Which of the following is a characteristic of an overflow dam?

An overflow dam is specifically designed to control and manage excess water flow by allowing water to spill over its crest. This mechanism is primarily utilized for managing water levels in rivers, lakes, and reservoirs while preventing flooding downstream. Holding excess water directly relates to the dam's primary function where it retains and regulates water that accumulates above it.

The reason "A" is not the correct choice is that an overflow dam is not concerned with the buoyancy of materials; instead, it focuses on managing the flow of water. The correct answer emphasizes the dam's function in holding back water and any other materials present, while also allowing controlled release when levels become too high. This process is essential for maintaining balance in aquatic ecosystems and preventing catastrophic failures, which could occur if water levels were not managed properly.