I'm trying to understand how to identify a stream as a mikveh. Clearly the stream has to flow. But what sources of the stream are permitted: drainage basin (a.k.a watershed), aquifer, and others?
The source of a Mikvah of flowing water must be a spring. Rainwater is only kosher for a mikvah when it is standing still. (The halacha discusses a mixture of the two).
Therefore, if the source of the flowing water is only rain or melted snow, it would not be good for a mikvah. It it was a mixture of spring water and rain/melted snow, it would only be acceptable in extreme circumstances, as detailed in that Halacha.