From the previous answers, a lot has been said regarding the potentials usefulness of dams, which is really good. I would like to adress in this answer, for the sake of discussion, some key points regarding why dams can be problematic to us.

It appears that during the past decades, wild river streams (ones that run free from headwaters to confluence) have nearly been wiped and then replaced by river dams. Fact is, dams disrupt natural systems and subsequently thwart the work of rivers. They block fish runs, seasonal flood patterns, affecting redistribution of nutrients (like marine nitrogen delivered by salmon to feed the inlands).

Dams encourage unsustainable growth, displace people, often indigenous people

Very often, dams are built through grants of institutions and rarely benefit the local people. Most of the generated hydropower (if any) is delivered to big cities, not the rural villages displaced by the dams.

Even from a design perspective, it appears dams are made to fail. A huge reservoir surfaces mean terrible annual evaporation losses. Silting is unavoidable. Even the largest reservoirs silt up. When they collapse, it is nightmarish.

Some researchers point out dams are not the solution to our energy crisis. Solar and wind can provide far more reliable, long-term energy than hydropower with far fewer environmental costs.

Dear Mannan,

These structures were built at different times and to meet various objectives: to control the flows (rolling of floods and support of low water levels), to constitute reserves in water, produce energy, raise fish, facilitate navigation, more recently create recreational spaces, etc. Nearly half of the listed works, however, have no use today identified. Dams disturb the functioning of rivers in different ways: They modify their hydrological regime, disturb the ecological conditions upstream and downstream of the reservoir, Decrease self-purification capabilities, alter erosion and solid transport processes, store sediments and pollutants, fragment habitat for aquatic species, and obstacle to the movement of highly migratory species. Finally, they constitute a danger in case of rupture and can aggravate the risk of flooding upstream. A reflection on what must be done is therefore essential.

Removing structures that have no proven use to justify their existence or whose utility is low is the first option. This is the simplest and most effective way to eliminate all the constraints imposed by the structure and to recover better quality habitats. However, this action is not always easy and a number of effects need to be taken into account. The removal of the threshold or the dam will indeed release polluted sediments and modify the artificial ecological conditions that the structure had instituted. It will often take a little time for a new equilibrium to take hold. Another possible solution is to adapt the structure to reduce some of its negative effects on the ecosystem: reduce its height, open gates, install crossing, etc. In all cases, the decision to cancel or develop must result from a reflection comparing, over time, the advantages and disadvantages of the different possible scenarios. It must be taken as part of a global reflection on the river and its watershed.

With my best regards

B. ACHOUR

Dear Bachir Achour Thanks a lot for your complete description

Every coin has two sides. Making dams may destroy the local ecosystem and environment. However, it makes benefit for human beings. What we can do is to enhance the advantages at most and reduce the disadvantages at least.

Dear Mannan,

You may get some useful information from the homepage of the International Commission on Large Dams (ICOLD/CIGB):

https://www.icold-cigb.org

here specifically:

https://www.icold-cigb.org/GB/dams/role_of_dams.asp

https://www.icold-cigb.org/GB/world_register/general_synthesis.asp

Regards,

R. Croissant

Dear Mannan

This is a very complex and involved question. There is lots of literature on this subject.

This report here: Technical Report Harnessing hydropower: Literature Review

The World Commission on Dam's report although published in 2001 is still relevant. You can download the executive summary here: https://digitalcommons.wcl.american.edu/cgi/viewcontent.cgi?referer=https://www.google.co.uk/&httpsredir=1&article=1253&context=auilr

The final report from this Commission is here: https://www.internationalrivers.org/resources/dams-and-development-a-new-framework-for-decision-making-3939

I hope that this is of help.

Darren

Dear Mannan

Dam construction today play an important role to supply water to human being

it also help to do water management. surface runoff

Mannan Rouhani Instead of disadvantages like you mentioned , Dams building can be used in the development of different levels of architects and flood controlling to reduce the harmful effects of flood and other extreme quality of water.

Construction of dams across rivers has advantages (control flood, produce energy, improve irrigation schemes, store water, etc.) and disadvantages (river bed and bank degradation, retained sediments upstream the dam cause less fertility of agriculture land, resettlement of people living upstream the dam reservoir etc.

Environment is for human being. Without existing of human, environment is worthless. Though dams have some negative impacts on environment like river system or ecology , it is very important for protecting valuable assets of people from disasters like flood, giving irrigation facility in the dry condition etc.

So we have to derive its usefulness giving less impacts on environment.

Eugene A. Simonov Thanks a lot, your words are exactly what i wanted to say.

I couldn't have said it better myself.

Interesting topic...interesting discussion.

From the previous answers, a lot has been said regarding the potentials usefulness of dams, which is really good. I would like to adress in this answer, for the sake of discussion, some key points regarding why dams can be problematic to us.

It appears that during the past decades, wild river streams (ones that run free from headwaters to confluence) have nearly been wiped and then replaced by river dams. Fact is, dams disrupt natural systems and subsequently thwart the work of rivers. They block fish runs, seasonal flood patterns, affecting redistribution of nutrients (like marine nitrogen delivered by salmon to feed the inlands).

Dams encourage unsustainable growth, displace people, often indigenous people

Very often, dams are built through grants of institutions and rarely benefit the local people. Most of the generated hydropower (if any) is delivered to big cities, not the rural villages displaced by the dams.

Even from a design perspective, it appears dams are made to fail. A huge reservoir surfaces mean terrible annual evaporation losses. Silting is unavoidable. Even the largest reservoirs silt up. When they collapse, it is nightmarish.

