Hearing the sound 'Po! Po! Tro! Tro!', one wishes a wave would appear if there were a river.

This couplet paints a picture of longing for a river. The sounds "Po! Po! Tro! Tro!" evoke a sense of boats or ships, perhaps distant horns, that one would hear by a river. The poet imagines, "If only it were a river, there would be waves." It expresses a deep wish for the presence of a flowing river, bringing with it not just the movement of water and gentle waves, but also the associated sounds and life that a river embodies. It's a beautiful expression of missing nature's serene yet lively presence.