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Dams are a commonly used technology for harnessing hydropower from flowing water. Hydropower dams vary in design, operation, and scale, and therefore have different impacts on local aquatic ecosystems, fisheries, hydrology, and geomorphology. Dams have the potential to negatively affect fish populations, especially for those species that migrate up and downstream as part of their life cycle (such as many salmon and trout species). Dams can block these migratory pathways unless there is a fish ladder or another mechanism which allows fish to move around the dam. Even with these mechanisms in place, water temperature and physical changes to rivers as a result of dams can continue to affect fish migration.
In some instances, the benefits of a dam no longer outweigh its maintenance cost, or the structure is worn down and weakened, so that removal of the dam is necessary for economic or safety reasons. Dam removal, or decommissioning, can be expensive and must be well-planned out. However, rivers and fisheries can recover relatively quickly from the effects of a dam, with normal ecological processes resuming soon after.
The Greenhouse Effect
[Image: NOAA Paleoclimatology]
Gases in the Earth's atmosphere that trap heat are referred to as "greenhouse gases". Some of these greenhouse gases occur naturally in the atmosphere but are augmented by inputs from human activities. Others are unnatural compounds that are only created and emitted into the atmosphere from human activities. The principal greenhouse gases released into the atmosphere from human activities are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases. The U.S. Environmental Protection Agency inventories greenhouse gas emissions and sinks and provides information about greenhouse gases and climate change.
Itaipu Dam, Paraguay/Brazil, The world's largest hydroelectric facility. [Image: USGS, Itaipu Binacional]
Hydroelectric power - or hydropower - is a renewable energy source that uses the energy of moving water to generate electricity. Earth's water constantly moves throughout the hydrologic cycle through precipitation, groundwater infiltration, surface water runoff, and evaporation from the ocean and other water bodies. The energy from the movement of water in this cycle, which is itself driven by energy from the sun, can be captured and converted to electricity. Hydropower has been harnessed by humans for thousands of years, but the technology to convert hydropower to electricity was first developed in the U.S. in the late 1800's and widespread by the 1930's.
There are two types of on shore hydroelectric facilities: storage systems, where water is stored in a reservoir and released when needed, and run-of-the-river systems, where the water's current is strong enough that water does not need to be stored. In both systems, the pressure of the flowing or released water is used to turn turbines, and is then converted to electricity. Also, many types of alternative water powered electricity generating systems are currently being developed all over the world, harnessing the energy of the rise and fall of ocean waves and tides.
Advantages: renewable and clean
Hydropower is considered a form of renewable energy because although the energy of moving water is used to create electricity, the water itself is not used up in the process. Hydropower is also a "clean" energy source, meaning the processing and usage of hydropower emits few or no greenhouse gases.
Disadvantages: effects on river processes and fish habitat
Although hydropower is a clean and renewable energy source, it comes with ecological costs. Hydropower energy project infrastructure such as dams can have adverse effects on aquatic resources, by altering and impeding habitat connectivity for certain types of migratory fish, and by altering physical properties of rivers, such as channel morphology and stream temperature. Although measures can be taken to mitigate impacts to aquatic habitat and fisheries, these impacts are often difficult to fully mitigate.
U.S. Energy Information Administration
Tidal and Offshore Hydropower
Tidal energy turbines [Image: Oak Ridge National Laboratory]
Dams are the traditional source of hydropower produced commercially in the United States. Now, research, development, and installation of other sources of hydropower are becoming more common, as the impacts of conventional hydropower are being considered along with its benefits, and most major rivers in the United States have already been developed for hydropower. These new sources include tidal power, wave power, and ocean thermal energy conversion systems.
Tidal power systems use the energy of the incoming or outgoing tide to generate electricity through tidal fences, tidal turbines, or tidal barrages. Tidal turbines can be used independently and can be sited nearly anywhere as long as there is a strong tidal flow, and follow the same premise as wind turbines. Tidal barrages are built across a marine inlet, and use a dam (or barrage) with a turbine system to generate electricity on the incoming tide, outgoing tide, or both, depending on the system used. Tidal fences have turbines mounted along a fence, and are more environmentally friendly and less expensive to install than tidal barrages.
Ocean waves have high energy potential that can be harnessed and used to produce electricity. Waves can be directed into narrower channels, and the concentrated energy can be used to spin turbines. Wave energy can also be harnessed using devices such as terminator devices, point absorbers, attenuators, or overtopping devices. Terminator devices capture or reflect the power of waves, and are placed perpendicular to the waves' traveling direction. Point absorbers are floating devices with two main components, one within another, that move relative to each other and drive energy converters. Attenuators are segmented devices that flex with different wave heights. Overtopping devices have reservoirs that are filled by the incoming waves which are then released back to the surrounding ocean through turbines.
Ocean thermal energy conversion systems take advantage of the temperature difference between the sea surface and the deep sea in low latitude tropical areas, and the water vapor resulting from the transfer or heat between the temperature gradient is used in a turbine generator to produce electricity. (Sources: U.S. Energy Information Administration, OCS Alternative Energy Programmatic EIS)
Hydropower's air emissions are negligible because no fuels are burned. However, if a large amount of vegetation is growing along the riverbed when a dam is built, it can decay in the lake that is created, causing the buildup and release of methane, a potent greenhouse gas.
Water Resource Use
Hydropower often requires the use of dams, which can greatly affect the flow of rivers, altering ecosystems and affecting the wildlife and people who depend on those waters. Often, water at the bottom of the lake created by a dam is inhospitable to fish because it is much colder and oxygen-poor compared with water at the top. When this colder, oxygen-poor water is released into the river, it can kill fish living downstream that are accustomed to warmer, oxygen-rich water. In addition, some dams withhold water and then release it all at once, causing the river downstream to suddenly flood. This action can disrupt plant and wildlife habitats and affect drinking water supplies.
Hydroelectric power plants release water back into rivers after it passes through turbines. This water is not polluted by the process of creating electricity.
Solid Waste Generation
The use of water to create electricity does not produce a substantial amount of solid waste.
Land Resource Use
The construction of hydropower plants can alter sizable portions of land when dams are constructed and lakes are created, flooding land that may have once served as wildlife habitat, farmland, and scenic retreats. Hydroelectric dams can cause erosion along the riverbed upstream and downstream, which can further disturb wildlife ecosystems and fish populations.
Hydroelectric power plants affect various fish populations in different ways. Most notably, certain salmon populations in the Northwest depend on rivers for their life cycles. These populations have been dramatically reduced by the network of large dams in the Columbia River Basin. When young salmon travel downstream toward the ocean, they may be killed by turbine blades at hydropower plants. When adult salmon attempt to swim upstream to reproduce, they may not be able to get past the dams. For this reason, some hydroelectric dams now have special side channels or structures to help the fish continue upstream.
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