Behold, I will do a new thing (comp. Isaiah 42:9, with the comment). It is, of course, quite possible that the novelty is not merely in the circumstances of the deliverance, but extends to all its results, among which is the Messianic kingdom—verily, a "new thing" (see Jeremiah 31:22).
Now it shall spring forth; rather, already it is springing up (comp. Isaiah 42:9). Things, however, are more advanced (to the prophet's eye) than when that passage was written. Events are shaping themselves—the deliverance approaches.
Shall ye not know it? rather, will ye not give heed to it? Will not the exiled people, whom Isaiah addresses, turn their thoughts this way, and let the idea of deliverance take possession of their minds, instead of brooding on past and present sufferings (see Isaiah 40:30; Isaiah 41:17; Isaiah 42:22)?
God is about to make a way in the wilderness, and rivers in the desert. As he led his people out of their Egyptian bondage, first through the Red Sea, and then through a "howling wilderness" (Deuteronomy 32:10), so now he will "make a way" for them through a still more desolate tract.
We are nowhere historically told by what route the Israelites ultimately returned. If they went by Tadmor and Damascus, they must have traversed a most arid and difficult desert. Even if they did not quit the Euphrates till they reached the latitude of Aleppo, still they must have had some wide tracts of wilderness to cross.