Adam Clarke on Psalms 77:9

Hath God - in anger shut up his tender mercies? - The tender mercies of God are the source whence all his kindness to the children of men flows. The metaphor here is taken from a spring, the mouth of which is closed, so that its waters can no longer run in the same channel; but, being confined, break out, and take some other course. Wilt thou take thy mercy from the Israelites, and give it to some other people? This he most certainly did. He took it from the Jews, and gave it to the Gentiles.