Some researchers point out dams are not the solution to our energy crisis. Solar and wind can provide far more reliable, long-term energy than hydropower with far fewer environmental costs.

Thank you dear Roland Yonaba for your great comment. you referred to another main problem with dams: annual evaporation. that is a very important point which we forgot in this discussion.

I believe that we need to stop constructing dams or replace them. in some countries like mine (Iran) dams destroyed, ruined the environment. we have many problems because of dams.

Dear Alam Zeb Khan you mentioned to flood controlling, but sometimes dams are responsible for causing floods. Because if dam is filled with water due to heavy rain then they open the gates and let the water flow then it causes flood. this can make disaster

https://www.internationalrivers.org/sites/default/files/attached-files/dams__floods_for_irn_final.pdf

Dear Sunil Lembhe as Roland Yonaba said: Very often, dams are built through grants of institutions and rarely benefit the local people. Most of the generated hydropower (if any) is delivered to big cities, not the rural villages displaced by the dams.

In many areas local people suffering from dams such as Gotvand Dam in Iran. Gotvand Dam, completed in 2012, is one of the biggest dams in Iran. It is built over the Karun River, the country’s only navigable river, stretches 950 kilometers, and empties into the Shatt al-Arab waterway. It is Iran’s biggest embankment dam and its reservoir has a capacity of 4.5 billion cubic meters, making it the second biggest artificial lake in the country. It turned Khuzestan’s water salty, around 19 percent of the basin behind the dam was filled with salt. Gotvand Dam has increased the salt in the Karun River by 35 percent. the dam had contributed to the death of 400,000 palm trees in one area of Khuzestan in 2014, due to the exponential increase in the salinity of the Karun River, which has made the already harsh environment unbearable for palm trees. The damage has severe implications on Iran’s agriculture and economy because 40 percent of Iran’s exported dates are produced in Khuzestan. according to studies carried out by Tehran University, Gotvand Dam is responsible for 25 percent of the pollution in Karun River.

Advantages and disadvantages of any structure should be studied before the construction, for this reason there is feasibility study. There are two ideas, are the hazards of constructing dams are more than their benefits?. Whenever we obtain this answer we can decide we need dams or not. But in regions where control flood, clean energy, irrigation are required, we have to think to build dams.

Mannan Rouhani yes it is but good planing, new technologies, flood control designing in of Dams, Culverts, Bridges and other different kinds of flood controlling devices for policy implications were going to mange.

Recommended please to have a slice look for further understanding. it might be helpful.

Chapter Flood Frequency Analysis Using Bayesian Paradigm: A Case Stu...

Yes, dam used for flood control design.

A lot have been said here in which I have equally learnt from. Few of such, is that dam is important, so long it's strategic, safety-oriented in design and effectively maintained. Thanks Mannan Rouhani for this question

Dear Roland Yonaba , evaporation and silting are two different phenomena which are only marginally related to one another. Silting of reservoirs is a consequence of the sedimentation of the solids carried in suspension by the river due to the reduction in the speed of the water when entering the reservoir behind the dam. The evaporation is linked to climatic conditions and to the surface of the reservoir, and its balance could be either way: you must take into account the vegetation that had to give way to the reservoir, and compare the evaporation surface after the construction and commissioning of the dam (the surface of the reservoir) with the pre-existing evaporation surface that is the sum of the surfaces of all the leaves of the plants and trees that occupied the area of the reservoir. Thus, if the dam was constructed in the desert, there would be an increase in the evaporation, while if the reservoir had displaced a forest, the net evaporation would actually have substantially decreased.

Very interesting discussion has taken place on the subject. If one analyses these answers, it can be concluded that there is advantages and disadvantages due to the construction of a dam. Hence it calls for a balancing act. The rule of nature is, "if you want to gain something, you have to loose something". This is why in the recent times it has been made mandatory to have Environmental Impact Analysis (EIA) before any larger intervention activities with the nature.

Nice topic.....interesting and valuable discussion.

There seems to be an open seaon on dams - most people seem to dam(n) them out of hand. So I would like to add some benefits of dams which have not been properly highlighted:

1. Recreational: In richer countries, sometimes dams are built purely for their recreation benefits; and dams built for other purposes also usually have recreational benefits such as swimming, boating, wind-surfing, lakeside walking, etc. This brings people into nature and makes them conscious of the need to conserve the environment.

2. Silting: In poor countries, silting can be a benefit rather than a problem. In the dry season as the reservoir water level goes down, the highly fertile silt at the head of the reservoir is exposed, and local people can use the area to plant high-value vegetables and other annual crops as the water recedes. Those crops draw water from the subsurface water table - the best kind of natural irrigation.

3. Providing electrical energy during peak demand periods: The disadvantage of other forms of renewable energy is that they may not be available when needed. For example, solar energy production is highest in mid-day, whereas electricity demand is highest in the evening. Wind energy is not generated on windless days. Tidal energy is maximal at a different hour each day. These problems can be solved by BUILDING NEAR EACH WIND FARM AND EACH SOLAR PANEL FIELD A DAM EQUIPPED WITH PUMP-TURBINES. During periods of excess power generation, the unused elecricity is used to pump water up into the reservoir. During periods of high demand, this water is let back down through the pump-turbines, generating electricity. The potential of this source of energy is grossly underdeveloped.

This said, it goes without saying that every dam project (like any other project) should undergo an environmental assessment to indicate measures for maximizing its positive and minimizing its negative impact on the environment.

Dams were built since the 3rd millennium BC in the Indian Subcontinent (present-day Pakistan mainly) to control monsoon floods. There is a lot of literature about them