1,114. (Not) Immersing in a River

162:12 Many great Torah authorities maintained that immersion in a river is not valid except when the river is small enough that we know for sure that it wasn't swollen by rainwater or melting snow. This is because rainwater and melted snow only purify when they are in a mikvah, not when they are flowing on the ground. Spring water does purify also when it's flowing. In a case of great need, in a place where there is no mikvah, the practice is to be lenient and to rely on the authorities who say that even if one saw the river was swollen from rain, nevertheless most of the water is from the river's source so the rainwater is annulled in the river and it purifies while flowing. In a place where there is a mikvah, however, one should never act leniently.  Even in a place without a mikvah, one should try to act stringently. If the river was swollen from rainwater, she can wait two or three days for it to return to its normal state and then immerse. If possible, it's better that she not immerse where the river has become wider, but in the place where it always flows. In such a case we can act a little more leniently.

162:13 Some rivers only exist because of rain and they dry up in between. Even though when it rains other rivers flow into it, nevertheless, because sometimes it stops completely, one may not immerse in it while it's flowing, only when the waters are gathered together.