Pe'ah Chapter 2, Mishnah 2. This mishnah continues and clarifies one detail from the previous mishnah. There we learned that one of the things considered a break, creating two separate fields, is the shelulis, which is like an irrigation channel and can also be called an ammas hamayim (water channel) - and that is the subject of our mishnah.
The text of the mishnah:
"Ammas hamayim she'eino yachol l'hikatzeir k'echad - Rabbi Yehudah omeir: mafsekes" - an irrigation channel so wide that it is impossible to harvest both of its sides at once. Only about such a channel does Rabbi Yehudah say that it creates a break and makes two separate fields. But if both sides of the channel can be harvested as one, it is not considered a break.
A dispute or an explanation?
Some understand that Rabbi Yehudah disagrees with the first opinion in the previous mishnah. According to that first opinion, a shelulis creates a break in any case, even if it is narrow and both of its sides can be harvested as one. Rabbi Yehudah then comes and disagrees, saying that if both sides can be harvested as one it does not create a break, and only a width that does not allow harvesting as one creates a break.
And some understand that Rabbi Yehudah is explaining the previous mishnah, and his words are a clarification of that very halachah: when we said shelulis, we meant only a channel this wide.
What is the definition of "she'eino yachol l'hikatzeir k'echad"?
Some commentators explain that it means that a person standing on one side of the channel cannot harvest the other side, because the channel is too wide for that.
Others explain that it means that a person standing in the middle of the irrigation channel cannot reach both of its sides. If it is that wide, it is considered a break, and if not, it is not a break.
The law of mountains:
The next halachah in the mishnah deals with hills and mountains: "Kol heharim asher b'ma'adeir ye'adeirun" - mountains that can only be worked with a hoe, "af al pi she'ein habakar yachol la'avor b'keilav" - the ox cannot pass there with the plowing implements, because the place is too steep and it is impossible to plow it with a plow drawn by an ox, so the soil must be broken up with a hoe - even so, this is not considered a break. Even though there is such a mountain or hill in the middle, one gives a single Pe'ah for the whole field, and it is all considered one field.
In summary: in this mishnah we learned the law of the water channel - that according to Rabbi Yehudah it creates a break only when both sides cannot be harvested as one. The commentators differ as to whether his words are a dispute with the previous opinion or an explanation of it, and also as to the definition of "cannot be harvested as one": whether it means a person standing on one side who cannot reach the other, or a person standing in the middle who cannot reach both sides. We also learned that mountains that can only be worked with a hoe, even though an ox cannot pass there with its implements, do not create a break, and the entire field is considered one field with regard to Pe'ah.