Environmental impact
Los proyectos de las represas grandes causan impactos ambientales irreversibles en una área geográfica grande, y, por lo tanto, tienen el potencial para causar daños importantes. Ha aumentado la crítica a estos proyectos durante la última década. Los críticos más severos reclaman que, como los beneficios valen menos que los costos sociales, ambientales y económicos, es injustificable construir represas grandes. Otros sostienen que se puede, en algunos casos, evitar o reducir los costos ambientales y sociales a un nivel aceptable, al evaluar cuidadosamente los problemas potenciales y la implementación de las medidas correctivas.[2].
El área de influencia de una represa se extiende desde los límites superiores de captación del reservorio hasta el estero, la costa y el mar. Incluye la cuenca hidrográfica y el valle del río aguas abajo de la represa.
Si bien existen efectos ambientales directos de la construcción de una represa (por ejemplo, problemas con el polvo, la erosión, el movimiento de tierras), los impactos mayores provienen del envase del agua, la inundación de la tierra para formar el reservorio y la alteración del caudal del agua, más abajo. Estos efectos tienen impactos directos para los suelos, la vegetación, la fauna y las tierras silvestres, la pesca, el clima, y, especialmente, para las poblaciones humanas del área.
Los efectos indirectos de la represa a veces pueden ser peores que los directos y se relacionan con la construcción, mantenimiento y funcionamiento de la misma (por ejemplo, los caminos de acceso, campamentos de construcción, líneas de transmisión de la electricidad) y el desarrollo de las actividades agrícolas, industriales o municipales, fomentadas por la represa.
Además de los efectos ambientales directos e indirectos de la construcción de la represa, deberán ser considerados los efectos que el medio ambiente produce en la represa. Los principales factores ambientales que afectan el funcionamiento y la vida de la represa son causados por el uso de la tierra, el agua y los otros recursos del área de captación encima del reservorio (por ejemplo la agricultura, la colonización, el desbroce del bosque) y este puede causar mayor acumulación de limos y cambios en la calidad del agua del reservorio y del río, aguas abajo.
Los beneficios de la represa son: se controlan las inundaciones y se provee un afluente de agua más confiable y de más alta calidad para el riego agrícola, y el uso domésticos e industrial. Además, las represas pueden crear actividades alternativas como energía eléctrica, turismo, pesca, piscicultura y navegación. La energía hidroeléctrica, por ejemplo, es una alternativa para la energía termoeléctrica a base del carbón, o la energía nuclear. La intensificación de la agricultura, localmente, a través del riego, puede reducir la presión sobre los bosques, los hábitats intactos de la fauna, y las otras áreas que no sean idóneas para la agricultura. Asimismo, las represas pueden crear una industria de pesca, y facilitar la producción agrícola en el área, aguas abajo del reservorio, que, en algunos casos, puede más que compensar las pérdidas sufridas en estos sectores, como resultado de su construcción.
Recientemente se está considerando el efecto beneficioso que pudiera tener el almacenamiento de agua en la tierra para compensar el crecimiento del nivel del mar, almacenando en forma líquida el agua que ahora permanece en tierra en forma de hielo en glaciares y nieves perpetuas de las montañas altas, que ahora se está derritiendo debido al calentamiento global. Los beneficios ambientales en las zonas costeras (muchas de ellas muy densamente pobladas) bien podrían compensar los problemas que pudieran producir en las tierras del interior.
Hydrological effects
By damming a river and creating a lagoon, the hydrology and limnology of the river system is profoundly changed. Dramatic changes occur in flow, water quality, quantity and use, biotic factors and sedimentation of the river basin.
The decomposition of organic matter (e.g. trees) from flooded lands enriches the reservoir's food. Fertilizers used upstream add to the food that accumulates and is recycled in the reservoir. These foods support not only fishing, but also the growth of aquatic grasses, such as water lilies and water hyacinths. Mats of grass and algae can be costly nuisances. If they block dam outlets and irrigation canals, they destroy fisheries, limit recreation, increase water treatment costs, impede navigation, and substantially increase water losses due to transpiration.
If the flooded land has many trees and is not properly cleared before flooding, the decomposition of this vegetation will deplete the dioxygen levels in the water. This affects aquatic life, and can cause large losses of fish. Anaerobic decomposition products include hydrogen sulfide, which is harmful to aquatic organisms and corrodes the dam's turbines, and methane, which is a greenhouse gas. Carbon dioxide, the main gas produced, also exacerbates greenhouse risks.
The suspended particles brought by the river settle in the reservoir, limiting its storage capacity and useful life, depriving the river of sediments downstream. Many floodplain agricultural areas have always depended on food-rich silts to sustain their productivity. As sediment is no longer deposited downstream in the alluvial terrain, this loss of food must be compensated by the addition of fertilizers to maintain agricultural productivity. The release of relatively sediment-free waters can wash downstream beds. However, sedimentation of the reservoir produces higher quality water for irrigation, industrial and human consumption.
Additional effects of changes in the hydrology of the river basin include variations in the water table, upstream and downstream of the reservoir, and salinization problems; These have direct environmental impacts and affect downstream users.
