Natural Bodies of Water
A woman in a state of niddah returns to a state of taharah (ritual purity) only after immersing in an halachically acceptable body of water. A natural spring (ma'ayan) or a natural pool may be acceptable, but as the laws governing them are quite complex, it is best to immerse in a recognized and well-maintained man-made mikveh. If no official mikveh is available, however, one may immerse in another body of water if it conforms to halachic requirements. While an ocean would normally be acceptable, a river may or may not be. Halachic consultation is important.