What is the structure called that is built to trap water in a reservoir?

The structure built to trap water in a reservoir is known as a dam. Dams are specifically designed to hold back water and create a reservoir, which can be used for various purposes such as water supply, flood control, hydroelectric power generation, and recreation.

Dams serve a critical function in controlling the flow of water in rivers and managing resources effectively. By creating a barrier that stretches across a river or stream, a dam raises the water level behind it, creating a large body of stored water. This stored water can then be released gradually, helping to maintain river flow downstream and providing a reliable water source when needed.

Other structures like barriers may provide some level of containment but do not function in the same way as a dam. Levees are primarily used for flood protection by keeping river water from overflowing onto adjoining lands, while embankments are generally earth or stone structures providing a foundation or support but do not specifically serve the purpose of trapping water like a dam does.