A dam as a water storage system: an explanation
Dams are normally built for one of two reasons, the first is to produce electricity, the second is to store water. There are several reasons that you would want to store water but by far the most common is for agriculture. The water that is stored will greatly increase the amount of land that can be farmed which is why dams have been around for so long.
The main reason that dams are built to store water is to allow it to be used for irrigation. There is a great deal of farmland that would not be usable if there were not dams to supply them with water. This water is normally sent from the reservoir through a series of canals so that it can get where it needs to be. The use of dams for irrigation has been around for centuries and is one of the most important developments in human history.
The reason that building dams to store water for irrigation is so important to history is that there would not be nearly enough farm land to support the number of people in the world without it. This is something that was recognized long ago which is why dams have been built for so long. Without them we would almost certainly never have developed the civilization that we have since agriculture would never have developed to the point that has. There is obviously still a long way to go in this regard in some parts of the world but at least we know that we can use dams to provide the necessary water.
While dams are critical for agriculture they are becoming a little bit controversial. The problem is that the dam does have an impact on wildlife. The reservoir will flood a lot of area and force the wildlife that lived there to move. Meanwhile the lower water level on the other side of the dam will cause problems for the wildlife that relies on the river. The result is that these days there is almost constant fighting between farmers and environmentalists about where dams should be built and what the water levels should be for the ones that already exist.
There are other uses for the water stored by a dam that should be considered as well. The most important of these is to provide drinking water. It is not all that common to use dams as to create reservoirs for drinking water since most of it comes right from lakes. However in some areas where there are not a lot of lakes this may be the best option. Regardless of what the water is to be used for the design of the dam will be similar. In these cases the dam is usually just a big wall with some spillways to allow the water level to be controlled.