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 'Bavel 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 a 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 (the outside of the Rivers), but between the Rivers it took for granted that they are Meyuchasin.

Explanation #1: Rashi explains that some say 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.): ... in which case Rav Ada bar Ahavah is 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 states 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 count the 'Kahal of Petzu'a Daka, which does not refer to Pesulei Yuchsin.

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