Rambam - 1 Chapter a Day

There are six categories of mikveot, one superior to another. The first is the water of pit, cisterns, storage trenches, storage caverns, and the like, where water is collected on the earth. Even though the water was "drawn" and even though there were less than 40 se'ah, since the water only contracts impurity through willful activity, as we explained, the prevailing assumption is that it is pure. It is fit to use such water to make dough from which challah must be separated or to wash one's hands, provided he washes his hands with a container, as we explained.

On a superior level is rainwater that has not ceased flowing, i.e., the rain is still descending and the mountains are still gushing with water and that water flows down and collects on the ground. It is not "drawn;" however, there are not 40 se'ah. Such water is acceptable for terumah, for the washing of hands, and for the immersion of water that became impure.

If the rains ceased descending, but the water was still flowing from the mountains, such water is still considered in this category. If water is no longer flowing from mountains and they are not gushing, it is considered as water collected in a pit.

When one digs at the side of the sea, a river, or a swamp, and water from these bodies flows into the pit that was dug, the water is considered like water that has not ceased flowing.

When one digs at the side of a spring, as long as the water emerges because of the spring, even though at times, its flow is interrupted, but then it flows again, it is considered as a spring. If it ceased flowing entirely, it is considered as water collected in a pit.

On a superior level is a mikveh that contains 40 se'ah of water that was not drawn. Every person can immerse in such a mikveh to regain purity except a male zav. All impure keilim and hands that must be immersed to touch sacrificial foods are immersed in such a mikveh, as we explained.

On a superior level is a natural spring whose water is minimal and drawn water was added to it. The laws governing it resemble those governing a mikveh in that it does not impart purity through immersion in the water that flows from it, only in water that is collected and stationary in a reservoir. And they resemble those governing a natural spring in that even the smallest amount of its water imparts purity. For there is no minimum measure required for the water of a natural spring. Even the slightest amount can impart purity.

On a superior level is a natural spring in which drawn water was not mixed, but its water was spoiled; it was bitter or salty. It imparts purity when its water is flowing, i.e., the water emerges and streams forth from the spring.

On a superior level is a natural spring whose water is "living water." In such springs alone may male zavim immerse and from them alone water may be taken for the purification of a person afflicted with tzara'at and for the sanctification of the water used for the ashes of the red heifer.

What are the differences between the laws governing a natural spring and a mikveh? A mikveh does not impart purity unless it contains 40 se'ah of water, while even the smallest amount of water from a natural spring imparts purity. And the water of a mikveh imparts purity only if it is collected in a reservoir. Any water flowing out from it does not impart purity. The water of a natural spring, by contrast, imparts purity when flowing. Also, immersion in a mikveh is not effective for zavim, while a zav may immerse in a natural spring of "living water."

When the waters of a spring flow outward and into a receptacle in a stone and afterwards emerge from the receptacle and flow further, all of the water in the receptacle and all that flowed out of it is unacceptable for immersion. If some of the water from the spring - even the slightest amount - was flowing over the edge of the receptacle, the water outside the receptacle is acceptable for immersion. The rationale is that a natural spring purifies water as long as it shares even the slightest amount of contact with it.

When water from a natural spring was flowing into a pool that was filled with water and collects there, that pool is considered as a mikveh. If the water flows outside the pool, that water is not acceptable for the purification of zavim and those afflicted with tzara'at or for the sanctification of the water for the ashes of the red heifer until one is certain that all of the collected water that was originally in the pool has flowed out.

When the waters of a spring flow over articles that do not have a receptacle, e.g., a table, a bench, or the like, the water is considered as a mikveh, provided one does not immerse above the articles themselves.

When there was a natural spring with small irrigation ditches flowing out from it and one poured water into the spring so that the water in the ditches would increase and flow powerfully, the water is considered as a spring in all regards.

If the water of the spring was stationary and not flowing and one added water until it flowed into the irrigation ditches, the water that flows into those ditches is comparable to a mikveh in that it imparts purity only when collected in a reservoir and it is comparable to a spring in that even the slightest amount imparts purity.

All of the seas impart purity despite the fact that their water is flowing, but they are invalid for the purification of zavim and persons afflicted with tzara'at and for the sanctification of the water used for the ashes of the red heifer.

Water that flows from a natural spring is considered as an extension of the spring in all contexts. Water that drips from a spring, even though the dripping is continuous, is considered only as a mikveh and does not impart purity unless there are 40 se'ah standing in one place. Such water is not acceptable for the purification of zavim and those afflicted with tzara'at or for the sanctification of the water for the ashes of the red heifer.

If the water flowing from the spring becomes mixed with water dripping from it, it is considered like a stream in all respects. If, however, the amount of the water of the stream that was dripping was greater than that which was flowing, or if an amount of rainwater greater than the water in a river flowed into it, the water does not impart purity when flowing, only when in a reservoir. Therefore one must use a mat or the like to surround the portion of the river where the water is mixed until the water will be gathered in a single place so that one can immerse in it.

When one caused water that was dripping to flow, e.g., one placed a smooth earthenware tablet next to a mikveh whose water was dripping and thus the water was flowing as it descended down the tablet, it is acceptable. Any article that is susceptible to impurity, even by Rabbinic decree, should not be used as a conduit for water.

When one uses the external green shell of a nut to change the direction of the flow of water the water is acceptable as water from a spring, as it was before. It is not disqualified, because a fresh nut shell which imparts color is not considered as a container.

When rainwater is cascading down a hill, flowing as it descends, even though there are more than 40 se'ah of water from the beginning of the torrent to its end, it is not acceptable for immersion while it is flowing. It must be collected in a reservoir containing 40 se'ah. If one surrounded the water with containers, having them serve as partitions, i.e., walls for a body of water to collect, and 40 se'ah of rainwater flowed into it and collected there, it is acceptable for immersion. The containers with which one surrounded the water are not considered to have been immersed.

When a wave becomes separated from the sea and falls on a person or on keilim, they are pure for ordinary purposes, provided the wave contains 40 se'ah, for a person who immerses does not have to have a specific intent, as we explained in Hilchot Sha'ar Avot HaTum'ah. If the person focused his intent on purifying himself for a specific level of purity, and was waiting in anticipation of the wave falling upon him, he is considered to have purified himself for the activity for which he desired to purify himself.

One may not immerse in a wave when it is in the air before it falls on the earth even if it contains 40 se'ah of water. The rationale is that just as we do not immerse in water that is flowing, how much more so do we not immerse in water that is in the air. When the two ends of a wave are touching the earth, we can immerse in it. We do not, however, immerse in its crest, because that water is in the air.