Mikvaos, Chapter 5, Mishnah 5. This chapter sorts out the various natural bodies of water and asks which laws each of them follows: the laws of a mikveh, a gathering of rainwater, or the laws of a ma'ayan, a spring. Our mishnah lays down the basic rule for flowing water as opposed to water that collected from rainfall, and then addresses a very practical problem: a mikveh that has begun to leak.
Flowing Water and Dripping Water
The opening words are "hazochlin kema'ayan": water that runs of its own accord, a river for example, is governed by the rules that apply to a spring. Two consequences follow from this. First, such water works bechol shehu, purifying no matter how small a quantity is present. Second, it serves for every case in which the Torah demands spring water: the immersion of a zav, the purification of a metzora, the sanctifying of the waters of the parah adumah, and the remaining instances of that kind.
Next comes "vehanotfin kemikveh": rain that trickles down and gathers in a hollow of the ground is governed instead by the rules of a mikveh, and it purifies only once forty se'ah have accumulated there. The mishnah is therefore separating two categories: a body of water that came into being on its own and runs of its own accord, as against water whose entire existence is owed to rain that happened to collect in one spot.
The Testimony of Rabi Tzadok
What happens when the two are mixed together? The mishnah records a testimony: "he'id Rabi Tzadok al hazochlin sherabu al hanotfin sheheim kesheirim" - where rainwater has flowed into a naturally formed body of water in the ground, and the majority of the water is the naturally flowing water that carries the halacha of a ma'ayan, the water retains its original status. It is valid, and it remains zochlin, purifying even while it runs.
A Mikveh That Springs a Leak
The reverse case is "hanotfin she'asa'an zochlin": we have a mikveh made of collected rainwater, and now a crack opens in one of its walls and the water begins running out. A mikveh purifies only as ashboren, standing water, so the question becomes how one may block the flow in order to restore the mikveh's standing status.
Rabi Tzadok's ruling is remarkably lenient: "somech afilu makel, afilu kaneh, afilu zav vezavah" - one may prop up the breach with a stick, or with a reed, and even a zav or a zavah may place a finger in the wall to stop the water. Although a zav and a zavah are themselves temei'im, they may hold back the flow, and by doing so the water once again counts as ashboren. Then, says the mishnah, the tamei person "yored vetovel", goes down and immerses. This is divrei Rabi Yehuda, the opinion of Rabi Yehuda, who is not troubled by the use of something tamei to keep the water from draining out.
A Question of the Mefarshim
The mefarshim raise an obvious difficulty. We have a rule that a mikveh may not come into being by means of something that is mekabel tumah, an object susceptible to tumah. Here, though, it appears that we are preserving the validity of the mikveh precisely through such an object.
Their answer is that there is a fundamental difference between forming a mikveh and maintaining one. When one is creating a mikveh and directing the water into it in the first place, that havayah may not be accomplished through an item that is mekabel tumah. In our mishnah, however, nothing is being brought in; we are only preventing water that is already there from running out. In that situation Rabi Yehuda is lenient and permits even an object that is susceptible to tumah.
The View of Rabi Yosi
The mishnah closes with a dissenting voice. "Rabi Yosi omer: kol davar shehu mekabel tumah ein mazchilin bo" - Rabi Yosi disagrees with Rabi Yehuda and holds that one may not hold back the leak with anything that is mekabel tumah, since in his view this too runs afoul of the principle that a mikveh's status may not be brought about through an object susceptible to tumah.