Mishnayos Mikvaos Perek 5 Mishnah 3

שהוא משוך כנדל (which are conducted in channels radiating like the feet of a centipede) – whose waters flow in many places like this moving creature/unclean reptile that has many feet and we call it in the foreign language – centipede.

ריבה עליו (if one added to it) – drawn water until they flowed further and the places where their waters flow there grew in size.

היה – the spring.

עומד – and its waters would not be flowing, and one added to it drawn water and the spring drew until its waters were flowing further and further on account of the drawn waters.

שוה וכו' למעין – these places where the water is flowing/running in them, are equivalent to a spring that ritually purifies in a miniscule amount, and are equivalent to a Mikveh/ritual bath that does not ritually purify other than in a pond/collection of water. Another interpretation: if one added to it and it was made to flow further, that the pools that would direct the water widened from this spring and enlarged them, behold it is like it was, that in the place that its waters flow prior to adding to them and widening them, there one ritually purifies with flowing waters, but from the sides, one does not purify other than in a pool/collection of water, but if the waters were not flowing but rather just standing, and one added to it, that widened the place of their standing, and flowed further, that he had the place of its standing as flowing, it is equivalent to a Mikveh to ritually purify in a pond/collection of water, and it doesn’t purify In flowing waters, for after that in the essence of the place of the spring they are not flowing.