Question:
The following inquiry by a soldier to his chaplain was transmitted to us. His wife is moving near the camp. There are no Jewish communities in the neighborhood with adequate mikvah facilities.
Answer:
In the absence of a mikvah it is quite permissible to use a river, provided it is not the kind of river which dries up entirely in the summer because it is fed entirely by rain water. (See Yore Deah 201, #2, the note of Isserles.)
Also, the ocean or any lake fed by streams (see m. Mikvoth V,4. Also, commentary of Tosfos Yomtov to m. Parah VIII 8. Also, Sefer Ha-Eshkol 50, #8, the teshuva of Natronai).
In all these cases there must be enough water at the bathing for complete immersion.
To which Dr. Jung adds: The bathing suit worn by the woman must be wide enough to allow the water unimpeded to reach every part of the skin. From every angle visit to a mikvah in a neighboring town is preferable. Also some other woman, or husband in the distance if immersion is covered by sheet, must be present to witness completeness of immersion.