Reference for Mikvaot 7:7
הִטְבִּיל בּוֹ אֶת הַמִּטָּה, אַף עַל פִּי שֶׁרַגְלֶיהָ שׁוֹקְעוֹת בַּטִּיט הֶעָבֶה, טְהוֹרָה, מִפְּנֵי שֶׁהַמַּיִם מְקַדְּמִין. מִקְוֶה שֶׁמֵּימָיו מְרֻדָּדִין, כּוֹבֵשׁ אֲפִלּוּ חֲבִילֵי עֵצִים, אֲפִלּוּ חֲבִילֵי קָנִים, כְּדֵי שֶׁיִּתְפְּחוּ הַמַּיִם, וְיוֹרֵד וְטוֹבֵל. מַחַט שֶׁהִיא נְתוּנָה עַל מַעֲלוֹת הַמְּעָרָה, הָיָה מוֹלִיךְ וּמֵבִיא בַמַּיִם, כֵּיוָן שֶׁעָבַר עָלֶיהָ הַגַּל, טְהוֹרָה:
If one immerses a bed in it [in a <i>mikveh</i> containing precisely forty <i>se'ah</i>], even if its legs sink into thick mud [at the bottom of the <i>mikveh</i>, which is not counted as part of its waters] it is pure, because the waters precede it. If the waters of a <i>mikveh</i> are shallow [such that one cannot immerse one's whole body at once], one may weigh down even bundles of wood or bundles of reeds [with weights, on one side of the <i>mikveh</i>,] so that the waters will rise, and then he can descend and immerse. Regarding a [impure] needle on the steps of a cavern [which houses a <i>mikveh</i>], if one was moving the waters back and forth [by stirring the waters around with one's hands or feet], once a wave passes over it [the needle], it is pure.