Mikvaot, Chapter Five, Mishnah Three
1) If [water from] a spring that flows into many channels was increased in quantity so that it was made to flow in abundance, it remains as it was before.
2) If it was standing and its quantity was increased so that it was made to flow, it becomes equal to a mikveh in that it can purify in standing water, and to a spring in that one may immerse [objects] therein however small its quantity.
Section one: The spring here flows gently and splits into many little channels. The Hebrew word used to describe this is “nadal” which is the Hebrew word for centipede! If he added drawn water to such a spring, it still counts as a spring and one can immerse in it or in its small tributaries.
Section two: This spring is completely standing still and not flowing into little rivulets like the one in section one. It is like a mikveh in that a mikveh need not be flowing for it to effect purification. And it is also like a spring in that it need not be forty seahs in order for it to be valid for immersion.