Mikvaot, Chapter Five, Mishnah Six
1) If a wave was separated [from the sea] and was forty seahs, and it fell on a man or on vessels, they become clean.
2) Any place containing forty seahs is valid for immersing oneself and for immersing other things.
3) One may immerse in trenches or in ditches or even in a donkey-track whose water is connected in a valley.
4) Bet Shammai say: one may immerse in a rain torrent.
a) But Bet Hillel say: one may not immerse.
b) They agree that one may block its flow with vessels and immerse oneself in it, but the vessels with which the flow is blocked are not thereby [validly] immersed.
Section one: The wave that was separated from the sea is equivalent to a spring; thus one can immerse in it even when it is flowing. But it is also similar to the mikveh in that it must contain forty seahs. So if one put vessels in such a wave, the vessels would be pure.
Section two: Any hole in the ground that has forty seahs of water that has not been drawn is valid for a mikveh. One can immerse in it and one can immerse vessels in it as well.
Section three: This section illustrates the previous one. As long as there are forty seahs and the water was not drawn, one can immerse in water found anywhere. One can immerse even in donkey tracks found in a valley whose little puddles have been connected so that together they add up to 40 seahs.
Section four: According to Bet Shammai, if the from the start to the end of the rain flow there are 40 seahs of water, one can immerse in them. In other words, one doesn’t even need any shape to the water whatsoever. As long as the source is rain (if the source was from the ground this would not be a mikveh but a spring) the water can serve as a mikveh.
Bet Hillel says that the water must be gathered together for it to count as mikveh. After all, that is the meaning of the word “mikveh”gathered waters.
Bet Hillel agrees that one may fence off the flow of rain water with vessels and thereby create a mikveh out of flowing rain water. But the vessels themselves don’t count as having been immersed because the outside of the vessel is not touching the mikveh that has been formed on the inside and the water that flows outside is not valid for use as a mikveh.