Which feature is typically associated with a flood-control dam?

A flood-control dam primarily serves the purpose of managing and regulating water flow to prevent flooding downstream. The most significant feature associated with such a dam is the reservoir it creates to store excess water during heavy rainfall or snowmelt. This reservoir can hold back water, allowing for gradual release to manage floods effectively and protect surrounding areas.

The reservoir serves not only to mitigate flooding but also can provide benefits like recreational opportunities, water supply, and enhanced ecological habitats. By temporarily holding excess water, the dam can reduce the severity of downstream flood events and help manage water resources more sustainably.

The other choices reference structures or features that do not directly align with the primary function of a flood-control dam. For instance, immediate overflow structures may not control water effectively, while maintaining an area completely dry contradicts the nature of flood management, which expects fluctuation in water levels. Lastly, a canal might help redirect water flow but is not an inherent feature of a flood-control dam itself. Thus, the reservoir is the defining characteristic that allows for effective flood management.