Dam and Reservoir – A barrier (made of earth, concrete or stone) built across a valley to interrupt river flow and create a man-made lake (reservoir), which stores water and controls the discharge of the river
Damn! That's a big dam. The dam was holding back the reservoir.
The advantages of dams and reservoirs, apart from often providing drinking water for large populations, can include a reduction in the risk of flooding during periods of heavy or prolonged rainfall, as flow can be regulated. As water flows through the dam, it might also be used to generate electricity. Storing water in reservoirs can also ensure a constant flow of water, even during extended periods of dry weather.
Of course, there can also be disadvantages. Large areas of land need to be flooded, which can lead to people being displaced, and dams are generally very expensive to construct.
Sediment, transported by the river from its upper reaches, can be deposited in the reservoir. This means sediment is not deposited further down the course of the river, which can reduce soil fertility on the floodplain.