Mikvaos, Chapter 7, Mishnah 7. This mishnah deals with a mikveh that holds exactly forty se'ah, not a drop more, and asks what a person is to do when the water, as it lies, will not manage to cover the object he needs to immerse. Three separate cases are presented, and each one teaches us something about what counts as a valid tevilah.
The bed whose legs sink into the mud
The first case: "Hitbil bo es hamitah", a person immersed a bed in such a mikveh. The bed had become tamei and now needs tevilah, but it does not fit into the water; the mikveh is simply not deep enough for it. The floor of this mikveh, however, is not solid rock but a bed of thick mud. So the man presses the bed downward, and as he presses, the legs of the bed bore their way into the muddy bottom until the whole bed is under the surface.
And what is the ruling? The mishnah declares the bed tahor: the tevilah holds, despite the fact that its legs finished up embedded in the packed sludge, "shok'os batit he'aveh", rather than surrounded by water. One would have assumed the opposite, since mud is not a mikveh, and whatever is lodged inside it can hardly be called immersed. The mishnah's explanation is contained in three words, "mipnei shehamayim mekadmin": the water precedes the leg. Driving a leg into the soft bottom hollows out a channel, and water pours into that channel before the leg itself arrives. Each leg therefore descends into a pocket already full of water, so the entire bed has in truth been immersed in the water of the mikveh.
A mikveh whose water is spread thin
The second case: "Mikveh shemeimav merudadin", a mikveh whose water is spread out over a wide area. There is a full forty se'ah here, so the shiur is not lacking, but the water lies shallow rather than deep, and as it stands a person cannot immerse himself in it. What may he do to raise the level?
The mishnah teaches: "Koveish", he may press objects down into the water so that the water is displaced upward and the level rises. "Afilu chavilei eitzim, afilu chavilei kanim", even bundles of wood, even bundles of reeds and twigs may be sunk into the water for this purpose, "kedei sheyispachu hamayim", in order that the water should swell upward, "veyoreid vetovel", and then he goes down and immerses.
It is worth noticing which materials the mishnah selected: timber and reeds, substances that cannot contract tumah in the first place. Pressing down keilim to achieve the same rise in the water would open an altogether different discussion. Were the level lifted by sinking vessels into it, the mikveh's very fitness would be standing upon a davar hamekabel tumah, an item susceptible to tumah, and we would find ourselves facing the familiar issue of havayah, the mikveh's coming into being, being brought about through such an item, which can spoil the whole arrangement.
The needle on the steps of the cave
The mishnah's final case: "Machat shenesenah al ma'alos ham'arah", a needle that is tamei and has been left lying on the stairs that lead down into a cave, at the bottom of which sits a mikveh. Dropping it straight into the water would certainly accomplish the tevilah, but its owner is afraid that the needle will vanish in the mikveh and never be recovered. His alternative, "hayah moleich umeivi bamayim", is to sweep his hand through the water one way and back again, raising a swell that surges up onto the stairs and passes across the needle. The mishnah's verdict: "keivan she'avar aleha hagal, tahor". Once the wave has washed over it, the needle is treated as having undergone a proper tevilah.
This raises an obvious difficulty. It is a halachah we know well that zochlin, water in motion, are unfit for tevilah: the water of a mikveh must be collected and at rest. Here, though, the man deliberately produces a surge, and at the instant the needle is covered, what covers it is water that has climbed out of the pool and is spilling back down the stairs. Why should such an immersion be effective at all?
The Chazon Ish points out that this mishnah serves as a proof for the position of the Rivash in siman 292. The Rivash holds that water which is in motion but is destined to return into the body of the mikveh is not classified as zochlin at all, and it therefore remains fit for tevilah. That is precisely our case: the wave rides up over the steps and then falls back into the mikveh from which it came.
Other answers to this question have been offered as well, among them the distinction between water that a person himself sets in motion and water that runs off on its own. One should look into the mefarshim on the mishnah to see the range of teirutzim that have been given.