what is the purpose of constructing a dam? is it for saving the water that is wasted and let into the sea or?

It is for saving water from wasting to sea and at the same time to preserve for own use by increasing ground water level.Naturally it prevents others to use.

There are several major reasons for dam construction.

1. Flood control - As an example, there are numerous dams on the Mississippi that are there to control the water surges that occur during the spring thaws. These dams help control the flow of water down stream, preventing surges that breach the levees and flood farmlands, towns and cities

2. Irrigation - Especially in the western USA, dams store the spring melt from the mountains and dole it out throughout the summer. This allows this stored water to be used to irrigate lands that would otherwise not get enough rain to grow large crops

3. Power - The movement of large amounts of water from a higher elevation to a lower elevation represents a huge amount of kinetic energy. Channeling that flow of water through turbines creates a huge percentage of the world's energy. The aluminum plants of the Pacific Northwest made the aluminum that built the planes that won WWII. The power for those plants came from dams

A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees (also known as dikes) are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are often used in conjunction with dams to generate electricity. A dam can also be used to collect water or for storage of water which can be evenly distributed between locations.

Underground dams are used to trap groundwater and store all or most of it below the surface for extended use in a localized area. In some cases they are also built to prevent saltwater from intruding into a freshwater aquifer. Underground dams are typically constructed in areas where water resources are minimal and need to be efficiently stored, such as in deserts and on islands.

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