Make an object reflecting on real water footage in AE CC 2017

This tutorial is for an older version of AE, but probably covers what you're looking for:

VIDEO COPILOT | After Effects Tutorials, Plug-ins and Stock Footage for Post Production Professionals

Layers, blend modes and possibly displacement mapping.

Duplicate the 3D layer you want to reflect in the river and rotate it and position it so that the perspective looks right and the tracking is perfect.

Change the blend mode to Multiply, Overlay, or Screen to see which one looks right.

Add a little blur.

Try adding Displacement Map using the footage as the gradient source to improve the composite. This will require some pre-composing and a bit of fiddling.

If Displacement Map does not work the same pre-composing and fiddling can be tried using Caustics.

If I saw a screenshot of your composition and the video I could explain how I would do it. It would look something like this:

I added a light to simulate the light in the scene on the reflection. This should give you the idea anyway. The hardest part is going to be accurately positioning and scaling the reflected graph layer so that it tracks with the camera.