A dam is a barrier that confines water; as a result, reservoirs are created by dams not only to withstand floods, but also dams provide water supply, recreational lakes, and waste management. However, dams has been used since the ancient times of Roman. Such info can explain why dams were created in the first place, and the different way they were are created compared to today’s dams. 
Types of dams
Many dams can be classified with different purposes and designs. Down below lists five different types of dams and what they are used for:
- Storage dams are built to provide a reliable source of water for short or long periods of time. The purpose for which the water is being stored, such as municipal water supply, recreation, hydroelectric power generation, or irrigation. .
- Diversion dams are typically designed to raise the elevation of a water body to allow that water to be conveyed to another location for use. Diversion dams also supply irrigation canals and transfer water to a storage reservoir for municipal or industrial use. 
- Detention dams are constructed to minimize the impact of flooding and to restrict the flow rate of a particular channel. In some cases, the water trapped by a detention dam is held in place to recharge the subsurface groundwater system. 
- Recreation dams provide prime recreational facilities throughout the United States. Boating, skiing, camping, picnic areas, and boat launch facilities are all supported by dams. 
- Debris dams trap sediment carried by floods and debris flows. 
- Operation of such multipurpose dams is complicated by sometimes opposing needs. The efficient and economic operation of storage and diversion dams requires that water levels be maintained at the highest possible levels. 
The dam wall itself blocks fish migrations, which in some cases and with some species completely separate spawning habitats from rearing habitats.  The dam also traps sediments, which are critical for maintaining physical processes and habitats downstream of the dam. Changes in temperature, chemical composition, dissolved oxygen levels and the physical properties of a reservoir are often not suitable to the aquatic plants and animals that evolved with a given river system. dams can affect land development, agricultural practices, industrialization, and commercial fishing  This also includes soil erosion (which can lower the water supply for wildlife), species extinction (which can cause the the food supply of fish and mammals to extinct), spread of disease (which can attract insects and create a breeding environment for them), and the changes to Earth’s rotation (the shift of water can change the rotation of the ocean and then the rotation of Earth). 
More negative impacts include:
- The physical environment is altered 
- Changing habitat caused by the flooding of land behind and below dams is certain to change the habits of nearby humans and animals. 
- Cycles and variation of flow downstream are effected. 
- Country/regional/global - Reduced fuel imports, improved exports, loss of biodiversity, reallocation of funding, sustainability. 
- Nutrients are unable to move downstream.
- Irrigation areas - Increased water availability and agriculture, water weeds, changes in flow and mineral content, pollution. 
- Coastal erosion is increased due to loss of sediment transportation.
- People are forcibly relocated due to the flooding of the land on which they live to create the reservoirs above the dams.
- Reservoir area - Inundation of land, presence of large man-made reservoir, pollution, changes in mineral content, decaying organic material, pollution. 
- Dams, on the average, can be expected to get clogged with sediment at a rate of about 0.5 to 1 percent per year. 
- Productivity and species diversity of estuaries can be reduced due to the reduction of freshwater flow.
In the beginning, hydroelectricity was long assumed to have less global-warming impact than electricity generated using fossil fuels.  In fact, CO2 is emitted during the manufacture of the cement, concrete, and steel that go into making a modern dam, these emissions, for a typical dam, amount to less than 10% of the emissions from generating the same amount of electricity by burning fossil fuels.  In addition, dams in the tropics, are significant sources of methane, a potent greenhouse gas which has about 23 times the global-warming effect of CO2, ton for ton 
Impacts of climate change can include:
- Tropical dams - the trees gets drowned by water; as a result the wood and other types of organic (carbon-containing) matter that decay under water are mostly broken down by anaerobic bacteria. 
- Rotting trees creates a new dam lake to rot due to the decaying trees.
- Dams may have to contend with decreased rainfall and water flow from changing climate. 
Dams have been thought of being unsafe the environment. However, dams can be beneficial to the environment such as:
- Flood protection dams help prevent the loss of life and property caused by flooding. Flood control dams impound floodwaters and then either release them under control to the river below the dam or store or divert the water for other uses. 
- Water Storage (Fire & Farm Ponds) dams create reservoirs throughout the United States that supply water for many uses, including industrial, municipal, and agricultural. 
- Irrigation ten percent of American cropland is irrigated using water stored behind dams. Thousands of jobs are tied to producing crops grown with irrigated water.
- Mine Tailings there are more than 1,300 mine tailings impediments in the United States that allow the mining and processing of coal and other vital minerals while protecting the environment 
- Electrical Generation the United States is one of the largest producers of hydro-power in the world, second only to Canada. Dams produce over 103,800 megawatts of renewable electricity and meet 8 to 12 percent of the Nation's power needs. Hydro-power is considered clean because it does not contribute to global warming, air pollution, acid rain, or ozone depletion.
