Reference for Mikvaot 5:5
הַזּוֹחֲלִין, כְּמַעְיָן. וְהַנּוֹטְפִים, כְּמִקְוֶה. הֵעִיד רַבִּי צָדוֹק עַל הַזּוֹחֲלִין שֶׁרַבּוּ עַל הַנּוֹטְפִים, שֶׁהֵם כְּשֵׁרִים. וְנוֹטְפִים שֶׁעֲשָׂאָן זוֹחֲלִין, סוֹמֵךְ אֲפִלּוּ מַקֵּל, אֲפִלּוּ קָנֶה, אֲפִלּוּ זָב וְזָבָה, יוֹרֵד וְטוֹבֵל, דִּבְרֵי רַבִּי יְהוּדָה. רַבִּי יוֹסֵי אוֹמֵר, כָּל דָּבָר שֶׁהוּא מְקַבֵּל טֻמְאָה, אֵין מַזְחִילִין בּוֹ:
Flowing [water] sources are like a spring, and dripping [water] sources are like a <i>mikveh</i>. Rabbi Tzadok testified regarding flowing sources that [their waters] were greater than [those of] dripping sources [in a case where the two became mixed], that they are valid. And regarding a dripping source that was made into a flowing source [i.e. if it was somehow breached], one may put even a stick, or even a reed, or even a <i>zav</i> or a <i>zavah</i> beside it [the breach, in order to seal it off], and one may [then] descend and immerse [as into a <i>mikveh</i> of gathered waters], according to Rabbi Yehuda. Rabbi Yose says: anything which can be rendered impure, one cannot use it to stop the flowing.