A Sage View
‘It Needs An Iron Partition To Divide It’
Our Gemara discusses the case of two neighboring cities whose respective techumim meet in the middle of a pond. R. Chiya rules that an iron partition must be constructed to prevent the water of the first city intermingling with the water of the second city. Otherwise, it’s prohibited for the inhabitants of either city to draw water from the pond.
Why? Because some water from side A of the pond might have moved on Shabbos to side B, and the people from city B will then be drawing water on Shabbos from outside their techum. The same is true of the water on side B of the pond. It might move to side A on Shabbos, and the people of city A will be drawing water that originated from outside their techum.
Tosafos (47b, s.v. “cherem,” following the textual reading found in our Vilna Shas at the beginning of 48a) explains that a partition in the pond is required only according to R. Yochanan b. Nuri who maintains that ownerless objects acquire shevisa (their “Shabbos residence”) at the onset of Shabbos. However, according to the sages (whom the halacha follows), no partition is necessary because the water in the pond is hefker and does not acquire shevisa until someone takes possession of it.
A Different View
The Mechaber (Orach Chayim 402) maintains that a partition is necessary even according to the sages. The Gemara (supra 45b) states that rainwater that gathers in a pool near a city is subject to the techum of that city even according to the sages. This water is not considered hefker because the townspeople already had its use in mind.
Therefore, the rainwater acquires the techum of the city and may not be removed beyond the city’s boundary. Hence even according to the sages, a partition is required to ensure that the waters on opposite sides of the pond don’t mingle.