Final answer:
The apparent narrowing of a river as it flows into the distance is an effect of perspective, where parallel lines seem to converge due to the way our eyes and brain interpret visual information.
Step-by-step explanation:
The apparent narrowing of a river as it flows directly away from you into the distance is an example of perspective. This visual phenomenon, where parallel lines seem to converge as they move away from the viewer, is a key principle in depth perception and is often observed in various natural settings. In the context of a river, the sides of the riverbanks, which are essentially parallel, will appear to come closer together or converge the further away they are from the observer. This effect is due to the way our eyes and brain interpret visual information.
In essence, the apparent narrowing does not represent the actual widening or slowing down of the river flow due to the increase in sedimentation as tributaries unite. Instead, it is a perception caused by the fact that objects appear smaller the farther away they are, thus creating the illusion of the narrowing river.