Another project to assist migratory fish is to build fish paths that allow the fish to pass through the dam unharmed. These paths, however, have difficulty attracting the fish to them, and they also expose fish to fishermen. As of now, no fish passage system exists that allows complete safe passage of fish through the dam while being practical to construct and operate.
When asked to name dams whose benefits clearly outweighed their monetary and environmental costs, Larry Stephens, the executive director of the United States Committee on Large Dams could only name two off the top of his head. (Those dams being Hoover Dam and Grand Coulee.) This being the case the obvious question that needs to be asked is, Why do we not demolish those dams whose benefits do not outweigh their environmental and monetary costs? The answer is communities are often too attached to the government subsidies and jobs that dams bring to eliminate dams already built, and also the demolition of dams is very costly. Demolition of two dams on the Elwha River in Washington would cost over $70 million dollars.
How then can the environmental problems of dams be solved? The answer is that in our society these environmental problems can not be completely solved, but wise management of dams can control the environmental impact of dams on river systems. Dam management for too long has just taken into account industrial, household, and agricultural needs without thinking about the needs of the river ecosystem. Dams are going to remain on many rivers around the world, but dam managers can reduce their environmental damage by balancing human utilization of the river with the need for environmental conservation. Experiments are currently underway studying the most effective way to reverse the changes to the environment that dams have made. Through the results of these experiments, the world will better understand how rivers can be utilized without destroying their natural beauty and deleting their influence on the environment.
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