1. Rebbe Meir: the ירדן separates between animals regarding מעשר בהמה

Rebbe Meir said on the previous Daf that the ירדן separates between animals on its two sides regarding מעשר בהמה. Rebbe Ami said this only applies where there is no bridge crossing the ירדן, but if there is a bridge, גשר מצרפן – the bridge combines them. He understands Rebbe Meir’s reason to be משום דלא מיקרבן הוא – because [the animals] cannot come close to each other; when a bridge enables them to, they do combine. [Accordingly, any river without a bridge would separate animals for מעשר בהמה.] This is challenged from a Baraisa which teaches that animals on two sides of the ירדן, or in two provinces (even within a mil of each other), do not combine, and certainly if some animals were inside Eretz Yisroel and some outside Eretz Yisroel, they do not combine. This last case is like a place with a bridge (since no river prevents the two groups of animals from joining), yet they do not combine!? Therefore, Rebbe Yochanan explains Rebbe Meir’s reason differently: the passuk says "והירדן יגבול אותו" – the ירדן bordered [the land of Binyomin]; הכתוב עשאו גבול בפני עצמו – the passuk made it an independent border, halachically dividing between its two sides.

1. The source of all water in the world is the פרת

The Gemara states: זכרותא דמיא פרת – the source of all the water in the world is the Euphrates River, because Rav said that one who vows not to drink ממימי פרת – from the waters of the פרת is אסור בכל מימות שבעולם – forbidden to drink any of the waters of the world. The Gemara clarifies that if he said, “I will not drink from the waters of the פרת,” it would only prohibit water taken from the river itself, but water from other rivers is permitted. Rather, the case is where he said, “I will not drink ממים דאתו מפרת – from water which comes from the פרת.” This includes all water in the world, because Rav said that all rivers are lower than the three great rivers which issue from עדן, and are thus fed by them, and these three rivers are lower than the פרת and are fed by it; thus, the פרת is the source of all water. Although there are springs which flow from mountains (from above the פרת), הנהו סולמי דפרת נינהו – these are fed by the “ladders” of the פרת, i.e., underground channels which carry the water up the mountains.

1. Do rivers swell from natural sources, or from rainwater?

Shmuel said: נהרא מכיפיה מיבריך – a river is increased from its bedrock, meaning a majority of the seasonal increase is from natural sources, not from rainwater. This statement disagrees with Rav, who said: מיטרא במערבא סהדא רבה פרת – When rain falls in the West (i.e. Eretz Yisroel), the Euphrates is a great witness thereof. The פרת increases in volume as a result of the rain in Eretz Yisroel. Shmuel’s father would construct mikvaos for his daughters during Nissan and did not allow them to immerse in the פרת itself. The Gemara explains that he holds like Rav, that the פרת swells as a result of rainwater in Eretz Yisroel. Therefore, he was concerned that שמא ירבו נוטפין על הזוחלין – the rainwater might increase during Nissan relative to the flowing water, והוו להו מי גשמים רובא – and the rainwater will be the majority. Rashi here explains that Shmuel’s father was concerned that his daughters were זבות, and a זבה can only be purified with מים חיים – water of a natural spring, not rainwater. Therefore, he constructed indoor mikvaos, located at natural springs, which would not be affected by rainwater.