Which of the following has the lowest visible elevation in a retention pond?

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The permanent water level in a retention pond represents the sustained water level that remains even during dry periods, establishing a baseline for water retention. This level is crucial for maintaining habitat and water quality in the pond.

In contrast, the high water level and maximum water level refer to elevations that occur during rain events or storm surges when the pond's capacity is temporarily exceeded. These levels can fluctuate and rise above the permanent water level, indicating a temporary state rather than a consistent feature of the pond.

The forebay water level typically refers to a specific area intended to catch sediment and other materials before entering the main part of the pond. It usually exists at a higher elevation than the permanent water level to facilitate this function.

Thus, among the given options, the permanent water level is the lowest visible elevation consistently present in a retention pond. It serves as a reliable reference point for assessing pond conditions and understanding hydrological dynamics.