Environmental effects of dams and hydropower

The production of electricity in hydroelectric plants has more than a century of history, has very well-developed technologies and dominates renewable energy production; but it begins to be an energy with more history than future for technical and environmental reasons. 

On the one hand, the large electricity-consuming countries have practically exhausted the possibilities of building large power plants using this technology (power plants with several thousand MW of power). This is so because it requires the conjunction of mighty rivers and geographical configurations that allow the construction of dams that collect large volumes of water and allow remarkable falls without requiring the construction of giant walls, or the occupation of immense areas. This type of location has already been used mostly in the most developed countries, so the construction of this type of dams and power plants now occurs mainly in economies in transition, with accelerated growth and few old infrastructures. This should not make us forget that projects of this type, with serious environmental repercussions, are being planned and built in the large basins of South America, Africa and Asia; often in remote locations where current human impact is still small.

On the other hand, the environmental repercussions of reservoirs and hydroelectric plants are intense, to the point that their suitability as an option for the future is questioned, and sometimes their dismantling is even considered. The greatest effects on biodiversity generate by dams are associated with the direct destruction of habitats, and the species that inhabit them, in the large areas that become flooded. Furthermore, this direct action is focused on some of the ecosystems most affected by human activities: those located in the valley bottoms and along the rivers, where the human population has historically been concentrated. 

Along with this direct effect, there are problems associated with the fragmentation of habitats and animal populations, those associated with aquatic environments, which are unable to move along the rivers (for example, fish populations such as sturgeons that cannot cross the dams), as well as those of terrestrial species for which the flooded area is an insurmountable barrier (e.g., bears and lynxes in the Iberian Peninsula). These changes in the most eminent organisms of the ecosystems (vertebrates, plants) are only the tip of the iceberg compared to the changes in the functioning of the affected ecosystems, especially the aquatic ones (nutrient cycles, changes in temperature, pH and concentrations of dissolved oxygen…). To these effects must be added the additions for the construction of electrical evacuation lines, which are especially extensive when the construction of the reservoirs is carried out in remote areas, very distant from the areas where the electricity demand is.

Finally, it should be noted that reservoirs are a known source of greenhouse gases (CO₂ and CH₄), mainly as a result of the decomposition of organic matter in the vegetation and soil present in the flooded area, but also as result of the substitution of a running water system for one of still water. The seriousness of the problem is even greater in tropical and subtropical areas than in temperate latitudes, so the balance in terms of greenhouse gases from new dams is worrying.