The United States started using dam building for hydropower in the early 1900s. While hydropower was largely viewed as a green and renewable energy source at the time, it would later be found that its use may come at the expense of the environment.
Dams are a barrier constructed to hold back water, typically found in rivers, and raise its level. The resulting reservoir is used in the generation of electricity, as a water supply or for other purposes.
Dam building in the U.S. peaked around the time that the National Historic Landmark the Hoover Dam was built in the 1930s.
The Hoover Dam was constructed to tame the Colorado River and provide water and hydroelectric power for the developing Southwest. It fueled the development of major cities, including Los Angeles, Las Vegas and Phoenix.
It is capable of irrigating 2 million acres. Its 17 turbines generate enough electricity to power 1.3 million homes.
This photo taken Tuesday, Oct. 2, 2012 shows the Hoover Dam and Mike O'Callaghan-Pat Tillman Memorial Bridge, as seen from the heliport in Boulder City, Nev. (AP Photo/Pablo Martinez Monsivais)
Dams provide a range of economic, environmental and social benefits. Dams are used for recreation, flood control, water supply, hydroelectric power, waste management, river navigation and wildlife habitat, according to the U.S. Federal Emergency Management Agency (FEMA).
The U.S. is one of the largest producers of hydropower in the world, second only to Canada.
Dams produce over 103,800 megawatts of renewable electricity. They produce 8 to 12 percent of the power needs for the U.S.
Hydropower is considered 'clean' energy because it does not contribute to global warming, air pollution, acid rain or ozone depletion, according to FEMA.
"It's renewable, but that doesn't mean better. As energy gets big, it gets different. That happens for wind, for solar, for hydro," Penn State Associate Professor of Energy Engineering Jeffrey Brownson said.
Hydropower is the first of the renewable energy sources used to grow enough that many of the previously unseen impacts are visible, according to Brownson.
While hydropower is considered green energy, many dams have negative environmental impacts.
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The Aswan Dam in Egypt demonstrates the negative environmental implications of dam building.
"It turns out that when a dam gets so big, huge environmental problems follow it," Brownson said using the Aswan Dam as an example.
The main benefit of the Aswan Dam is its ability to control the annual flooding of the Nile River. The dam has helped the agricultural industries in the area, has provided much needed water for irrigation and has produced electricity from the hydroelectric output of the river.
The dam has helped Egypt to reach its highest ever level of electric production as well as to achieve numerous economic achievements.
While it has allowed many small villages the luxury of using electricity for the first time, some agricultural fields have become waterlogged as a result of silt deposits in the reservoir. Other fields have been slowly eroded, particularly along the coastline.
In addition, the delta has lost much of its fertility because the Nile River no longer carries nutrients all the way to the mouth of the river.
Construction of the Aswan High Dam over the river Nile in Egypt, April 1964. (AP Photo)
While larger dams are still built in the U.S., smaller dams are not built as often in the U.S.
"What we've seen happen over the course of time is that a lot of those small dams that were originally built for a specific purpose have been abandoned on the landscape," River Restoration Director at American Rivers Lisa Hollingsworth-Segedy said.
These dams have numerous negative environmental impacts, especially those not maintained properly.
Stagnant reservoirs and rivers facing reduced water levels from river-draining diversions heat up in the sun. These temperatures may be deadly to certain local freshwater species.
The pool behind the dam creates what Hollingsworth-Segedy calls the 'Kiddie Pool Effect.'
"Think, when you're a little kid and you want to get into in your kiddie pool in the backyard in the summertime. At first it's really cold because the water's right out of the hose but after it sits in the sun for awhile, the water warms up and that's the same kind of thing that happens behind the dam," Hollingsworth-Segedy said.
The water warms up. Thus, the oxygen levels in the water drop. The water is no longer the appropriate temperature, or "thermal profile," for the native species.
The warmer water trapped by the dam dissolves contaminants a lot faster than colder water, causing a chemical change that is unhealthy for the native species.
This trapped, warmer water also causes toxic algae blooms that create poor water quality.
In this Wednesday, Oct. 14, 2015 photo, a tourist looks at Lake Mead on the Colorado River at Hoover Dam near Boulder City, Nev. The bathtub ring shows how far water level has dropped in recent years. (AP Photo/Jae C. Hong)
The section of the river that's upstream of the dam is different from other sections of the river, in terms of temperature, flow, sediment and water chemistry. This section is no longer a streaming environment.
"You have a pond or a lake environment that supports invasive species that wouldn't live in a flowing stream, but they're happy to thrive in that pond environment," Hollingsworth-Segedy said.
Dams have contributed to an average freshwater wildlife population decline of 81 percent since 1970, with some species even sent to extinction, according to the World Wildlife Fund's Living Planet Report.
Dams also trap beneficial sediment, preventing its deposition on floodplains and deltas. This material is needed to sustain fertile agricultural lands, wetlands and beaches.
Climate change impacts will significantly affect dam resources and will exacerbate the current environmental issues, according to the World Watch Institute.
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