The construction of dams and other hydraulic structures is one of the oldest branches of engineering. Human civilization started around rivers, and as we have spread and advanced across the globe tens of thousands of dams and reservoirs have been constructed on virtually every watershed upon the earth. These engineering works, large and small, have impounded millions of cubic meters of water and transformed the ecologies of the global riverine environment in ways still not completely understood, and it is only comparatively recently that we have come to a fuller understanding of the total impact of the effects of dams on the environment as a whole.

Dam
Erosion and Sedimentation

The effects of dams on rivers can have dramatic consequences both upstream and downstream as the natural flow and drainage of the land is altered. One of the most obvious of these effects is a profound altering of the natural sediment load carried by the waters of the previously free-flowing river. As the sediment-laden upstream waters flow into the impoundment behind the dam, suspended sediments drop out and form thick layers of silt at the bottom of the impoundment. Also as a result, when water is released through the dam it is relatively sediment-free, and hungrily picks up a sediment load as it moves downstream, leading to increased erosion of the riverbanks and streambed for dozens and sometimes hundreds of kilometers downstream from the dam.
 
http://www.internationalrivers.org/node/1637
 
The Effects of Dams on Riverine Ecosystems
 
One of the adverse effects of dams that has been poorly understood until quite recently is the impact the fragmentation of watercourses has had on riverine ecosystems. The interconnected ecologies of riparian environments are profoundly altered as the cycles and rhythms of the natural flow of rivers are interrupted. Plant and animal populations are thrown out of all balance as invasive species move into the disrupted riparian ecologies and native species are displaced, reduced and in some cases eradicated.

http://www.hydroreform.org/sites/www.hydroreform.org/files/Fragmentation%20Of%20Riparian%20Floras.pdf
 

Water Temperature
 
The reservoirs impounded behind dams alter the temperature regimes both within the upstream reservoir and the downstream water channels as the water is released through the dam. Within the impounded water the natural thermodynamics of a free flowing river can be replaced by stratified temperature gradients, which can have profound effects upon the aquatic life both upstream and downstream of the impoundment. Many aquatic planktons, invertebrates, mollusks and fish are extremely sensitive to these thermal changes and must either adapt, relocate or perish.

https://www.msu.edu/~hayesdan/PDF/lessard.pdf
 
The Effects of Dams on Coastal and Marine Ecosystems
 
The construction of large dams can have adverse effects on coastal and marine environments hundreds and even thousands of kilometers downstream. The impoundment of not just the water, but also the blocking of the riverine silt and nutrient load as well, has altered the ecologies of many river deltas, estuaries, coastal wetland and marine environments.

Without the annual burden of silt from floodwaters, many delta wetlands have become subject to severe erosion, and the reduced dispersal of organic nutrients from river outflow has severely stressed many marine populations from phytoplankton up through the food chain to many fish populations. Reduced outflow has also increased the salinity of estuarine and coastal wetland ecosystems, having a severe impact on the delicate ecostructures of these environments.

http://ocean.tamu.edu/Quarterdeck/QD3.1/Elsayed/elsayed.html
 
The Social Effects of Dams
 
The building of dams can also have far-reaching and often unintended social consequences as well. It is estimated that almost a quarter of a million square kilometers of land has been inundated by the impoundment of river waters over the last century.

The World Commission on Dams estimates that 40-80 million people have been displaced by dam construction in living memory. There are also increased health risks associated with the construction of large dam and reservoir systems, especially in tropical and sub-tropical areas where the disruption of the natural drainage ecologies provides fertile ground for the growth of waterborne disease vectors.

The increased transmission of malaria has been directly linked to the construction of dam impoundment reservoirs in Southeast Asia and Africa. A further health risk associated with dam reservoirs is the accumulation of toxins that can leech into impounded waters and be released downstream into the water supply used by people.

http://tinyurl.com/3eddj54