Mikvaot, Chapter Five, Mishnah One
1) [Water from] a spring which is made to pass over into a trough becomes invalid.
2) If it was made to pass over the edge in any quantity, [what is] outside [the trough] is valid, for [the water of] a spring purifies however little its quantity.
3) If it is made to pass over into a pool and then is stopped, the pool counts as a mikveh.
4) If it is made to flow again, it is invalid for zavim and for those with skin disease and for the preparation of the hatat waters until it is known that the former [water] is gone.
Section one: The trough that is set into the rock in the ground is considered a vessel. Therefore, when they direct the water from the spring into and through the trough, it gains the status of “drawn water” and cannot be used for a mikveh. The trough in this case is not above the spring, as it was in the case in 4:5.
Section two: In this case, most of the waters from the spring pass into the trough but some go over the edge, outside of the trough. Even if the part of the spring that goes outside of the trough is only the smallest amount, it still validates the remainder of the water, because the living waters of the spring purify the drawn water from the trough even in the smallest amount. The only thing one couldn’t do in this case is immerse directly into the water in the trough.
Section three: If the spring is made to flow into a pool and then the spring is stopped up, the pool becomes a mikveh. When there are forty seahs in the mikveh, it will purify.
Section four: If he lets the spring flow again the water that immediately flows after the point at which it had been stopped cannot be used for those people who require “living waters”zavim, people with tzaraat (skin disease) and for preparing that hatat waters (red heifer waters). The spring cannot be used until the water that had been temporarily stopped is gone, because once the water was stopped, it is no longer living waters.