Implied Question: The reason that the Gemara does not ask how far the border extends beyond the P'ras, Rashi explains, is ...

Answer: Because the P'ras flows into the Chidekel, and 'aBavel extends below that point until Apamya Tata'a, as the Gemara explained earlier.

Explanation #1: Which is why he slandered them.

Explanation #2: Others explain that it pertains to Rav Papa.

Refutation: This is not correct however ...

Reason: Since it is not feasible to say that that Tzadik would slander falsely.

Clarification: It must be referring to two different rivers ...

Reason: Because if it was referring to Chidekel and P'ras, the Gemara would not have asked 'So what if it is between the Rivers?' ...

Reason (cont.): Seeing as it is unanimously agreed upon that they are Meyuchasim ...

Proof: Since above too, the Gemara only asked the She'eilah concerning the sides (i.e. the outside of the Rivers), but between the Rivers it took for granted that they are Meyuchasin.

Explanation #1: Some say, Rashi explains, that he died, in which case this is not the Rav Ada bar Ahavah mentioned (often) in the Gemara.

Explanation #2: It is more likely however, that 'Yoshev be'Cheiko shel Avraham' means that he entered into the B'ris Milah ...

Explanation #2 (cont.): And Rav Ada bar Ahavah is then the one who lived very long - from the time that Rebbi died until the latter generations, till the days of Rav Nachman bar Yitzchak ...

Source: As the Gemara says in Bava Basra (Daf 22a).

Support for Explanation #1: The Ri however, explains that there were two Rav Ada bar Ahavahs.

Explanation #1: Rashi explains that this refers to Pesulei Yuchsin, and that it does not therefore include Petzu'a Daka, since it is not talking about Pesulei Yuchsin.

Explanation #2: Furthermore, says the Ri, 'Kahal' is not applicable to a Petzu'a Daka.