Why will the water ascend mountains?
Radak: "Ya'alu" and "Yerdu" are future tense in place of past. It fled from Hashem.
Malbim: Initially, the water ascended to the Raki'a against its previous nature. Hashem rebuked it, and it fled its place. It ascended mountains, against its nature.
Did the water ascend mountains and descend to valleys?
Radak: It fled from Hashem until it found the place that He established for it.
Malbim: First, Hashem rebuked it and it ascended mountains, against its nature. From the voice of His thunder, clouds exploded and rain fell to valleys below, like its nature.
What is "El Mekom Zeh Yasadta Lahem"?
Radak: "Mekom" is Samuch to "Zeh" - it hints to the Mikveh (gathering of) ha'Mayim.
Malbim: It is the Okainus.
