Parah, Chapter 8, Mishnah 11. Throughout this chapter the Mishnah has been sorting out which sources of water may be drawn for the mei chatas, the water into which the ashes of the parah adumah are placed. Our mishnah continues that project: it names two particular bodies of water, rules on water whose appearance has changed, discusses water that reaches us through a channel from a distant spring, and closes with a dispute about a well that has become muddy.
The Mishnah begins by naming two specific bodies of water and declaring each of them "kesheirah," fit for this purpose. The first is called "Be'er Achav." The second is "Me'aras Pamyas," a cave situated southwest of Mount Chermon, in the area of the Banias River. Regarding the first of the two, we no longer have any record of where it was to be found.
"Hamayim shenishtanu v'shinuyan meichamas atzman kesheirin": water that has changed, where the change came about of its own accord, is fit. In other words, the water's appearance is no longer what it was, but nothing was mixed into it from the outside to cause that. Such water is still valid for the chatas.
"Amas hamayim haba'ah meirachok kesheirah": a water channel that arrives from far away is fit. Even though the water in front of us has traveled a long distance, and the aqueduct is fed by a spring somewhere else entirely, the water may be drawn for the chatas.
To this the Mishnah attaches a condition: "u'vilvad sheyishmerena shelo yafsikenah adam," provided that one watch it, so that no person cut it off. Someone must be able to keep an eye on the run of water from the spring until the point where it is drawn. The concern is that a person might interrupt the flow, and then the water drawn would no longer be connected to its spring, which would render it unfit. One has to know that what he is drawing is flowing without a break from the spring itself.
"Rabi Yehudah omer, harei hi b'chezkas muteres": Rabi Yehudah holds that the channel stands in a presumption of permissibility. According to him one need not actually station someone to watch it; we assume it is intact and the water is fit.
The Mishnah now turns to a well that has had something drop into it: "charsis o adamah," bits of broken pottery or clumps of soil. When such material falls in, it churns up the silt and grit that had settled at the well's floor, and the water turns cloudy. Rabi Yishmael's position is "yamtin lah ad shetitzal": one holds off until the sediment sinks again and the water regains its clarity, and only then may it be drawn.
Rabi Akiva, however, holds that "eino tzarich l'hamtin": there is no requirement to hold off at all, and the water may be drawn even while it is still cloudy. The halachah is decided in accordance with Rabi Akiva.