Horns to a River

Why the poets give horns to rivers, must be sought for in the poet's book, nature. I like the interpretation given by a glance up some sinuous and shelving valley, where the mighty stream, more than half lost to the eye, is only seen in one or two of its bolder reaches, as it tosses itself here to the right, and there to the left, to find a way for its mountain waters. The third question about horns I am not able to answer. It would be interesting to know where your correspondent has found it in late Greek.