The beginning of chapter 2 of Pea says:
These things divide a field [into two] for purposes of pea [requiring a separate pea from each subdivision]: …, a שלולית, ….
The Bavli, Bava Kama 61:1, cites a difference of opinion as what a שלולית is. According to one view, it's a gully or place where rainwater flows; according to the other, it's a main irrigation canal that, on overflowing, feeds side canals.
The commentaries on the mishna in Pea — Rash, Rambam, Rav, P'ne Moshe, Tif'eres Yisrael, K'hati — all accept the latter explanation: a שלולית is a main irrigation canal (some clarify that it's manmade) that feeds lesser canals on overflowing.
Why does Rashi translate שלולית differently from almost everyone else?
(Note that, whereas the aforementioned commentaries are commenting on the mishna itself, Rashi is commenting on the quotation of the mishna in a g'mara. Maybe, then, the answer to my question is some way in which the context in M'nachos demands a different translation of שלולית. (Same for Rashbam, incidentally.) On the other hand, maybe the answer to my question is a difference of opinion about how to read the conclusion of the g'mara in Bava Kama. Or maybe something else.)