QUESTION: Rebbi Chiya reported (47b) that a Tana said that if there is a reservoir between two cities, it is forbidden for each city to draw water from the reservoir on Yom Tov (and certainly on Shabbos, when there is the additional problem of carrying in Reshus ha'Rabim). This is because at the onset of Yom Tov, the water near one city was Koneh Shevisah with that city and became limited to that city's Techum. When the residents of the second city draw water from the reservoir on Yom Tov, they might be drawing water that drifted over from the side of the other city and is now beyond its permitted Techum. However, if a Mechitzah of iron was placed in the reservoir separating the water of each city, then each city may draw water from the reservoir on Yom Tov.
The Gemara concludes that an iron Mechitzah is not necessary. Rather, the Chachamim were lenient with regard to Mechitzos in water and ruled that any Mechitzah that is at least ten Tefachim high serves to create a division between the waters of each city. The Gemara mentions that this ruling is in accordance with the opinion of Rebbi Yochanan ben Nuri, who maintains that objects of Hefker (such as water) are Koneh Shevisah.
The RIF and ROSH, as well as the SHULCHAN ARUCH (OC 402), rule that a reservoir between two cities needs a Mechitzah in order for the cities to be permitted to draw water on Yom Tov, as the Gemara says.
Why do they rule like the Gemara when the Gemara's statement follows only the opinion of Rebbi Yochanan ben Nuri? The Gemara earlier (46a) explicitly says that the Halachah follows the opinion that objects of Hefker are not Koneh Shevisah, which is not the opinion of Rebbi Yochanan ben Nuri! (Based on the Gemara earlier, a number of Rishonim, such as the RITVA, rule that no Mechitzah is necessary because the water in the reservoir is not Koneh Shevisah, as the Rabanan who argue with Rebbi Yochanan ben Nuri maintain. The RAMBAM, too, makes no mention of any requirement for a Mechitzah in the reservoir.)
(a) The MAGEN AVRAHAM (OC 402) suggests that the Gemara's statement that the requirement for a Mechitzah follows the view of Rebbi Yochanan ben Nuri was only an initial, tentative assumption (which was suggested due to the necessity to explain why Rebbi Yosi b'Rebbi Chanina laughed at the Halachah that Rebbi Chiya taught). In truth, the Rabanan agree with this Halachah. Just as the Rabanan agree that rain water that fell near a city acquires the Shevisah of the city because the people there intend to use that water, here, too, the water in the reservoir acquires the Shevisah of the city adjacent to it.
(TOSFOS (47b, DH Cherem) actually asks this as a question on the Gemara: Why does the Gemara suggest that this Halachah follows only the opinion of Rebbi Yochanan ben Nuri? Even the Rabanan would agree in this case, because it is similar to rain water that fell near a city! Tosfos answers that since the water in the reservoir near the city tends to flow away towards the other city, the people of the first city do not have their minds on that water, and it therefore becomes like any other object of Hefker which is not Koneh Shevisah. The Magen Avraham's answer, however, is not the simple way to understand the Gemara, because the Gemara shows no indication that it retracts its original statement that the Halachah of the iron wall applies only according to the opinion of Rebbi Yochanan ben Nuri.)
(b) The VILNA GA'ON suggests that the text of the Gemara of the Rif and Rosh did not include the three lines that say that this Halachah conforms specifically with the opinion of Rebbi Yochanan ben Nuri. (However, this suggestion requires further elucidation, because the text that appears in our edition of the Gemara is the same text that appears in all known editions of the Shas; see DIKDUKEI SOFRIM.)
The BI'UR HALACHAH concludes that this ruling of the Shulchan Aruch remains difficult to understand.