Mikvaos, Chapter 1, Mishnah 7. Our perek has been climbing a ladder. Each mishnah presents a body of water that carries some advantage over the one listed before it, until the Mishnah has laid out six ascending levels of water. Our mishnah brings us two more rungs on that ladder.
Before we begin, one fundamental distinction should be kept in mind, since the whole mishnah turns on it. A rainwater mikveh purifies only when its waters are gathered and at rest, and only when it holds the required measure of forty se'ah. A ma'ayan, a spring welling up out of the ground, purifies even while its water is flowing along, and it purifies in any quantity at all.
The first rung: a mikveh of forty se'ah
"Lema'alah meihen," higher than the levels the Mishnah has already listed, is a "mikveh sheyeish bo arba'im se'ah," a gathering of rainwater that contains forty se'ah. What is its advantage? "Shebo tovlin umatvilin": in such a mikveh a person who has become tamei may immerse himself and emerge tahor, and one may also immerse keilim that have contracted tumah. All of this, however, on condition that the water is standing still and gathered in its place, not running.
The second rung: a small spring enlarged by drawn water
"Lema'alah meihen," higher still, is a "ma'ayan shemeimav mu'atin": a genuine spring, but one whose own flow is meager, less than forty se'ah. "Vehirbu alav mayim she'uvin," and a person increased its volume by adding mayim she'uvin, drawn water that had been collected in a vessel. Ordinarily such water is the ruin of a mikveh; poured into a rainwater mikveh it disqualifies it. Here it was poured into the spring, and between the two together there are now forty se'ah.
What is the halachic character of this mixture? The Mishnah rules that it is a hybrid, and it draws from each of its two parents. "Shaveh lamikveh letaheir be'ashboren": in one respect it is like an ordinary mikveh, in that it purifies only be'ashboren, when its water lies gathered and at rest, and not while it runs. "Vehama'ayan letaheir bechol shehu": in another respect it retains the strength of a spring, in that it purifies with whatever quantity it contains, without needing the full measure of forty se'ah.
So the ruling comes out as a compromise. Since part of this water is drawn water, one may immerse in it only where the water is standing; but since part of it is spring water, it needs no complete shiur of forty se'ah.
A question the mefarshim raise
Many of the mefarshim linger over precisely this compromise and ask what its logic is. If we are treating this body of water as an ordinary mikveh, why should it be exempt from the requirement of forty se'ah? And if we are treating it as a spring, seeing that it purifies bechol shehu, in any amount, why should it be limited to purifying only be'ashboren, only when the water is at rest? The Mishnah teaches the halachah as a blend of the two, and the reasoning behind that blend is a matter the commentators take up at length.