Charles Ellicott Commentary Isaiah 42:15

"I will lay waste mountains and hills, and dry up all their herbs; and I will make the rivers islands, and will dry up the pools." — Isaiah 42:15 (ASV)

I will make waste mountains ... —The whole description is symbolic, and points to the subjugation of the heathen nations, the “rivers” and “pools” probably representing the kingdoms of the Tigris and Euphrates (Isaiah 8:7).

All this seems a purely destructive work, but through it all mercy and truth are working, and a way is being opened for the return of Israel. In portraying this, as elsewhere, the literal melts into the spiritual, as in a dissolving view. (See Note on Isaiah 40:4.)

“These things” include the whole work of judgment and of mercy.