The Gemora on Daf Hay says that the chidush of Rav Yochanan is that mayim al gabay mayim is a hanocho. But on Daf vav the gemorah brings the case of pasei biraos from airuvin where it says that the chachomim instituted the pasim to prevent people from taking mayim from reshus hayachid to the reshus harabim. So don't we see from the mishna in eiruvin that mayim al gabay mayim is a hanocho? Why then do I need Rav Yochanan's Chidush?
A very interesting and excellent question. The CHASAM SOFER asks the exact same question.
(a) The Chasam Sofer answers that the Gemara here is referring to a case where the person gathered ("Kalat") the water before it was all resting in the pit, but while the flow was coming down (and "Mayim Al Gabay Mayim" means that a flow of water was on top of, and pouring into, a puddle of water. All of the water is considered to be touching and connected, even the uppermost drop of water at the top of the flow, because of the uninterrupted stream of water.). For this reason, we might have thought that the upper water is not considered resting (Munach) on the lower water. But water that is actually resting on top of other water, such as in the case of Pasei Bira'os, is certainly considered Munach.
(b) It seems to me that from the Gemara on the previous Amud it is clear that Rebbi Yochanan here is talking about one who grasped the water ("Kalat") in his hand (as it was falling) from on top of the pit and brought it into his Reshus, and not one who merely picked up the water from the pit. It seems that if the water would have come to rest in the person's hand, and then he would have brought it into his Reshus, it is obvious to Rebbi Yochanan that he would be Chayav. Rebbi Yochanan's Chidush is that even when the water did not rest in his hand, but he merely grasped it and right away brought it into his Reshus, he is Chayav. We might have thought that the water was never resting in its place and since it was not Munach, he never transferred from one Reshus into another. If so, the same is true for water that is in a bucket lowered into a well; when the person brings the water into a new Reshus, he is certainly Chayav.
Rabbi Kornfeld AMV"S,
Perhaps by the Mishnah of Pasai Biroas the water could have been draw from within 3 Tefachim of the Karkah of the Boar and hence would be consideredresting on the ground and not on the water. We see from 5a. that within 3 Tefachim of the ground is considered resting when Abai wants to answer the question of the Gemorah that resting of 4 by 4 is needed by saying it is talking about the hand being within 3 Tefachim of the ground. Also on 100a. Rabbi Yochanan says the case of the Mishnah of throwing further than 4 Amos and returning within 4 Amos is talking about the object resting in the air and Rashi states D"H S'nach Al Gavi Mashaho that its considered like resting if its within 3 tefachim of the ground., Kol Tov, Yehudah Relis
Excellent! Thank you!