When the waters fled, the mountains appeared again, and the valleys became visible.
This is how God acted during creation (Genesis 1:9) and after the flood (Genesis 8:5).
Everything happened according to God’s plan, and so, the author of this psalm praises the Lord (see Psalm 104:1a and Psalm 104:35c!). The author encourages us to support him in his praise.
8 The mountains rose, the valleys sank down to the place that you appointed for them.