God is the One Who pours down water from the sky causing the valleys to overflow. The river then produces lots of foam as the water runs fast. The metals, melted to make ornaments or utensils, too, produces lots of scum. By these examples the Lord wants to explain how the truth (like the water or the melted metal) which is useful for the people stays and how the falsehood (the foam or the scum which is good for nothing) disappears.