What visual cue indicates that a boater is nearing a low-head dam?

The visual cue of a smooth horizon line indicates that a boater is nearing a low-head dam because this feature can create a false sense of safety. Low-head dams often create a smooth surface above the dam, which can mislead boaters into thinking the water is calm and safe to navigate. However, just beyond this seemingly tranquil area lies a significant drop, and the water can be extremely hazardous.

In contrast, other options like rough water surface, changes in water color, and unusual wave patterns may be indicative of various water conditions, but they do not specifically point to the presence of a low-head dam in the same way that a smooth horizon line does. Recognizing this visual cue is crucial for ensuring boater safety, as it serves as an important warning sign to approach the area with caution.