The construction of reservoirs, especially large ones, has significant environmental and social impacts. The Worldwatch Institute, an American think tank specialized in sustainability, denounces that reservoirs cause the disappearance of forests and arable land and the displacement of millions of people around the world when their villages are flooded.
They also alter the territory; reduce biodiversity; they hinder the emigration of fish, river navigation and the transport of nutrients downstream; they decrease the flow of rivers; they modify the level of underground currents, the composition of the dammed water and the microclimate; and they carry the risk of disease in the area . They also claim that they can alter tectonic activity, although they recognize that the probability that they produce seismic activity is difficult to predict.
Given these consequences, March 14 was declared the International Day of Action against dams and for rivers, water and life , in 1997, during the First International Encounter of those affected by Dams in Curitiba, Brazil. Those responsible point out that, since 1950, at least 45,000 dams have been built, fragmenting and transforming rivers (today almost half of the world's rivers have at least one large dam), and displacing between 40 and 80 million people.
