In flat basins large dams cause flooding of large tracts of land, destroying local animals and habitats.
People have to be displaced causing change in life style and customs, even causing emotional scarring. About 40 to 80 million people have been displaced physically by dams worldwide.
Large amounts of plant life are submerged and decay anaerobically (in the absence of oxygen) generating greenhouse gases like methane. It is estimated that a hydroelectric power plant produces 3.5 times the amount of greenhouse gases as a thermal power plant burning fossil fuels.
The migratory pattern of river animals like salmon and trout are affected.
Dams restrict sediments that are responsible for the fertile lands downstream. Farmers use chemical fertilizers and pesticides to compensate for the loss in productivity.
Salt water intrusion into the deltas means that the saline water cannot be used for irrigation.
Large dams are breeding grounds for mosquitoes and cause the spread of disease.
Farmers downstream who used to wait for the flooding of the fields to plant their seeds are affected.
Dams serve as a heat sink, and the water is hotter than the normal river water. This warm water when released into the river downstream can affect animal life.
Peak power operations can change the water level thirty to forty feet in one day and can kill the animals staying at the shorelines.
Around 400,000 km2 of land worldwide has been submerged due to the construction of dams.