This question seems to have been addressed at

http://www.epa.gov/cleanenergy/energy-and-you/affect/hydro.html

The consensus seems to be that the environmental impact is negligible.

@ C.P. Ravikumar, ya I had also seen this, On general basis, You can say this is ok, But when you will analyse in detail (practically ), then Result will different, So i am not agree with this information...

There may be no source of 100% clean energy. Even to harvest solar energy, you need panels and electronics and in manufacturing these, you may pollute the environment. You may wish to see the utilization of these components and define a measure of environmental impact that they create.

No. Hydro power plant is not responsible for pollution. It does not produce any pollution material at all. It only converts water power into electricity. It may have impact on watershed ecosystem.

Hydro power plant is not only responsible for pollution, but are responsible for disasters too(Earthquake and landslide with, instant heavy flooding in branches of river Gengis of Himalaya, near Kedarnath tample,in year 2012, took tool of thousands in hour). If the sight for resarevoier, is decided, without proper detailed study of geothermal and sesmmic(earthquake) activity of area, resarevoier can't utilize to it full capacity due to, danger of earthquake (Koina Dam in Maharastra, Tehri dam in Himalyas, India). Huge valuable,forest(industrial pollution will increase) area and agriculture land with existing villages land are dipped in water, on upper stream side(Narmada river, hydro project,India).

The use of hydropower prevents the burning of billion barrels of oil or million tons of coal each year and for this reason cannot be considered a polluting energy, but this doesn't mean that the use of hydropower is free of problems that are affecting the environment and the population.

The advantages of hydropower are convincing, but there are some serious drawbacks that are causing people to reconsider its overall benefit.

Since the most feasible sites for dams are in hilly or mountainous areas, the faults that often created the topography pose a great danger to the dams and therefore the land below them for thousands of years after they have become useless for generating power. In fact, dam failures do occur regularly due to these terrain conditions, and the effects are devastating.

When a new dam's reservoir floods the countryside, people who live in the area have to move and relinquish their former lifestyles in order to make way for the project. This is very stressful and often controversial, especially if a community has maintained a particular way of life on the same land for generations.

The construction of a dam not only affects the people nearby, it can severely alter a river's natural functions by diverting water for power. Dams remove water needed for healthy in-stream ecosystems. Stretches below dams are often completely de-watered." This may not seem like a significant problem until animal species are studied. Birds that have migrated to a specific riparian environment for generations no longer have enough insects on which to pray when the water level drops. If they have few migration alternatives, that could mean the endangerment of species that once flourished. Fish species such as salmon "depend on steady flows to flush them down the river early in their life and guide them upstream years later to spawn. Stagnant reservoir pools disorient migrating fish and significantly increase the duration of their migration." Native populations of fish may decrease or disappear altogether due to temperature changes caused by dams. Slower water flow means warmer temperatures, and bottom-release of cold water means cooler temperatures. Several of hydropower's disadvantages focus on fish. It is easy to forget how important fish and other aquatic life are, some of which reside at the bottom of the food chain.

The environmental changes caused by hydroelectric projects may be obvious to the local biologist, but elude the average person. Most people will more readily notice a smoggy haze developing in an area where a coal plant is operating than a smaller population of a particular bird species where a hydropower facility functions. Such oversights lead people to believe that nothing is wrong.

Hydroelectric companies and organizations often emphasize their "clean" manufacture of electricity and neglect to mention the long-term environmental hazards. "Dams hold back silt, debris, and nutrients." Silt collects behind the dam on the river bottom, accumulating heavy metals and other pollutants. Eventually this renders the dam inoperable, leaving the mess for future generations, who will either have to remove the collected debris or live with a potentially catastrophic mudflow poised to inundate the area below the dam.

Hydropower may be better on the environment than fossil-fuel sources, but its future is so uncertain that we may need to focus on other alternatives.

Unlike other renewables such as wind and solar power that receive more praise than criticism, hydropower is a highly controversial issue. While it does have many merits, it too is like so many other sources of energy if we ignore the critics' warnings, we may not realize its full impact on our natural resources until it is too late.

