A reservoir is a man-made lake or large freshwater body of water. Many people think of a reservoir as a lake and might even use the words interchangeably. However, the key difference is that reservoirs are artificial and made by humans, while lakes are naturally occurring bodies of water. Reservoirs are great because they provide a supply of water for when naturally occurring bodies of water, like lakes or rivers, run dry.

mainly there are 3 types of reservoirs.

1.valley-dammed reservoirs

Valley-dammed reservoirs are created in valleys between mountains. Usually, there is an existing lake or body of water. The mountain sides are used as the walls of the reservoir to hold the water. A dam, or artificial wall in the reservoir, is built at the narrowest point to hold in the water. To create a valley-dammed reservoir, the river that will fill the reservoir must be diverted, so the ground can be cleared to lay a foundation for the dam. Next, a concrete lining is put in place, and dam construction can begin. It can take years to build a dam, but once it's done, the water pools in the valley, and a large source of water becomes available.

2.bank-side reservoirs

Bank-side reservoirs are reservoirs that are made by diverting water from local rivers or streams to an existing reservoir. Although this can be applied to many different geographical areas, unlike the valley-dammed reservoir, which requires a valley, diverting water from a river can create problems. The River Thames in London is one example of a bank-side reservoir.

3.service reservoirs

Service reservoirs are entirely man made. They are usually stored in concrete basins above or below ground. You might be familiar with the large water towers in the countryside. This water tower is a service reservoir. In some areas, people dig cisterns, or service reservoirs that are underground. Cisterns must be in a place of higher elevation to allow the water to flow where it needs to go, whereas water towers are already at a higher location than the surrounding land.

Multipurpose reservoir

The 'Multipurpose Reservoir' is a reservoir constructed and equipped to provide storage and release of water for two or more purposes such as flood control, power development, navigation, irrigation, recreation, pollution abatement, domestic water supply, etc'.