Dams are built across the river in order to store water for drinking, agricultural, industrial purpose. Now days they are mainly used for the hydropower production.
Big dams are plain bad. They flood people out of their homes and off their lands; wipe out endangered habitats and species; spread water-borne diseases; deprive flood plains of the water and sediments of life-giving floods (while increasing the damage floods cause to people); ruin beautiful landscapes and submerge places of great cultural or spiritual importance. And that’s just a partial charge sheet. Big dams even cause earthquakes (because of the weight of water in reservoirs), release greenhouse gases (because of the rotting of flooded vegetation), destroy marine fisheries (because they disrupt river-borne flows of freshwater and nutrients into oceans) and lead to coastal erosion (because the sediments that eventually fill reservoirs would previously have flowed out through estuaries and then been washed back by waves to protect the shoreline). Occasionally, they collapse and drown people. In the world’s worst dam disaster – a mega-catastrophe that struck central China in 1975 when two large dams burst – as many as 230,000 people died.