Which of the following best describes an underflow dam?

An underflow dam is specifically designed to manage materials that are lighter than water. This feature allows it to effectively contain and retain substances such as oils, fuels, and other lighter liquids that may float on the water's surface. By creating a physical barrier, the underflow dam prevents these materials from spreading and helps in the management and remediation of hazardous spills. This containment is crucial for protection against environmental contamination and serves as a critical component in spill response efforts.

The other options do not accurately capture the purpose and function of an underflow dam. Materials heavier than water would not be effectively contained by such a system, which is tailored for lighter substances. Capturing spills from water's surface misrepresents the primary function, as underflow dams are geared toward preventing lighter materials from escaping rather than simply capturing surface spills. Lastly, an underflow dam does not act as a barrier for all liquid levels, as its design and functionality are specifically focused on managing lighter-than-water materials.