Psalms 104:8 - Joseph Benson’s Commentary on the Old and New Testaments

They go up by the mountains Rather, They went up mountains: they went down valleys, &c. They went over hill and dale, as we say; they neither stopped at the former, nor lodged in the latter, but made the best of their way to the place founded for them. The psalmist is “describing the motion of the waters in mountains and valleys, when, at God's command, they filed off from the surface of the earth, into the posts assigned them.” Some interpret the psalmist's meaning to be, that, in that first division of the waters from the earth, part went upward and became springs in the mountains, but the greatest part went downward to the channels made for them. Thus Dr. Waterland: They climb the mountains; they fall down on the valleys. The Hebrew, however, may be rendered, (as it is by some, both ancient and later interpreters,) The mountains ascended; the valleys descended; that is, when the waters were separated, part of the earth appeared to be high, and formed the mountains, and a part to be low, and constituted the valleys or low grounds. So Bishop Patrick: “Immediately the dry land was seen, part of which rose up in lofty hills; and the rest sunk down in lowly valleys, where thou hast cut channels for the waters to run into the main ocean, the place thou hast appointed for them.” But the former sense seems most agreeable to the context, because he speaks of the waters both in the foregoing and following verses.