Do you mean HRTF?  If you mean HRTF, then speaking as the resident HRTF expert, it kind of can't be done.  You can do a good enough job that sighted people will be satisfied when combining it with sight and find it realistic, but no one has yet shown me a volumetric demo where I go "wow, that actually sounds like a river and it's totally right there", the best you get is "it's big and vaguely to the left".  overall it's interesting to talk to sighted people about this because they combine it with graphics without even knowing, then start in on "but your HRTF library isn't good" and it's like "if you closed your eyes when looking at the Oculus demo..." but that's beside the point.  There's a reason that everyone does a Swamp-style radar and, though I've tossed a lot of ideas around about this, I don't think it's possible to do better.

There are ways to fake it. Onslaught has audio walls you can sort of fly along. I have an invisible square of lines around the field, and raycast out from the player in each cardinal direction, positioning a sound source wherever the ray intersects the square. That was for actual navigation, so in that sense it made sense to have 4 rays. For sound-emitting shapes, you could probably do some sort of nearest point calculation between the player and the geometry, then position the sound source for the object at that point. That would get you a river that emits sound directionally and loudly, but that dropped off and changed direction as you moved away from its exact shape.

Another way to fake it would be positioning sound sources every few units along the shape of the object, and tuning their parameters so they drop off somewhat quickly as you move away. You have to manage your sound resources a bit more carefully and not just spawn a thousand sounds for your river (unless your library can handle that, of course.) If you think about it, though, a river is rarely a uniform wall of sound, and this lets you play with tuning your audio environment. Your river might be one loud waterfall that can be heard from further away, and bunches of quieter sections of water on rocks, maybe a few rapids, another smaller waterfall, water running under a bridge, etc. You can make an ocean shore sound much nicer if you, say, place your basic crashing waves sound every few units, and stagger their start times such that the samples start at different times and you actually hear multiple waves crashing at once. I was amazed at how this transformed a basic "crashing waves" sample from something that sounded a bit uninspired, to a beach with actual waves that I could hear crashing in front of me, then shifted to my right as I walked along.

Hope that helps. I think I posted something similar a few years back and got answers similar to my first suggestion about faking it, so that may be the state of the art.