Dams can bring both benefits and problems to the environment around us. However, the problems greatly outweigh the benefits. The benefits are only that the regulated water in reservoirs can help in irrigation, water supply and water for industrial purposes. Also, reservoirs produce flood control as they are the only measure that can store water in a very significant manner, modifying the hydrographs and reducing the peak flows. However, the problems caused by dams are many too. The construction of dams can have a deep impact on downstream fishes, as they obstruct the movement of migratory fishes. Fish ladders have been constructed but they were proved helpful only in a few cases. They also directly affect downstream wetlands as severity of floods decreases and water quality and sediment loads altered. Biodiversity loss is also caused by dams as many habitats are destroyed. Large dams have forced 40 to 80 million people from their homes and lands, as well as destroyed many wildlife habitats. Dams' impacts on ecosystems are mostly negative. Large dams have led to the extinction of many fish and other aquatic species, the disappearance of birds in floodplains, huge losses of forests, wetlands and farmland, erosion of coastal deltas, and many other impacts. Most reservoirs emit greenhouse gases, some in fairly high quantities. In the initial stages, the emissions are mainly due to the rotting of flooded vegetation, but this process usually continues with the deposition of organic matter by in-flowing streams. Water quality is also degraded as salinity increase can make the water unusable for consumption and irrigation. Dams can trigger earthquakes due to an effect known as 'reservoir induced seismicity'., where the weight of water combined with water seeping deep into the pores of the underlying rock strata changes the area's response to existing stresses.
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