אורח חיים ת״בOrach Chayim 402

Se’if 1א׳

חָרִיץ מָלֵא מַיִם מְכֻנָּסִים שֶׁהוּא בֵּין שְׁנֵי תְּחוּמֵי שַׁבָּת, מִקְּצָתוֹ בִּתְחוּם עִיר זֶה וּמִקְּצָתוֹ בִּתְחוּם עִיר אַחֶרֶת, שְׁתֵּי הָעֲיָרוֹת אֲסוּרוֹת לְמַלְּאוֹת מִמֶּנּוּ אֲפִלּוּ בְּתוֹךְ הַתְּחוּם, שֶׁהַמַּיִם שֶׁבְּתוֹךְ הַתְּחוּם קוֹנִים שְׁבִיתַת הָעִיר וּמִתְעָרְבִים אֵלּוּ עִם אֵלּוּ וְצָרִיךְ לַעֲשׂוֹת מְחִצָּה בְּסוֹף הַתְּחוּמִין לְהַפְסִיקָן, וַאֲפִלּוּ הִיא תְּלוּיָה שֶׁאֵינָהּ מַגַּעַת לְקַרְקַע הֶחָרִיץ וְשֶׁל קָנִים בְּעָלְמָא, סַגֵּי. הַגָּה: וּבוֹר שֶׁל תֶּבֶן הָעוֹמֶדֶת בֵּין שְׁנֵי תְּחוּמִין, אֵלּוּ מַאֲכִילִין מִכָּאן וְאֵלּוּ מַאֲכִילִין מִכָּאן וְאֵין חוֹשְׁשִׁין שֶׁמָּא יִקַּח אֶחָד מֵחֵלֶק חֲבֵרוֹ. (גְּמָרָא פֶּרֶק חַלּוֹן) וְאִם הַמַּיִם מוֹשְׁכִים, אֵין צָרִיךְ שׁוּם תִּקּוּן שֶׁאֵינָם קוֹנִים שְׁבִיתָה וַהֲרֵי הֵם כְּרַגְלֵי הַמְמַלֵּא.

The Laws that Apply to a Trench Filled with Water Located Between the Shabbos Limits of Two Towns: When water collected within a trench located between the Shabbos limits of two towns, and filled it, with part of [the trench] located in the Shabbos limits of one town and part in the Shabbos limits of the other, [the residents of] both towns are forbidden to draw [water] from it [on Shabbos], even though [the residents of] each [town] are drawing water from within their own Shabbos limits. [The rationale is that] the residents within a techum (Sabbath limit) acquire ownership of the city, and these residents can intermingle with each other. However, it is necessary to make a partition at the end of the boundaries to separate them. Even if the partition is suspended and does not touch the ground of the ditch, it is still valid. As for the ownership of the non-permanent structures, it is considered insignificant. Rema: And a pit of straw standing between these two domains, those eat from here and these eat from here, and there is no concern that one will take from the portion of his fellow. (Chulin) And if the water is drawn, no correction is needed, as they do not acquire a dwelling and they are like the feet that fill [the space].