1. The Gemara discusses the case of a tributary of an ocean that breached the wall of a courtyard.
2. Rav Yehudah discusses the case of throwing an object from Reshus ha'Rabim into a Mavoy that has only two parallel walls.
3. If a Mavoy is square-shaped, one may not carry in it on Shabbos, even if it has a Lechi or a Korah.
4. The same law (#3) applies to a circular Mavoy.
5. The Gemara discusses how much longer must a Mavoy be than it is wide in order for a Lechi or Korah to permit carrying in it.
A BIT MORE
1. The Gemara explains under what circumstances the water may be used, and under what circumstances it is forbidden to carry in the courtyard because of the breach in the wall.
2. If a Lechi was placed at the entrance of the Mavoy the person is liable according to Torah law. If he had placed only a Korah atop the entranceway, he is exempt. The Gemara deduces that Rav Yehudah maintains that a Lechi acts as a third wall, while a Korah acts only as a reminder and not as an actual wall, and thus the Mavoy with a Korah is not a Reshus ha'Yachid because it has only two walls.
3. A Lechi or Korah permits carrying in a Mavoy only when the Mavoy's length is greater than its width. In this case, a four-Tefach-wide Korah must be placed at the entranceway (as is required in order to permit carrying in a yard).
4. This law applies to any Mavoy that is not longer than it is wide (see #5).
5. Shmuel assumed that the length must be twice as much as the width in order for a Lechi or Korah to permit carrying in it. However, Rav told him that Rebbi Chiya had ruled that the length must be only slightly more than the width for it to be permitted with a Lechi or Korah.