Bechorot 55 - Jordan and Other Rivers
Rabbi Meir said that the animals on two sides of the Jordan river are not counted together for giving tithes. Rav Ashi noted that if there is a bridge there, they do combine. But, by the same token, animals on two sides of the Israeli border should combine, because they can cross - and we know that they don't! Rather, as Rav Chiya explained, Jordan is called a border by the Torah. But then animals on different sides of the tribes' borders should not combined, and they do!? Rav Chiya has to say that the "land" combines all tribes, but "Jordan" is not land.
Jordan is called that way because it is "yored mi-dan" - comes from Dan. Its source is the cave of Pameas. Why does it matter? - One who vows not to drink water of Pameas should not drink any Jordan water. Jordan's source - as is the source of all water in the world - is the river Euphrates. Why does that matter? For one who vows against the water of Euphrates. But we judge by common meaning, and that includes only Euphrates' water?! - Rather, if he vows against all water that comes from Euphrates.
But how can all water come from Euphrates seeing that there are springs in the high mountains? - They are fed by underground channels. And why is it called Euphrates? Because its waters multiply ("parim" in Hebrew). This teaching supports Shmuel, who said that a river is primarily fed from its bedrock, and disagrees with the father of Shmuel, who would build a separate mikvah for his daughter in Euphrates, because he felt that its water comes at times mostly from rain.
Art: John Robert Keitley Duff - Highland Bridge with Shepherd and Flock