Siman 402

The Tur and the Shulchan Aruch wrote: "A trench full of gathered water that is between two Shabbat boundaries, part of it within the boundary of this city and part of it within the boundary of another city—both cities are forbidden to draw from it, even within the boundary, for the water that is within the boundary acquires the city's place of rest, and these mix with those. And one must make a partition at the end of the boundaries to separate them, and even if it is suspended, such that it does not reach the ground of the trench, and is made of mere reeds—it is sufficient. And if the water is flowing—no correction is needed, for they do not acquire a place of rest, and they are like the feet of the one who draws" thus far their words.

And as for that which they were required to say, that part of it is within the techum of another city—behold, even if they are not within the techum of another city, they are outside of this techum, and the water that is within the techum mixes with the water that is outside the techum—for this is no prohibition at all, since the water that is outside the techum is hefker and does not acquire a shevitah, and it is like the feet of the one who draws it. And one should not ask that if so, all water should have no shevitah since it has no owners, for the truth is that the people of the city within the techum are its owners because their intent is upon it. And the partition must be either nine tefachim above the water and one tefach within the water, or nine in the water and one outside the water, as written in siman 356, and they did not feel the need to explain, for they relied on what was written there. And in flowing water, no rectification is needed, for since it moves, it does not acquire a shevitah, as written in siman 397, and there it was explained that even standing springs, like our wells, are called moving and do not acquire a shevitah, and they are like the feet of the one who draws it, see there.

It is stated in the Gemara (79a) that a pit full of straw or a bundle of straw standing between two Shabbat boundaries—these may feed their animals from here and those from here, and we do not fear that each might take from what is outside his boundary. And it appears from the Gemara there that this is specifically to stand the animals so that they may eat, but to place it into a basket is forbidden; see there. And the reason was explained in siman 372 regarding a straw-shed between two courtyards; see there.