Some of the advantages and disadvantages in the use of hydropower for the generation of electricity are the following:

Advantages

•Once the dam is built, the energy is virtually free.

•No waste or pollution produced.

•Much more reliable than wind, solar or wave power.

•Water can be stored above the dam ready to cope with peaks in demand.

•Hydroelectric power plants can increase to full power very quickly, unlike other power plants.

•Electricity can be generated constantly.

Disadvantages

•The dams are very expensive to build. However, many dams are also used for flood control or irrigation, so building costs can be shared.

•Building a large dam will flood a very large area upstream, causing problems for animals that used to live there.

•Finding a suitable site can be difficult - the impact on residents and the environment may be unacceptable.

•Water quality and quantity downstream can be affected, which can have an impact on plant life.

Hydel power may be sometimes seen as a environmental threat and practically thinking, in some cases wrong planning have impacted negatively but with proper planning and after project management can reduce the effect.

Hydel is one of the major sources of power generation without pollution with additional benefits from the dam and reservoir also.

It wont be nice to say that these structures are pollution creators.

See this link:

https://water.usgs.gov/edu/wuhy.html

@ i am agree with all authors.

But at one place i found, Regarding air pollution from Hydro plant ... that is ....

* Emission of methane and carbon dioxide from Hydro power plant:

The reservoir of water for hydroelectric power releases a large amount of carbon dioxide and methane. The area around the dam is filled with water. The plants and trees in them start rotting and decompose by other method without the use of oxygen. So this type of decomposition dumps a great amount of methane and carbon dioxide which increase pollution.

- See more at: http://www.conserve-energy-future.com/Disadvantages_HydroPower.php#sthash.Un0AUir1.dpuf

yes,

(a) GHG pollution: dams/reservoirs emit greenhouse gases (GHG) in a number of ways such as, from breakdown/decomposition of soil and plant carbon, anaerobic conversion of organic carbon to CH4 and rotting/decay of vegetation at the bottom of dams/lakes.

(b) Cold water Pollution: Older dams release water that is stored at the bottom of the dam, which is typically colder and adversely affects species adapted to warmer temperatures. Such an effect is sometime referred to as ‘cold water pollution

Power generation by either thermal (coal and oil), hydro or nuclear methods are not completely free from polution and other problems that may affect health and wellfare of our civilization. Natural resources like solar or wind enrgy are only two that may not affect much. So one has to optimize. In that sense hydroelectric is a lesser evil than coal or oil based generators.

Dear Rasbnidu,

Nuclear power creates radioactive waste which are harmful to living organisms (can cause cancer, genetic mutation),  also require substantial amount of cooling water.  Solar and wind energy may be good in the subcontinent and can create local jobs  and supply of local electricity 

Dear Golam,

You are absolutely right- but it is 'cleaner' and more efficient.

We have to wait for major breakthrough for solar energy.

The best breakthrough for using solar energy would be, not to seal surfaces and also to construct less roofs, to let life grow at the earth surface.

Wind energy is less harmfull than photovoltaic devices ( and both a good choice in general ) and is worth to be considered first.

The consequences of large river dams appearently are not evaluated well, as to be seen from the megadams in China or Brazil: The Belo Monte ( in the Amazonas basin ) hydroelectric project  does not care much about the huge area submerged nor the indigenous people living in harmony with nature.

The construction companies attract people, who tend to settle there, knowing nothing of the environment, thus continuing destructive work.

In Germany some river beds are planned now to make deeper, likewise bad.

You forget the mega dam,Ashvan(Nail) and Narmada(Sardar srovar)popular sunny area,the huge water surface, can be utilized for solar power plant, and can reduced the evaporation of water also. Same is done, by Govt. of Gujarat for the main canal from Sardar sarovar. The solar roof will reduced the  exposer of water to sun, and will reduced the growth of vegetation and elgie, responsible for pollution of water/environment.

no

The problem of increased amount of mercury in the water in flooded areas of hydro developments has forced the authorities to issue "mercury advisories" for those consuming fish (basically, it is safe to eat fish only once a week) in many parts of USA and Canada. That would tell you part of the story. It wasn't planned that way, but it turned that way.

