Eruvin, Chapter 8, Mishnah 7. Much like the previous mishnah, which dealt with a well standing between two courtyards, our mishnah discusses a stream that passes through a single courtyard - just one courtyard. The problem with such a stream: when it is ten handbreadths deep and four handbreadths wide, it is considered a domain in its own right, and its status is that of a karmelis. Because of this, a problem arises with carrying - that is, with drawing water from the stream, which is a karmelis - into the courtyard, which is a private domain.
The words of the mishnah:
"Amas hamayim shehi overes bechatzer" - a channel of water that enters the courtyard on one side and exits on the other.
"Ein memal'in himenah beShabbos" - one may not draw water from it on Shabbos.
"Ela im ken asu lah mechitzah gevohah asarah tefachim bichenisah uviyetziah" - unless a partition ten handbreadths high was made at the point where it enters the courtyard and at the point where it exits, thereby removing its status as a karmelis.
Again it must be stressed: all of this applies specifically when the measurements are ten handbreadths in the depth of the channel and four handbreadths in its width. With less than that, the water channel is considered part of the private domain itself, and there is no problem at all.
The view of Rabbi Yehudah:
Here Rabbi Yehudah applies the same principle he stated in the previous mishnah, that an existing wall above the water can serve as a partition: "Kosel shelegabah tidon mishum mechitzah" - the existing wall of the courtyard, standing above the place where the stream enters and above the place where it exits, is considered the partition needed to separate the stream inside the courtyard from the stream outside, which is certainly a karmelis, and no additional partition is required.
To support his view, Rabbi Yehudah cites an incident: "Amar Rabbi Yehudah: maaseh be'amah shel Avel, shehayu memal'in mimenah al pi chachamim beShabbos" - there was a stream in a place called Avel, and apparently it flowed through a courtyard, and people drew water from it in the courtyard on Shabbos on the authority of the Sages.
The reply of the Sages:
"Amru lo" - the Sages answered Rabbi Yehudah regarding that incident, that the reason there was different: that stream "lo hayah bo kashiur", meaning it did not have the measurements required to be considered a karmelis - its depth was less than ten handbreadths, or its width less than four handbreadths.
A further point from the Gemara:
It is worth noting that according to the Sages, who disagree with Rabbi Yehudah, an additional partition is required in both places - at the entrance and at the exit alike. If a partition was made at only one of them, that is not enough, since the stream is still connected to the rest of the channel outside, and consequently all of it is considered a karmelis just as the outside portion is.
In summary: in this mishnah we learned that a water channel passing through a courtyard, when it is ten handbreadths deep and four handbreadths wide, has the status of a karmelis, and one may not draw water from it on Shabbos unless a partition ten handbreadths high was made at its entrance and at its exit. Rabbi Yehudah holds that the existing wall above it counts as a partition, and he brought proof from the incident of the channel of Avel; the Sages rejected his proof by explaining that that channel did not have the required measurements.