Mishnah Mikvaos 1:5: When does their purity [that of the waters of shallow pools during the rain] begin? Beit Shamai say: from when they [the rain waters] exceed [the amount of the pools' waters] and cause overflow. Beit Hillel say: [from when] they exceed, even if they do not cause overflow. Rabbi Shimon says: [from when] they cause overflow, even if they do not exceed. [Such waters are] valid to be used for challah [a portion of a batch of bread dough given to a priest which becomes holy upon separation, and can only be consumed by priests or their household], as well as for washing [literally: taking] one's hands with them.

מֵאֵימָתַי טָהֳרָתָן. בֵּית שַׁמַּאי אוֹמְרִים, מִשֶּׁיִּרְבּוּ וְיִשְׁטֹפוּ. וּבֵית הִלֵּל אוֹמְרִים, רַבּוּ אַף עַל פִּי שֶׁלֹּא שָׁטְפוּ. רַבִּי שִׁמְעוֹן אוֹמֵר, שָׁטְפוּ אַף עַל פִּי שֶׁלֹּא רַבּוּ. כְּשֵׁרִין לַחַלָּה וְלִטֹּל מֵהֶן לַיָּדָיִם:

Mishnah Mikvaos 1:6: Above these [mikvaot described, and next in the hierarchy of mikvaot,] are runoff waters that have not stopped [flowing down from the mountains]. If an impure person drank [from them], and a pure person drank [afterwards], he is [still] pure. If an impure person drank, and someone filled up a pure vessel [from those waters afterwards], it is pure. If an impure person drank, and then a loaf of terumah fell in, even if one rinsed [the loaf], it is pure. If one filled an impure vessel [from the runoff waters], and a pure person drank [from it], he is pure. If one filled using an impure vessel, and then filled a pure vessel [from the first vessel], it is pure. If one filled an impure vessel, and then a loaf of terumah fell in [the runoff waters], even if one rinsed it, it is pure. If impure waters fell [into the runoff waters], and a pure person drank [from it], he is pure. If impure waters fell in, and one filled a pure vessel [from it], it is pure. If impure waters fell in, and then a loaf of terumah fell in, even if one rinsed it, it is pure. [And these waters are] valid for [use with] terumah, as well as for washing [literally: taking] one's hands with them.

לְמַעְלָה מֵהֶן, מֵי תַמְצִיּוֹת שֶׁלֹּא פָסְקוּ. שָׁתָה טָמֵא וְשָׁתָה טָהוֹר, טָהוֹר. שָׁתָה טָמֵא וּמִלֵּא בִכְלִי טָהוֹר, טָהוֹר. שָׁתָה טָמֵא וְנָפַל כִּכָּר שֶׁל תְּרוּמָה, אַף עַל פִּי שֶׁהֵדִיחַ, טָהוֹר. מִלֵּא בִכְלִי טָמֵא וְשָׁתָה טָהוֹר, טָהוֹר. מִלֵּא בִכְלִי טָמֵא וּמִלֵּא בִכְלִי טָהוֹר, טָהוֹר. מִלֵּא בִכְלִי טָמֵא וְנָפַל כִּכָּר שֶׁל תְּרוּמָה, אַף עַל פִּי שֶׁהֵדִיחַ, טָהוֹר. נָפְלוּ מַיִם טְמֵאִים וְשָׁתָה טָהוֹר, טָהוֹר. נָפְלוּ מַיִם טְמֵאִים וּמִלֵּא בִכְלִי טָהוֹר, טָהוֹר. נָפְלוּ מַיִם טְמֵאִים וְנָפַל כִּכָּר שֶׁל תְּרוּמָה, אַף עַל פִּי שֶׁהֵדִיחַ, טָהוֹר. כְּשֵׁרִים לַתְּרוּמָה וְלִטֹּל מֵהֶם לַיָּדָיִם: