What is "Nahar Pelagav"?
Rashi: It is the river of Gan Eden.
Radak #1: It is when the mountains will clamor and fall, and the sea will clamor. I.e. there will be great wars among the kingdoms. This is like the metaphor "u'Sh'avtem Mayim b'Sason mi'Ma'aynei ha'Yshu'ah" (Yeshayah 12:3).
Radak #2: It is literally a river - "Yetze'u Mayim Chayim mi'Yerushalayim" (Zecharyah 14:8).
Why does it say "Yismechu Ir Elokim"?
Radak: Yisrael will not fear; they will rejoice.
Malbim: Not only will the river not destroy Ir Elokim - just the contrary, it will gladden them! It will satiate their field for Brachah.
What is "Kedosh Mishkenei Elyon"?
Radak: Yerushalayim is the Kadosh place of all Eretz Yisrael, which is Mishkenei Elyon (Hashem's dwelling place).
