There is a disagreement in the Mishna on our daf about how to arrange for water that is channeled through a courtyard to be used on Shabbat. According to the Tanna Kamma (first), ten-ama high walls must be built around it at the entrance to the courtyard and its exit in order to be allowed to draw water from it. Rabbi Yehuda rules that it is not necessary to build such walls, since the walls of the courtyard itself act as a separation between the water outside the hatzer and the water inside the hatzer.
Rabbi Yehuda said: There was an incident involving a water channel that passed through the courtyards of the town of Avel, from which the residents would draw water from it on by the authority of the Elders, relying on the courtyard wall suspended above it. They said to him: It is due to the fact that channel was not of the size that requires a partition, i.e., it was less than ten handbreadths deep or less than ten handbreadths wide, it was permitted to draw water from it even without a partition.
The halakha, in fact, makes this distinction. If the water channel is more than four tefahim wide and ten tefahim deep, it is considered a karmelit (an intermediate domain, neither public nor private – see the Introduction to Massekhet Eiruvin) and one cannot draw water from it into a public or private domain. If it is less than four tefahim wide and ten tefahim deep, it has the status of whatever area it is in.
The specific water channel mentioned by Rabbi Yehuda flowed a few kilometers from the village of Avel to the larger city of Tzippori in order to bolster the water supply in the larger city.