Mikvaos, Chapter 5, Mishnah 1. The new chapter turns to the mayan, a natural spring or well, and to the special powers that a mayan enjoys. Two of them stand behind everything in this mishnah. First, a mayan is metaher b'chol shehu: it purifies with any amount whatsoever and does not require the forty se'ah needed for a mikvah. Second, it is metaher b'zochlin: its water purifies even while it is running and flowing, and need not be gathered and standing.
Leading a mayan over a trough
The mishnah begins: "Mayan she'he'eviro al gabei hashokes, pasul". A person channels the water of a spring so that it runs into a shokes, a hollowed stone trough of the sort that was fashioned as a vessel before being fastened down to the ground, and therefore holds the halachic status of a keli. The water emerging from that trough is disqualified. Having passed through a vessel, it takes on the character of mayim she'uvim, drawn water, and its ongoing physical continuity with the spring is not accepted as a valid connection for these purposes.
The mishnah then narrows the ruling: "He'eviro al gabei sefah kol shehu, kasher chutzah lah". If not all of the spring water entered the trough, but even the smallest trickle passed along its rim and around the outside, the picture changes entirely. That bypassing trickle remains untouched spring water, and once it joins the stream flowing out of the trough, the two are united. This union is hashakah, the point of contact between valid mikvah water and water that on its own would be unfit, and through it all of the water becomes fit for immersion, purified by the power of the spring.
A justification is attached to this ruling: "shehamayan metaher b'chol shehu". No minimum measure governs the purifying strength of a spring; even a trickle too small to measure carries its full force. That is why the sliver of water that skirted the trough loses nothing of its potency, and when the disqualified water merges with it, the spring lifts the whole quantity into validity.
Leading a mayan into a pool
The mishnah now presents a different arrangement: "He'eviro al gabei bereichah v'hifsiko, harei hu k'mikvah". A person leads the spring water into a pool where it gathers, and then he cuts off the channel that connected the pool to the spring. That standing water now carries the status of ordinary mikvah water. Since it was never brought there by human hands or through a vessel, it is perfectly valid water, but it is valid as a mikvah and no longer as a mayan. It is like rainwater: it purifies only while gathered and standing, and it will not purify b'zochlin, while flowing.
Suppose he then opens the channel again. The mishnah rules: "Chazar v'himshicho, pasul l'zavim u'l'metzoraim u'l'kadesh mei chatas". Reconnecting the pool to the spring does not give the water back its former standing as mayim chayim, living water taken directly from its source. The moment the link was broken, the water was redefined as mikvah water, and reopening the channel cannot undo that redefinition. Consequently it is unusable wherever the Torah insists specifically on mayim chayim: the taharah of a zav, the taharah of a metzora, and the preparation of mei chatas, the waters sanctified together with the ashes taken from a Parah Adumah.
The disqualification, however, is not permanent. "Ad sheyeid'u sheyatz'u harishonim", until it can be established that the original water has entirely flowed out and been displaced by the fresh water now streaming in from the spring. Once the pool has been fully replenished by the living source, it once again holds the status of mayim chayim, and it may serve for all of these purposes.
Our mishnah thus teaches how easily the identity of water can shift. A spring is powerful enough to rescue drawn water through the slenderest trickle that bypasses a trough, and yet a single severing of its channel can reduce that same spring water to the level of an ordinary mikvah, until fresh living water comes and takes its place.