Mikvaot, Chapter Five, Mishnah Five
1) Flowing water is considered like a spring and dripping water is considered like a mikveh.
2) Rabbi Zadok testified that if flowing water exceeded dripping water [with which it was mixed] it was valid [as flowing water].
3) If dripping water became flowing water, its flow may be blocked by a stick or by a reed or even by a zav or a zavah, and then one may go down and immerse oneself in it, the words of Rabbi Judah.
a) Rabbi Yose says: one may not stop the flow of water with anything which is liable to uncleanness.
Section one: If one sees flowing water, then one can assume it comes from a spring and it can be treated like spring water for matters of purity. But if the water is dripping slowly on the ground, then it probably came from rain water and it must be treated like a mikveh. There will need to be forty seahs for it to be a source of purity.
Section two: If most of the water comes from flowing water, then it counts as flowing water and there need not be forty seahs for it to be purificatory.
Section three: There is a stream of dripping water which one directed downhill such that it now became flowing water and it now flows into a river or wadi. If one wants to gather some of in is a specific place so that he can fully immerse in it, he can put a stick or reed to stop the water up. Even a zav or a zavah who require living water can direct the flow of this water and immerse in it.
Rabbi Yose says that if one wants to immerse in flowing water one can’t stop the flow with anything that is liable to uncleanness. All the more so, a zav or zavah cannot use such water because it is no longer considered living water. This principle was stated in Parah 6:4.