What do you imagine when you hear the word "dam"? Maybe you will visualize a huge structure or an artificial lake that stores lots of water. This overview introduces what a dam is and what roles it plays. First, we would like to describe the basics of dams, such as the difference of types and their functions.

In the river law in Japan, the dam is defined as "a structure to store or take water from a river, the height from its foundation to the top must be 15 m or more". The height of the dam should be 15 meters or more by law. A dam of less than 15 meters in height is legally not considered as a dam, and it is called "a barrage" or "a pond" to distinguish it from a dam.

There are several types of dams. The type of a dam is dependent on the conditions such as topography, geological structure, ground strength in the planned site before construction. It is roughly classified into two categories, "a concrete dam" or "a fill dam."

A concrete dam is a dam constructed from concrete and can be sub classified into "a gravity type" and "an arch type." A gravity type dam resists the water pressure using the weight of the dam itself. An arch type dam resists the water pressure transferring pressure to the right and left abutments and the foundation.

A fill dam is a dam constructed from rock and soil materials. This type of dam allows the construction of a large scale dam even on relatively soft ground. Depending on the materials used, a fill dam can also be further classified. A dam constructed using rock, gravel, or soil materials, and stabilized with rock, and stop water from seeping through the dam with impermeable material is called "a zone type" fill dam. In contrast, a dam constructed using uniform material is called "an uniform type" fill dam, which type corresponds historically to a pond built in ancient times.

The purposes of dams are summarized as follows.

Rivers in Japan are generally shorter and steeper compared to those in other countries. Following heavy rains, water flows downstream rapidly. The flow's strong momentum often floods rice and vegetable fields with large volumes of water, damage crops, inundate homes, and sometimes with loss of life. A dam temporarily stores that massive amount of inflow water and plays a leading role in reducing downstream flood damage by controlling the water flow.

To maintain river flow throughout the year for navigation, fishing, sightseeing, groundwater preservation, conservation of water environment, or securing vested water and other uses, a dam releases its stored water when the downstream river flow is insufficient.

A dam also plays a role in storing water necessary for irrigation. It helps maintain a steady supply of water to paddy fields from the river even during the times of low river flow.

Another function of a dam is to store water necessary for domestic water supply. The stored water is discharged to a river when water in the river is scarce. It helps supply water stably to homes for drinking, a bath, and washing via water purification plant.

A dam also stores water for industrial use and discharges it to a river when the river water is insufficient. It helps supply water stably to plants and other industrial facilities.

Water stored in a dam is discharged to the downstream to generate electricity by turning the turbine of the generator by the momentum of the water. Since hydropower does not use fossil fuels at all because it is operated by the acquired momentum of the potential energy or the gravity of water. For this reason, it does not generate carbon dioxide, and it is clean energy and friendly to Mother Nature.

A dam has varied facilities for executing works such as flood control and water supply.

A dam has facilities for discharging water – water outlet, which makes water flow to the downstream of the river. It releases water to meet assumed purposes. This is one of the most fundamental functions of a dam.

These are facilities to observe the water level and precipitation running into a river to perform proper management of a dam to meet the rainfall or whatever situations. It acts as the eyes of a dam, operating 24 hours a day, and automatically transmits observed data to the dam management office.

When a dam discharges water to the downstream, the water level of the river may rise suddenly. It is very dangerous to stay in or on the river, so it is necessary to inform people staying along the river or anglers or other visitors of such information by broadcasting or texting. Thus, warning equipment and information display panels are installed at certain spots in the downstream area of the dam. At the same time, JWA conducts patrol using a patrol vehicle equipped with red lights and loud speakers.

There are some other facilities necessary for managing a dam. Some of them are shown below.