No matter what you are going to need quite a bit of post work to make that look good.
IMO filming real assets is always better than faking them in AE.
As far as faking it (or for compositing the filmed drips), I would look into the Caustics effect. It won’t simulate the drips, but it will add a realistic water refraction effect that will help it transition into the river. Then again, it depends on how the river is animated, and what kind of look you’re going for.
If all you are looking for is the shape of the drips, then you could make a matte using Mr. Mercury, but it’s a pretty lame effect with minimal controls.
Also, if I’m following you right, you could film the drip transition in high contrast, use it as a matte, and composite it with the matte you’re currently using. That way you won’t have to repaint the last frame, or worry about it matching.
Good luck.