Question: Still, they stood miraculously! How can something 20 [wide] stand in a gap of 20?

Answer: In any case, Abaye asks that it did not stand miraculously. (The miracle was only that they were able to enter it, but not that it stood there.)

Implied question: Why couldn't we say that here is when Yisrael do Hash-m's will?

Answer: Presumably, they initially stood them up according to (the situation at that time), when they were doing Hash-m's will.

Explanation: According to texts that say "and an Amah", we can explain "two Amos inside", i.e. to widen the irrigation channel so it will be two Amos wide.

It is called "Amah" because it is one Amah deep.

Implied Suggestion: Amah is Nigra (mentioned in Bava Metzi'a 107b).

Rejection #1: It connotes in Bava Metzi'a (107b) that we require four Amos for Nigra.

Rejection #2: Here it says that according to one opinion, we plant trees, and according to [the other] opinion, we seed it, and there Rashi explained that we do not seed [the banks of Nigra]!

Explanation: Rather, Nigra is a river, like Rashi explained above (13a), and it is bigger, for everyone in the valley irrigates fields from it, so four Amos are needed.

Observation: Based on this, there was no reason for our Mishnah to say that a public path went through his field, and he gave to them...

The same applies even if he gave to them for no reason, since one may not take the law into his own hands.