What day is this?
Rashi: The day when Har ha'Zeisim will split from east to west 1 .
It is not "Yom Echad" of the previous verse - Rashi said that that is 1000 years. (PF)
To where will the water go?
Rashi: [Half] will go to the east via that cleavage, until the eastern sea in the east of the world, and half will go to the west, to the western sea 1 . Yo'el prophesized about this "u'Mayan mi'Beis Hashem Yetzei" (4:18), and Yechezkel said "va'Yamad Elef va'Ya'avireni va'Mayim" (47:4). Radak - it becomes a great river (ibid. 5).
Radak: This is like it says there "u'Va'u ha'Yamah El ha'Yamah" (Yechezkel 47:8).
Above (13:1), "Makor Niftach l'Veis David... ul'Yoshevei Yerushalayim" implies that it will not leave Yerushalayim. Here it says that the water will leave!
Malbim: (a) Here discusses after the final war. (b) Here also hints to the spread of water of knowledge and the river of true Emunah.
What do we learn from "ba'Kayitz uva'Choref Yihyeh"?
Radak: The rivers will be in summer and in winter; they will not dry up. Ibn Ezra says, these are dry seasons. Summer is dry, and winter is cold 1 ; rivers diminish then.
Ibn Ezra explains based on Bereishis 8:22. Kayitz and Choref are two of six seasons; each is two months. Kayitz begins in the middle of Sivan (or a half month before or after this, according to two other Tana'im in Tosefta Ta'anis 1:7) and Choref begins six months later. Why do rivers dry when it is cold? Perhaps it is so cold that water freezes, and does not flow. (PF)
