Mikvaos, Chapter 5, Mishnah 6. This chapter deals with the various natural gatherings of water and which of them may serve as a mikveh, and our mishnah carries that discussion further, beginning with a case involving the ocean.
A wave that separates from the sea
The mishnah opens: "Gal shenitlash uvo arba'im se'ah venafal al ha'adam ve'al hakeilim, tehorim." A wave broke away from the body of the ocean, and that detached wave held forty se'ah of water. It then came down upon a person, or upon keilim. The ruling is that they are tahor. The forty se'ah descended upon them all at once and enveloped them, and that itself counts as a tevilah, so the person and the keilim emerge in a state of taharah.
The Gemara adds an important qualification. This works only where the taharah was wanted, that is, where the person had kavanah for the water to bring him taharah. A tevilah is not something that simply happens to a person by accident; his intent that these waters purify him is part of the act.
Forty se'ah, wherever they are found
The mishnah then states a general principle: "Kol makom sheyesh bo arba'im se'ah, tovel umatvil." Anywhere that a gathering of forty se'ah is to be found, a person may immerse himself there, and he may also immerse his keilim there. The same water serves for both.
From here the mishnah moves to some unlikely looking places that nevertheless qualify: "Matbilin bacharitzin uvinetzitzin uv'parsas hachamor hame'urevet babik'ah." One may immerse in a trench dug out in the ground, and in cracks and fissures in the earth, once rainwater has collected in them. And one may immerse even in the hoofprints of a donkey in the valley. Picture soft ground that animals walked across, leaving depressions in the earth; the ground later hardened and the depressions filled with rainwater. So long as all of these little pockets of water are joined to one another, their water is combined and reckoned together as a single mikveh of forty se'ah.
Of course, all of this assumes that the little pools open into one another through a gap measuring two fingers in space and width, the shiur called k'shfoferes hanod, the width of the tube that plugs a wineskin. That is the opening the Torah treats as enough to merge waters that would otherwise be separate, making them count as one mikveh.
Rainwater streaming down a slope
Now the mishnah records a dispute. "Beis Shammai omrim: matbilin b'chardalis shel geshamim." A chardalis shel geshamim is rainwater running down the side of a mountain in a stream. Beis Shammai permit immersing in it. Although the accepted view is that rainwater does not bring taharah while it is zochlin, that is, while it is in motion, Beis Shammai maintain that flowing rainwater does purify.
"Beis Hillel omrim: ein matbilin." According to Beis Hillel one may not use such a stream for tevilah, and two separate difficulties stand in the way. One is zochlin, and this is the halachah we follow: moving rainwater does not purify a person, for the water must be at rest. The other difficulty is katafres, water that sits along a downward grade. Regarding any such water the rule is katafres eino chibbur, sloping water is not joined; each stretch of the stream is legally its own body of water, so forty se'ah will never be assembled in a single spot.
Damming the water with keilim
The mishnah concludes with a point of agreement: "Umodim shehu goder keilim vetovel bahen." Beis Hillel concede that a person may go down to the foot of the mountain and construct a makeshift barrier out of keilim, and then immerse in the water that piles up behind that barrier. Once the keilim block the flow and hold the water back, it is no longer running water on a slope but a standing gathering of forty se'ah in one place.
But there is a limitation: "V'keilim shegadar bahen lo hutbelu." Suppose the keilim he used to build the barrier were themselves tamei. They do not thereby undergo a tevilah, because they are being immersed from one side only. The far side of the barrier is not in contact with the water; it will meet water only when the water spills past it, and at that stage the water is zochlin, running water, which does not purify. Therefore the keilim used to dam the stream remain as they were and do not become tahor.
So this mishnah teaches us that forty se'ah of water can qualify in the most unexpected forms, a wave falling from the sea, a crack in the earth, the hollow left by a donkey's hoof, provided the water is truly gathered together and standing in one place.