Summary

Yosef Karo Shulchan Aruch — Yoreh Deah 201…

Paragraph 1 — A woman does not leave her status of [ritual] impurity through washing in a bath, and even if all of the waters in the world went over her, she remains in her status of impurity, and one would be liable for Karet for [cohabiting with] her, until she immerses her entire body at once in the waters of a mikvah or a spring which contains 40 seah.
Source: Wikisource

Yosef Karo Shulchan Aruch — Yoreh Deah 201…

Paragraph 10 — A spring that ran into a pool of water that is pooled and standing [still] , has the rule of a spring; but if the head of the trickle has stopped, it returns to having the rules of a mikvah; but if the trickle of the spring returns and continues into it, it returns to having the rules of a spring.
סעיף י — מעיין שהמשיכו לבריכת מים שהם נקוים ועומדים, יש לו דין מעין; ואם הפסיק ראש הקילוח, חזר להיות לה דין מקוה; ואם חזר והמשיך קילוח המעין לתוכה, חזרה לדין מעין.
Source: Wikisource

Yosef Karo Shulchan Aruch — Yoreh Deah 201…

And there are those that say that running does not hold unless it is necessarily on ground that is able to absorb it, but if it continues on a stone floor which is not able to absorb it, on all the more so on a plank or vessel, even though it does not invalidate the mikvah, it does not help. And it is good to be strict initially.
סעיף מו — אין המשכה מועלת אלא על גבי קרקע או על גבי צנור שלא היה עליו שם כלי בתלוש, אבל אם המשיך על גבי כלים, אפילו כלי גללים וכיוצא בהם, לא הוי המשכה (הראב"ד) .
Source: Wikisource

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