Mikvaos, Chapter 5, Mishnah 3. This chapter is devoted to the halachos of a mayan, a natural spring, and how it differs from a mikveh. A mayan is metaher bezochlin, it purifies even while its waters are running, and it requires no minimum quantity, while a mikveh purifies only be'eshboren, with the water standing still, and only with forty se'ah. Our Mishnah continues that discussion and examines what happens when drawn water, mayim sheuvim, is poured into a spring and thereby extends its flow.
A Spring Drawn Out Like a Centipede
The Mishnah begins: "mayan shehu moshech kenadal," a spring whose waters run in the shape of a centipede. The picture is of one central channel, a single main artery of water, from which many smaller branches spread out to the sides and receive the water spilling into them.
In such a case, "ribah," a person added water that was sheuvim, drawn water, "vehemshichu," and the addition strengthened and extended the flow. Rules the Mishnah: "harei hu kemoshehayah." This body of water retains its status as a mayan. It is metaher bezochlin, purifying while it runs, and it does not require the shiur of forty se'ah. The reason is that the water was going to be there in any event, and so the fact that he added mayim sheuvim to it does not alter the standing of the mayan.
When the Spring Was Standing Still
The Mishnah now turns to a different case: "hayah omed," the spring did not have such a large volume of water, and as a result its water was simply standing in place rather than flowing onward. "Veribo," he added a considerable amount of drawn water, "vehemshicho," and now the water is moving and reaching areas that it would never have reached had he not added the mayim sheuvim.
Here the halacha is a compromise. "Shaveh lemikveh," it is like a mikveh "letaher be'eshboren," in that it purifies only when the water is at rest, so in this respect it has lost the standing of a mayan. Yet it is also like a mayan, "lehatbel bo bechol shehu," that one may immerse in any amount of it whatsoever, and the shiur of forty se'ah is not needed.
Why the Halacha Splits This Way
Why should we treat this water like a mayan regarding the measure, exempting it from forty se'ah, while treating it like a mikveh regarding zochlin, requiring the water to be standing? The Steipler Gaon, in his sefer on Taharos, Siman Nun Alef, addresses this. He notes that there are mishnayos which yield the reverse combination, where the water does require forty se'ah and yet remains kasher bezochlin. So what determines the ruling here?
The answer he gives is that in our case the zechilah, the flowing itself, is not produced by the mayan. It does not come mikoach hamayan, from the strength of the spring, but from the strength of the water that the person poured in. A flow generated in that way cannot carry the privilege of a mayan, and therefore this water purifies only be'eshboren.
This stands in contrast to the halacha of gal shenitlash, which we will learn later on: when a wave detaches itself from the sea, the movement it carries is still the momentum of the sea's own current, and for that reason such a wave remains metaher bezochlin.
Our Mishnah has thus drawn a fine line. Where drawn water merely reinforces a flow that existed on its own, the spring keeps its full standing as a mayan. Where the flow itself is the creation of the added water, the resulting body of water keeps only part of the leniency of a mayan: no minimum measure is required, but purification is valid only when the water lies still.