Reference for Mikvaot 1:7
לְמַעְלָה מֵהֶן, מִקְוֶה שֶׁיֶּשׁ בּוֹ אַרְבָּעִים סְאָה, שֶׁבּוֹ טוֹבְלִין וּמַטְבִּילִין. לְמַעְלָה מֵהֶן, מַעְיָן שֶׁמֵּימָיו מֻעָטִין וְרַבּוּ עָלָיו מַיִם שְׁאוּבִין, שָׁוֶה לַמִּקְוֶה לְטַהֵר בְּאַשְׁבֹּרֶן, וְלַמַּעְיָן לְהַטְבִּיל בּוֹ בְּכָל שֶׁהוּא:
Above these [<i>mikvaot</i> described thus far, and next in the hierarchy of <i>mikvaot</i>,] is a <i>mikveh</i> containing forty <i>se'ah</i> [of water], in which one can immerse one's self and immerse other item [for purification]. Above these [<i>mikvaot</i>, and next in the hierarchy of <i>mikvaot</i>,] is a natural spring whose waters are few, and are exceeded by a greater quantity of drawn waters [which were added]. This [type of spring] is equivalent to a <i>mikveh</i> in that it purifies [only] when it is contained [in one place, standing still and not flowing], but [it is equivalent] to a spring in that one can immerse something in it [to purity it] when there is any amount of water [i.e. there is no minimum measure required].