A reservoir is an artificial lake, usually built for the purpose of storing clean water which can be supplied to people, who use it for drinking, washing, irrigation, and other purposes. Reservoirs have been used for thousands of years and are crucial for modern-day cities. While the way they are built has changed - today we have diggers, concrete and cement, whereas in the past people had pickaxes, mortar, and stone or bricks - the way they work hasn't changed much in hundreds or thousands of years.
To construct a reservoir, a river is diverted so that it flows into a basin. The usual way to divert a river is to build a dam, blocking the river from flowing along its normal path, and digging a trench that the river can flow through instead, which leads the river water into the basin. The basin may be naturally occuring, such as the floor of a valley, or it may be constructed by digging out a bowl-shaped depression in the earth and building up high walls at the sides.
Water flows out from the reservoir through a gate that people can control.
Since water always flows downhill, a reservoir must be located uphill from the town or city that it supplies water to. For this reason reservoirs are often built on hills or mountains.