Mikvaot, Chapter Seven, Mishnah Seven
1) If he immersed a bed in it, even though its legs sink into the thick mud, it still becomes clean because the water touched them before [the mud].
2) The water of a mikveh which is shallow, one may press down even bundles of sticks, even bundles of reeds, so that the level of the water may rise, and then go down and immerse oneself.
3) If an [unclean] needle is placed on the step [leading down to a mikveh] in a cavern, and the water is put in motion, once a wave has passed over it, [the needle] becomes clean.
Section one: Since the legs of the bed are immersed in the mikveh’s waters before they sink into the mud at the bottom of the mikveh, they are clean. Note that this mikveh has at least forty seahs of water. Furthermore, immersing only in water thick with mud would not suffice to make something clean (see mishnah one of this chapter).
Section two: This mikveh is shallow but still has exactly forty seahs of water. The problem is that the water is too shallow to immerse his whole body. He may take bundles of wood or reeds and place them on the sides of the mikveh to create a narrower deeper space. He is allowed to do this even though water will creep into the sides of the mikveh in between the spaces within the bundles of the wood or reeds and thereby reduce the volume of the other water in the mikveh. The mishnah teaches that the water in these spaces counts in constituting the requisite forty seahs for a mikveh.
Section three: The wave of water that reaches up from the mikveh and immerses the unclean needle is sufficient to purify the needle.