Bekhorot 55B

עינתא דמידליין! אמר רב משרשיא: הנהו סולמי דפרת נינהו.

springs that are higher in the mountains than the Euphrates; how can their water come from the Euphrates? Rav Mesharshiyya said: These are ladders of the Euphrates, i.e., the waters of the Euphrates seep through the ground and are drawn upward to emerge from these springs.

והא כתיב: ״והנהר הרביעי הוא פרת״!

The Talmud asks: But isn’t it written: “And a river went out of Eden to water the garden; and from there it was separated, and became 4 heads…And the name of the third river is Tigris; that is the one that goes toward the east of Ashur. And the 4th river is the Euphrates” (Genesis 2:10, 14)? This indicates that the Euphrates, which is mentioned last, is the least of the 4 rivers, not the source of the other 3.

אמר רב נחמן בר יצחק, ואיתימא רב אחא בר יעקב: הוא פרת דמעיקרא.

Rav Naḥman bar Yitzḥak said, and some say it was Rav Aḥa bar Ya’akov who said: It is the Euphrates that the verse mentions initially as the river that went out of Eden, which divided into all the other rivers. After the other 3 branched out from it, the Euphrates continued to flow.

תניא, רבי מאיר אומר: ״יובל״ שמו, שנאמר: ״והיה כעץ שתול על מים ועל יובל ישלח שרשיו״, ולמה נקרא שמו ״פרת״? שמימיו פרים ורבים.

It is taught in a baraita that R' Meir says: Yuval is the name of the Euphrates River where it emerges from Eden, as it is stated: “For he should be as a tree planted by the waters, and that spreads out its roots by the river [yuval], and should not see when heat comes, but its foliage should be luxuriant; and should not be anxious in the year of drought, neither should cease from yielding fruit” (Jeremiah 17:8). And why is it named Euphrates [perat]? Because its waters are fruitful [parim] and multiply without the need for rainfall.