Reference for Mikvaot 5:1
מַעְיָן שֶׁהֶעֱבִירוֹ עַל גַּבֵּי הַשֹּׁקֶת, פָּסוּל. הֶעֱבִירוֹ עַל גַּבֵּי שָׂפָה כָל שֶׁהוּא, כָּשֵׁר חוּצָה לָהּ, שֶׁהַמַּעְיָן מְטַהֵר בְּכָל שֶׁהוּא. הֶעֱבִירוֹ עַל גַּבֵּי בְרֵכָה וְהִפְסִיקוֹ, הֲרֵי הוּא כְמִקְוֶה. חָזַר וְהִמְשִׁיכוֹ, פָּסוּל לַזָּבִים וְלַמְצֹרָעִים וּלְקַדֵּשׁ מֵהֶן מֵי חַטָּאת, עַד שֶׁיֵּדַע שֶׁיָּצְאוּ הָרִאשׁוֹנִים:
Regarding a spring that was led to pass over a [man-made] trough, it [a <i>mikveh</i> filled from its waters] is invalid. If any [even minimal] amount [of water] was led to pass over its edge, it is valid [for immersion] outside of it [anywhere outside of the trough, and even on its edge], since a spring purifies [even] with a minimal amount. If it was led to pass over a pool and [then] it was stopped, it [the spring water in the pool] is thereby [considered to be] like a <i>mikveh</i> [a gathering of water collected by natural means and immersed in for purification]. If one went back and drew it [the spring] to continue [flowing into the pool again], it is invalid for [the purification of] <i>zavim</i> [males who had certain types of atypical genital discharges which render them impure, and who require immersion in naturally flowing water], and for lepers, and to sanctify <i>chatat</i> waters from them [with the ashes of a red heifer, as part of a purification ritual], until the original waters have departed.