What is a wall built across a river that stops the river's flow known as?

A dam is specifically designed to stop or control the flow of a river or other body of water. By constructing a wall across a river, a dam creates a reservoir or lake behind it, which can serve several purposes including flood control, water supply for irrigation and drinking, hydroelectric power generation, and recreation. The water is held back by the structure, allowing for management of flow rates downstream and the balanced usage of water resources.

In contrast, a levee is primarily used to prevent flooding by acting as a bank along a river, an embankment is often a raised structure that may help in controlling water or serving as transportation routes, and barriers serve broad purposes including redirecting water but do not specifically refer to a structure that completely halts flow. Thus, the precise function of stopping a river's flow uniquely defines a dam.