Mikvaos 7:7-8:1

Mikvaos 7:7

If one immersed a bed in a mikvah, it is purified even if the legs of the bed sink into the thick mud at the bottom because the water already touched them. If the water of a mikvah is spread out (and therefore shallow), one may press bundles of sticks or reeds down (i.e., on one side) so that the water level will rise, after which one may immerse. If a ritually unclean needle is placed on one of the steps leading to a mikvah in a cave and someone sets the water in motion, the needle is purified when a wave passes over it.

Mikvaos 8:1

The land of Israel is ritually clean and its mikvahs are ritually clean. The mikvahs of other nations are valid for those who had a seminal emission even if they were filled using a well pump. Mikvahs in Israel that are outside the city gates are fit even for a niddah, while those inside the gates are fit for those who had a seminal emission but unfit for those who are otherwise unclean. Rabbi Eliezer says that mikvahs that are close to a city or a road are unfit because of laundering (which causes drawn water to be poured into them) but those that are far away are ritually clean.