מקואות ה׳Mikvaot 5

Mishnah 1א׳

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 mikveh 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 mikveh [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] zavim [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 chatat waters from them [with the ashes of a red heifer, as part of a purification ritual], until the original waters have departed.

Mishnah 2ב׳

הֶעֱבִירוֹ עַל גַּבֵּי כֵלִים אוֹ עַל גַּבֵּי סַפְסָל, רַבִּי יְהוּדָה אוֹמֵר, הֲרֵי הוּא כְמוֹ שֶׁהָיָה. רַבִּי יוֹסֵי אוֹמֵר, הֲרֵי הוּא כְמִקְוֶה, וּבִלְבַד שֶׁלֹּא יַטְבִּיל עַל גַּבֵּי הַסַּפְסָל:

If it [the spring] was led to pass over vessels or over a bench, Rabbi Yehuda says: it is thereby [still] as it was [i.e. the status of the spring is unchanged]. Rabbi Yose says: it is thereby like a mikveh, so long as one does not immerse over the bench.