Charles H. Spurgeon on Psalms 126:4

As the streams in the south. Some render it, As the mighty waters in the south. Why would they have their captivity turned like those mighty floods in the south? The reason is this, because the south is a dry country, where there are few springs, scarce a fountain to be found in a whole desert. What, then, are the waters they have in the south, in those parched countries? They are these mighty strong torrents, which are caused by the showers of heaven: so the meaning of that prayer in the psalm is, that God would suddenly turn their captivity. Rivers come suddenly in the south: where no spring appears, nor any sign of a river, yet in an hour the water is up and the streams overflow. As when Elijah sent his servant toward the sea, in the time of Ahab, he went and looked, and said, "there is nothing"; that is, no show of rain, not the least cloud to be seen; yet presently the heavens grew black, and there was a great rain: 1Ki 18:44. Thus let our captivity be turned thus speedily and suddenly, though there be no appearance of salvation, no more than there is of a fountain in the sandy desert, or of rain in the clearest of heavens, yet bring salvation for us. We use to say of things beyond our supply, Have we a spring of them? or can we fetch them out of the clouds? So though no ground appears whence such rivers should flow, yet let our salvation be as rivers in the south, as rivers fetched out of the clouds, and dropped in an instant immediately from the heavens. —Joseph Caryl, 1602-1673.

Verses 4-6. The saints are oft feeding their hopes on the carcases of their slain fears. The time which God chose and the instrument he used to give the captive Jews their gaol delivery and liberty to return home were so incredible to them when it came to pass (like Peter whom the angel had carried out of prison, Ac 12:1-25), it was some time before they could come to themselves and resolve whether it was a real truth, or but a pleasing dream. Now see, what effect this strange disappointment of their fears had upon their hope for afterward. It sends them to the throne of grace for the accomplishment of what was so marvellously begun. "The Lord hath done great things for us; whereof we are glad. Turn again our captivity, O Lord": Ps 126:3-4. They have got a handhold by this experiment of his power and mercy, and they will not now let him go till they have more; yea, their hope is raised to such a pitch of confidence, that they draw a general conclusion from this particular experience for the comfort of themselves or others in any future distress: "They that sow in tears shall reap in joy", etc., Ps 126:5-6. —William Gumall, 1617-1679.