Isaiah 42:15 - Matthew Poole's English Annotations on the Holy Bible

I will make waste mountains and hills; not dry and barren ones, for these were waste already, but such as are clothed with grass and herbs, as the following words imply; which is to be understood metaphorically, of God's destroying his most lofty and flourishing enemies, who are oft compared in Scripture unto mountains and hills. I will dry up the pools; I will remove all impediments out of the way; which is expressed in the prophetical dialect, by drying up Euphrates, that the way of the kings of the east might be prepared, Revelation 16:12. He seems to allude to that which God did in drying up first the Red Sea, and then Jordan, to give his people passage into Canaan. And this exposition is confirmed by the following verse.