We are going to be in a country where there is no mikvah for 3 months. But there are a lot of big and small rivers there. How can I know or check, which river is valid to be used as a mikvah? There is no rabbi there.
First, please double check the Global mikveh directory, on the chance that there is a mikveh available. It is also worthwhile to contact Chabad, accessible through that site, which has reached most corners of the globe and may well have the information you seek.
There are a number of halachic requirements for the use of a "natural" mikveh. For a spring or river itself to meet the requirements for immersion, the water must originate from a spring and not from a collection of rainwater. Similarly, a natural lake that is fed by river water and springs can be used for immersion.
However, if most of the water in a spring, river, or lake is from rainwater, then only stagnant water would be permissible. Thus, the fitness of a spring or lake for immersion may vary seasonally. Pipes through which the water passes might also invalidate the water.
The water must be deep enough (stomach-height) so that the entire body can be immersed at one time, but not so deep that the immersing woman would be afraid.
As you can see, these halachot are sufficiently complex that a woman should not immerse in a "natural" mikveh without first checking its fitness with a rabbi. It is ideal to have a local rabbi who can actually inspect the river. But if that is impossible – as in your case – you can ask about specific bodies of water long-distance, using the phone or email.
Even when the body of water is appropriate, considerations of modesty and safety need to be taken into account. See our question on immersion in springs for more details.
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