What did the sea see that caused it to flee?
Sotah 37a: It saw Nachshon enter [before it split].
Radak: This is a metaphor. The water was split; it was as if it feared and fled when Yisrael passed through.
When did the Yarden go back?
Rashi (from Mechilta Beshalach 4): At the time of Keri'as Yam Suf, all waters of Bereishis 1 split.
Radak, Malbim: This was when Yisrael passed through the Yarden (Yehoshua 3:16-17).
Mechilta d'R. Yishmael (Beshalach, Masechta deva'Yhi Pharaoh 4) says 'all waters in the world.'
Why does it say regarding the sea "va'Yanos", and regarding the Yarden "Yisov l'Achor"?
Malbim: The miracle of the Yarden was only where Yisrael crossed - the water piled up. It flowed normally (and the water ceased) downstream from there. The Yam Suf 'fled' from the middle in both directions.
