English Explanation of Mishnah Eduyot 7:3

Rabbi Zadok testified concerning flowing water which exceeded in quantity dripping water; that it was valid.
There was such a case at Birath Hapilya, and when the case came before the Sages they declared it valid.
This mishnah discusses water which is used for a mikveh, the bath used by Jews to achieve ritual purification. This mishnah also appears in tractate Mikvaoth, where it is preceded by a distinction between running water and dripping water. Water which is flowing on the ground, such as a stream, creek or river, is valid for a mikveh even if there are less than 40 seahs in one place. However, dripping water, such as rain, must reach a minimum measure of 40 seahs in one defined place in order to be valid as a mikveh. [Note that water drawn to the mikveh is invalid in any case].
Our mishnah discusses a potential mikveh that has some flowing water and some dripping water. The question is, does this mikveh need to have 40 seahs?
According to Rabbi Zadok, as long as the quantity of flowing water exceeded the quantity of dripping water, the mikveh is valid, even if there are not 40 seahs in the place where the person immerses. If, however, there was more dripping water, then the mikveh would need to have 40 seahs of water in order to be valid. The mishnah then brings in an actual case where such a question arose and the Sages declared the mikveh to be valid.