Where did the Kohanim stand?
Rashi: They were [inside the empty river,] near the bank.
What is "Hachen"?
Rashi: It is in line, one opposite the other.
Radak: This is Mekor (not a conjugation, rather, like an infinitive or gerund) - ready and prepared, like the Targum. And so "mi'Matzav Raglei ha'Kohanim Hachin" means that the place was prepared.
Malbim: The ground was totally dry. This was a miracle; by nature, water should have remained in crevices, and the ground would be muddy. 1
Malbim: There were many other miracles. (a) Also Yisrael stood on dry ground. (b) The miracle lasted until the entire nation crossed.