- Debris Control in some instances, dams provide enhanced environmental protection, such as the retention of hazardous materials and detrimental sedimentation.
- Navigation dams and locks provide for a stable system of inland river transportation throughout the heartland of the Nation.
Impacts on human health
For people who may live near dams can be affect greatly for example:
- Upstream catchment and river - Changes in flood security, water-related diseases, difficulties with transportation and access to health facilities. 
- Reservoir area - Involuntary resettlement, social disruption, vector-borne diseases, water-related diseases, reservoir-induced seismicity.
- Downstream river - Food security affected on floodplains and estuaries (farming and fishing), water-related diseases, dam failure and flooding. 
- Irrigation areas - Changes in food security, vector-borne and water-related diseases. 
- Construction activities - Water-related diseases, sexually transmitted diseases, HIV/AIDS, accidents and occupational injuries. 
- Resettlement areas - Communicable diseases, violence and injury, water-related disease, loss of food security. 
- Country/regional/global - Macro-economic impacts on health, inequitable allocation of revenue, health impacts of climate change. 
Other closely related articles in this wiki include:
- Wallace, J. (n.d.). Gale Virtual Reference Library - Document - Dams. Retrieved September 22, 2016, from http://go.galegroup.com/ps/retrieve.do?tabID=T003&resultListType=RESULT_LIST&searchId=R2&searchType=BasicSearchForm¤tPosition=1&userGroupName=psucic&inPS=true&sort=RELEVANCE&contentSegment=&prodId=GVRL&contentSet=GALE%7CCX3727600183&&docId=GALE%7CCX3727600183&docType=GALE
- Benefits of Dams | FEMA.gov. (2016, January 6). Retrieved September 22, 2016, from https://www.fema.gov/benefits-dams
- Goings, D. B. (n.d.). Gale Virtual Reference Library - Document - Dams. Retrieved September 22, 2016, from http://go.galegroup.com/ps/retrieve.do?tabID=T003&resultListType=RESULT_LIST&searchId=R4&searchType=BasicSearchForm¤tPosition=2&userGroupName=psucic&inPS=true&sort=RELEVANCE&contentSegment=&prodId=GVRL&contentSet=GALE%7CCX3727800709&&docId=GALE%7CCX3727800709&docType=GALE
- Environmental Impacts of Dams | International Rivers. (n.d.). Retrieved September 17, 2016, from https://www.internationalrivers.org/environmental-impacts-of-dams
- Masters Evans, K. (n.d.). Gale Virtual Reference Library - Document - Threats to Aquatic Environments. Retrieved October 1, 2016, from http://go.galegroup.com.ezaccess.libraries.psu.edu/ps/retrieve.do?tabID=T003&resultListType=RESULT_LIST&searchId=R4&searchType=BasicSearchForm¤tPosition=1&userGroupName=psucic&inPS=true&sort=RELEVANCE&contentSegment=&prodId=GVRL&contentSet=GALE%7CCX2686100010&&docId=GALE%7CCX2686100010&docType=GALE
- Environmental Impacts of Dams. (n.d.). Retrieved September 17, 2016, from http://www.arch.mcgill.ca/prof/sijpkes/arch374/winter2001/dbiggs/enviro.html
- Mendelow, K. (n.d.) Dams: Their Uses, Benefits, Negative Effects and Far-Reaching Impacts on Our Waters Future « WGI Blog. (n.d.). Retrieved September 22, 2016, from http://www.wildgooseimaging.com/blog/wordpress/dams-their-uses-benefits-negative-effects-and-far-reaching-impacts-on-our-waters-future
- Negative Impacts of Dam Construction on Human Populations Can Be Reduced, Author Says | Caltech. (n.d.). Retrieved September 22, 2016, from http://www.caltech.edu/news/negative-impacts-dam-construction-human-populations-can-be-reduced-author-says-957
- Lerer, L. B. WHO | Health and social impacts of large dams. (n.d.). Retrieved September 22, 2016, from http://www.who.int/hia/examples/energy/whohia020/en/
- Gilman, L. (n.d.). Gale Virtual Reference Library - Document - Dams. Retrieved September 22, 2016, from http://go.galegroup.com/ps/retrieve.do?tabID=T003&resultListType=RESULT_LIST&searchId=R1&searchType=BasicSearchForm¤tPosition=9&userGroupName=psucic&inPS=true&sort=RELEVANCE&contentSegment=&prodId=GVRL&contentSet=GALE%7CCX3079000084&&docId=GALE%7CCX3079000084&docType=GALE
Relevant online sources to this wiki article include:
- Effects of dams in river networks on fish assemblages - by USGS
- Threats to the recovery of Atlantic Salmon? - by NOAA
- Problems With Big Dams - by International Rivers
- 4% of Global Warming Due to Dams - by International Rivers