What constitutes a kosher mikvah? The word ‘mikvah’ means ‘gathering of
waters’. We first encounter the term in the Torah on the third day of creation,
when G-d creates the land and sea. “Let the waters be gathered together
(yikavu hamayim) … and the gathering of waters (mikveh
hamayim) He called seas.” (Gen.1:9-10) According to Rashi**, this
primordial mikvah was the Mediterranean Sea.
The waters of a mikvah must gather together naturally. One may not just use tap water. They must come either from an underground spring or from rainwater; which may then be joined with tap water. If spring water is used, then the water can be flowing. However if the source is rainwater, then the water must be stationary. The ocean, halachically, is considered a spring. Thus, even though the water is flowing, it can still be considered a kosher mikvah. Rivers and lakes are more complicated, because the source could be either rain or spring water. If the river dries up during a drought, then its source is rainwater. Since a mikvah which comes from rainwater must not be flowing, therefore such a river would not be kosher. A river which does not dry up could probably be used as a mikvah. If a woman is actually in a situation where she needs to use the ocean or a river as a mikvah, a competent Rabbi must be consulted. It is always preferable to use a kosher man-made mikvah.
A mikvah must contain a minimum of forty se’ah (around 750 liter or 198 gallons) of water. Our Sages have determined that this is the minimum amount needed in order to immerse comfortably. The forty se’ah corresponds to the forty days and nights that the earth was covered in the waters of the Great Flood. Just as the earth needed to be totally submerged for forty days in order to become cleansed and to begin anew, so too a woman immerses herself in forty se’ah of water. Forty se’ah is the minimum requirement. The height of the water is usually about 1.2 m. (4 ft.) above floor level – not too shallow and not too deep.
**Rashi is the acronym for Rabbi Shlomo Yitzchaki, French scholar of 11th century France and primary exponent of the Talmud, the core of Judaism’s vast intellectual heritage.
Reference: S.Z.Lesches, Understanding Mikvah : An Overview of Mikvah