Social issues
Very often, city people, agricultural interests and people who live far away enjoy the benefits of dams. But those who bear the brunt of the environmental and social costs do not always benefit to a similar degree, and in many cases do not benefit at all. The inhabitants of the area flooded by the reservoirs, and those who live in the alluvial lands can receive benefits, but they almost always have to assume the damages of the works and the reservoirs.
Filling the reservoir usually requires the involuntary displacement of a variable number of people—which in some cases can reach hundreds of thousands—which requires profound social readjustment, not only on the part of the displaced but also of the people already established in the resettlement areas (see the “Involuntary displacement” section).
For people who remain in the river basin, access to water, land and biotic resources is often restricted. Artisanal fishing and traditional agriculture (recession type) of alluvial lands are interrupted, due to changes in flow and reduction in silt settlement. The alluvial terrain of many tropical rivers are areas of great importance for the human and animal populations; As floodplains reduce, there must be a change in land use; Otherwise, the populations will be forced to move.
Often, especially in warm areas, reservoirs increase the incidence of water-related diseases, such as malaria or schistosomiasis.
Conflicts also occur between the people who reside in the area and those who enter it from the construction, such as construction workers, temporary day laborers for agriculture and other activities induced by the dam, with consequences such as overwhelm of public services, competition for resources and social conflicts. These conflicts can be even more serious if the ethnicity of the local population differs from that of the newcomers.
Among the positive consequences can be mentioned: much greater demand for work during the construction of the dam, benefits for commercial and service activity in the area, a moderate increase in demand for work for post-construction maintenance, improvements in roads and energy provision, possible improvements in river transportation. In many cases, the population also takes advantage of the infrastructure that has been created for the construction of the dam once it is completed, such as the builders' homes. It is not unusual for governments to pay special attention to the infrastructure of a marginal area only when it comes to the fore due to the construction of a major project, with which the population of the area can obtain benefits that they would not normally have obtained.
Finally, large and medium-sized reservoirs are usually used to promote tourism to the region.
Fishing and wildlife
As previously stated, fishing usually deteriorates due to changes in river flow or temperature, degradation of water quality, loss of spawning sites, and barriers that prevent fish migration. However, fishing resources are created in the reservoir, which are sometimes more productive than those that previously existed in the river.
In biologically productive steron-bearing rivers, fish and mollusks suffer due to changes in flow and water quality. Variations in the flow of freshwater, and therefore, in the salinity of the estuary, change the distribution of species and the reproduction models of fish. Variations in the amount of food and deterioration in river water quality can have profound effects on the productivity of the estuary. These changes can have important results for marine species that feed or spend part of their life cycle in the estuary, or that are influenced by changes in the quality of coastal areas.
The greatest impact for fauna will originate from the loss of habitat, which occurs when the reservoir is filled and changes in the land use of the basin occur. They can affect fauna migration patterns, due to the reservoir and the development that is related to it. Illegal hunting and the eradication of species considered agricultural pests, a clandestine activity related to it, have a more selective effect. Wildlife and waterfowl, reptiles and amphibians can thrive thanks to the reservoir.
Seismic threat
Large reservoirs can alter tectonic activity. The likelihood of seismic activity is difficult to predict; However, the full destructive potential of earthquakes, which can cause landslides, damage to dam infrastructure, and possible dam failure, must be considered.
Watershed management
Dams are often made as an effective remedy to the periodic floods of certain rivers, frequently disastrous floods.
Frequently the population increases under the dam, both those that lived in areas flooded by the reservoir and a new population called by the possibilities of cultivating new irrigated lands provided by the dammed water. Environmental degradation sometimes occurs, and water quality deteriorates, and reservoir sedimentation rates increase, as a result of forest clearing for agriculture, pressure on pastures, land use such as downstream watershed areas.
• - Annex: Reservoirs in the world by surface area and volume of water.
• - hydroelectric power station.
• - filling of a reservoir.
• - dam "Dam (hydraulic)").
• - Hydraulics of Open Canals. Come Te Chow. Editorial Diana, Mexico, 1983. ISBN 968-13-1327-5.
• - Reference Book for Environmental Assessment (Volume I, II and III). Technical Work of the Environment Department of the World Bank.
• - Manuale dell'Ingegnere. Edition 81. Edited by Ulrico Hoepli, Milano, 1987. ISBN 88-203-1430-4.
• - (in Portuguese) Water Resources Engineering. Ray K. Linsley & Joseph B. Franzini. Edit.
• - (in English) Handbook of Applied Hydraulics. Library of Congress Catalog Card Number 67-25809.ra da Universidade de Sao Paulo e Editora McGraw-Hill do Brasil, Ltda. 1978.
• - (in English) Handbook of Applied Hydrology. A Compendium of Water-resources Technology. Ven Te Chow, Ph.D., Editor in Chief. Publisher McGraw-Hill Book Company. 1964. ISBN 07-010774-2.
• - Wikilibros hosts a book or manual on Environmental impacts/Hydraulic dam.
• - Coordinator of People Affected by Large Reservoirs and Transfers.
• - SEPREM - Spanish Society of Dams and Reservoirs.
• - Interview with Engineer Dulce María Camejo Corrales, pioneer of the construction of reservoirs in Cuba. Conversations about water, Chapter I.
• - Inventory of dams and reservoirs in Spain, Ministry of the Environment.
• - State of the reservoirs in Spain.
• - [2].