וְצִוָּה֙ הַכֹּהֵ֔ן וְשָׁחַ֖ט אֶת־הַצִּפּ֣וֹר הָאֶחָ֑ת אֶל־כְּלִי־חֶ֖רֶשׂ עַל־מַ֥יִם חַיִּֽים׃
Y mandará el sacerdote matar la una avecilla en un vaso de barro sobre aguas vivas;
Mishnah Mikvaot
Superior to them are "smitten waters" which can purify even when flowing [on the ground]. Superior to them are "living waters" for in them there is immersion for zavim and sprinkling for metzoraim, and they are valid for the preparation of the hatat waters.
Mishnah Mikvaot
[Water from] a spring which is made to pass over into a trough becomes invalid. 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. If it is made to pass over into a pool and then is stopped, the pool counts as a mikveh. 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.