Do you consider her as a zava? That is so to most Rishonim, but not to the Rambam who thinks there is a continuous cycle of 18. Once any girls sees even one time, the 18 cycle begins. And so any woman that sees even once might be a small zava. Then if she see three days she is a great zava who needs a flowing spring. Do rivers fed from springs count as springs? This seems to me from what I recall to be an argument between the Raavad and Rambam. [That is an argument as to the status of most rivers. To the mishna only four in Israel are not valid but all other rivers have the status of a spring.
[But if you go with most Rishonim including the Raavad most women that see blood are only nidot, not zavot so any body of natural water is Ok, e.g., a sea or river
[I know I am being short here. So just for information's sake: In the book of Leviticus you see a difference between a woman who sees blood and a woman who sees blood "not in her time". The one that sees not in her time needs "living waters" to get pure. That is a spring. So what is ""Not in her time". Well the simple idea would be seeing her regular time up to seven days. That is if she sees one day or two or three etc up until seven, she simply goes into a sea or river after the seventh day [or even during the seventh day if she has already stopped seeing.] But if she sees again on three consecutive days after the seventh day, then that is "not in her time". She needs then to count seven clean days and go into a spring [or river fed from springs] and bring a sacrifice [once there will be a Temple]
