What is the meaning of "Yehemu Yechmeru Meimav"?
Rashi: "Va'Yigreshu Meimav Refesh va'Tit" (Yeshayah 57:20; it will deposit mud), like it normally does.
Rashi citing Menachem: This is like "Me'ai Chamarmaru" (Eichah 1:20). (They were clouded - Rashi there.) 1
Radak: This is like "Chomer Mayim Rabim" (Chabakuk 3:15), "Chamarim Chamarim" (Shemos 8:10). I.e. the water will become great tall waves when it clamors strongly.
Malbim: This refers to the water of the river, which will be mentioned soon.
Radak: Some say that this is like "Be'eros Be'eros Chemar" (Bereishis 14:10), i.e. the water will become cloudy - "va'Yigreshu Meimav Refesh va'Tit" (Yeshayah 57:20; i.e. it will be cloudy due to mud - PF)!
Why does it say "Yir'ashu Harim b'Ga'avaso"?
Rashi: They will clamor amidst the pride of Hashem, who was mentioned at the beginning of the Mizmor.
Malbim: Even though the mountains will be noisy, we will not fear.