It is difficult to say that hydropower is responsible for the current level of pollution that we are suffering, but this not mean that the use of hydropower is not producing any type of contamination. In my opinion, the main disadvantages of the Hydro power systems are the following:

• Ecosystem damage and loss of land. Hydropower plants that use dams would submerge large areas of land due to the requirement of a reservoir. Large reservoirs required for the operation of hydropower plants result in the submersion of extensive areas upstream of the dams, destroying biologically rich and productive lowland and riverine valley forests, marshland and grasslands. The loss of land is often exacerbated by habitat fragmentation of surrounding areas caused by the reservoir;

• Hydroelectric projects can be disruptive to surrounding aquatic ecosystems, both upstream and downstream of the plant site. Generation of hydroelectric power changes the downstream river environment. Water exiting a turbine usually contains very little suspended sediment, which can lead to scouring of river beds and loss of riverbanks. Since turbine gates are often opened intermittently, rapid or even daily fluctuations in river flow are observed;

• Siltation and flow shortage. When water flows, it has the ability to transport particles heavier than itself downstream. This has a negative effect on dams and subsequently their power plants, particularly those on rivers or within catchment areas with high siltation. Siltation can fill a reservoir and reduce its capacity to control floods along with causing additional horizontal pressure on the upstream portion of the dam. Eventually, some reservoirs can become full of sediment and useless or over-top during a flood and fail;

• Changes in the amount of river flow will correlate with the amount of energy produced by a dam. Lower river flows will reduce the amount of live storage in a reservoir, therefore, reducing the amount of water that can be used for hydroelectricity. The result of diminished river flow can be power shortages in areas that depend heavily on hydroelectric power. The risk of flow shortage may increase as a result of climate change. One study from the Colorado River in the United States suggests that modest climate changes, such as an increase in temperature in 2 degree Celsius resulting in a 10 % decline in precipitation, might reduce river runoff by up to 40 %. Brazil in particular is vulnerable due to its heaving reliance on hydroelectricity, as increasing temperatures, lower water flow and alterations in the rainfall regime could reduce total energy production by 7 % annually by the end of the century;

• Methane emissions (from reservoirs). The Hoover dam in the United States is a large conventional dammed-hydro facility, with an installed capacity of 2,080 MW. Lower positive impacts are found in the tropical regions, as it has been noted that the reservoirs of power plants in tropical regions produce substantial amounts of methane. This is due to plant material in flooded areas decaying in an anaerobic environment, and forming methane, a greenhouse gas. According to the World Commission on Dams report, where the reservoir is large compared to the generating capacity (less than 100 watts per square meter of surface area) and no clearing of the forests in the area was undertaken prior to impoundment of the reservoir, greenhouse gas emissions from the reservoir may be higher than those of a conventional oil-fired thermal generation plant;

• Relocation. Another disadvantage of hydroelectric dams is the need to relocate the people living where the reservoirs are planned. In 2000, the World Commission on Dams estimated that the dams had physically displaced 40-80 million people worldwide;

• Failure risks. Because large conventional dammed-hydropower plants hold back large volumes of water, a failure due to poor construction, natural disasters or sabotage can be catastrophic to downriver settlements and infrastructure. Dam failures have been some of the largest man-made disasters in history. The Banqiao dam failure in southern China directly resulted in the deaths of 26,000 people and another 145,000 from epidemics. Millions were left homeless. Also, the creation of a dam in a geologically inappropriate location may cause disasters such as 1963 disaster at Vajont dam in Italy, where almost 2,000 people died. Smaller dams and micro hydropower plants create less risk, but can form continuing hazards even after being decommissioned.

Chapter Thermal Pollution Caused by Hydropower Plants