Mischna zu Bamidbar 19:17

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.

[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.