A reservoir (etymology from French réservoir a "storehouse ) or an artificial lake is used to store water. Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.
The term reservoir may also be used to describe underground reservoirs such as an oil or water well.
A dam is any barrier that holds back water; dams are primarily used to save, manage, and/or prevent the flow of excess water into specific regions. In addition, some dams are used to generate hydropower. This article examines man-made dams but dams can also be created by natural causes like mass wasting events or even animals like the beaver.
Another term often used when discussing dams is reservoir. A reservoir is a man-made lake that
is primarily used for storing water. They can also be defined as the specific bodies of water formed by the construction of a dam. For example, the Hetch Hetchy Reservoir in California's Yosemite National Park is the body of water created and held back by the O'Shaughnessy Dam.
TYPES OF RESERVOIRS AND CONSTRUCTION
Like dams, there are different types of reservoirs as well but they are classified based on their use. The three types are called: a valley dammed reservoir, a bank-side reservoir, and a service reservoir. Bank-side reservoirs are those formed when water is taken from an existing stream or river and stored in a nearby reservoir. Service reservoirs are mainly constructed to store water for later use. They often appear as water towers and other elevated structures.
The first and usually largest type of reservoir is called a valley dammed reservoir. These are
reservoirs that are located in narrow valley areas where tremendous amounts of water can be held in by the valley's sides and a dam. The best location for a dam in these types of reservoirs is
where it can be built into the valley wall most effectively to form a water tight seal.
To construct a valley dammed reservoir, the river must be diverted, usually through a tunnel, at the start of work. The first step in creating this type of reservoir is the pouring of a strong foundation for the dam, after which construction on the dam itself can begin. These steps can take months to years to complete, depending on the size and complexity of the project. Once finished, the diversion is removed and the river is able to flow freely toward the dam until it gradually fills the reservoir.
In addition to the high cost of construction and river diversion, dams and reservoirs are often controversial projects because of their social and environmental impacts. Dams themselves affect many different ecological components of rivers such as fish migrations, erosion, changes in water temperature and therefore changes in oxygen levels, creating inhospitable environments for many species.
In addition, the creation of a reservoir requires the flooding of large areas of land, at the expense
of the natural environment and sometimes villages, towns and small cities. The construction of China's Three Gorges Dam, for example, required the relocation of over one million people and
flooded many different archaeological and cultural sites.
Main Uses of Dams and Reservoirs
Despite their controversy, dams and reservoirs serve a number of different functions but one of the largest is to maintain an area's water supply. Many of the world's largest urban areas are
supplied with water from rivers that are blocked via dams. San Francisco, California for example, gets the majority of its water supply from the Hetch Hetchy Reservoir via the Hetch Hetchy Aqueduct running from Yosemite to the San Francisco Bay Area.
Another major use of dams is power generation as hydroelectric power is one of the world's major sources of electricity. Hydropower is generated when the potential energy of the water on
the dam drives a water turbine which in then turns a generator and creates electricity. To best make use of the water's power, a common type of hydroelectric dam uses reservoirs with
different levels to adjust the amount of energy generated as it is needed. When demand is low for instance, water is held in an upper reservoir and as demand increases, the water is released into a lower reservoir where it spins a turbine.
Some other important uses of dams and reservoirs include a stabilization of water flow and irrigation, flood prevention, water diversion and recreation.