What do we call a structure built to hold water back from flowing downstream?

A dam is specifically designed to hold back water and create a reservoir or alter the flow of a river. By obstructing the natural flow of water, a dam serves multiple purposes such as water supply, flood control, recreation, and hydroelectric power generation. The construction of a dam involves a significant structure that can withstand the pressure of water behind it, allowing for controlled water release downstream.

While levees, barriers, and embankments also serve to manage water, they typically do not create a significant water storage area like a dam does. Levees are primarily used to prevent flooding by acting as walls along riverbanks, while barriers are often employed for purposes like redirecting water flow rather than holding it back. Embankments are earthen structures but can have varied uses depending on their design and purpose. In contrast, a dam is fundamentally about retaining a large body of water and managing its flow.