Radak: Mountains and hills are a metaphor for the kings of the seven Kena'ani nations. Also "Esbam" is a metaphor for the seven nations, and also rivers and lakes. The nations are full of all good, like rivers and lakes; now they will lack all that good.

Malbim: Verse 14 depicted Hashem's actions like the motions of a Yoledes. This is a metaphor for the earthquake that Hashem will bring against His enemies. Earthquakes come via many vapors in the land due to sulfur. When they flame up, and the interior of the land is full of them, the land opens its womb quickly to let the fetus out. The mountains and hills will roll into the valley; they and all the grass and vegetation on them.

Rashi: It is an expression of dryness, for something that is [usually] moist, e.g. grass or rivers.

Radak: The rivers and lakes will dry and became islands, i.e. places to dwell.

Malbim: The mountains will roll into the valley and fill the rivers in the valley, and form islands.