If your intention is realistic and not ultra-saving on resources I think that you conceived the fundamentals wrongly.
It’s about natural-sounding environments after all, right?
Your initial solutions, the one you posted in your thread start, is correct.
You should create, as you did, a point that travels the river line and follows the player, and set up the emitter’s parameters to create the natural distance that the river flow’s power would transmit the sound.
Then, when the listener intercepts another part of the river or any other river, create another instance of the river sound emitter.
This is what would happen in real-life and it can be localized correctly by the player too.
So, in your first drawing, the player would hear both river sounds (probably have 1-3 different loops and change playback starting points to avoid phasing), and if you set things up correctly you will have a far more immersive experience because as the listener moves towards the second point the first point’s emitter will gradually fade-out. It goes without saying that you should use attenuation over distance for this design.
If you need to control voices for optimization, you can add some control to your algorithm. Separate the circle around your listener to four quarters and once per 3 seconds check if there are more than four emitters playing from various parts of the river because the listener has entered their trigger zone, see if the emitters are coming one from each quarter and if yes don’t spawn another instance until this state goes away.
Another optimization would be to check if the new emitter instance that is going to get triggered is within 15 degrees of another emitter instance already playing and if yes, don’t spawn it.
The 3 seconds time and 15 degrees values that I state above can be changed according to your game’s needs. The time depends on the speed the listener moves through the world and the degrees to the need for accuracy (i.e. FPS vs adventure games).
A nice trick that you can add to this system is that you can get the width, depth, and particle flow speed of the river, each time you spawn a new emitter, and control the emitter’s range, volume, and mix 3 different loops in real-time to simulate the river’s strength and flow. That way, when the listener is between one part of the river that has a weak stream and another part of the (same or other) river that has stronger flow, the experience will be accurate and natural.
How’s that sound?