Albert Barnes Commentary Psalms 104:8

"(The mountains rose, the valleys sank down) Unto the place which thou hadst founded for them." — Psalms 104:8 (ASV)

They go up by the mountains ... - That is, when they were gathered together into seas. They seemed to roll and tumble over hills and mountains, and to run down in valleys, until they found the deep hollows which had been formed for seas, and where they were permanently collected together. The margin here is, The mountains ascend, the valleys descend. So it is translated in the Septuagint, in the Latin Vulgate, by Luther, and by DeWette. The more natural idea, however, is that in our translation: They (the waters) go up mountains; they descend valleys.

Unto the place - The deep hollows of the earth, which seem to have been scooped out to make a place for them.

Which thou hast founded for them - Where you have laid a permanent foundation for them on which to rest; that is, which you have prepared for them.