Hi, I want to tell you about my pain.
The reason is: there is no rivertool in UE4.
So to make a river you should: 1) set spline, 2) add physicvolume and match it with spline, 3) add postprocessvolume and match it with spline, 4) make material to see waves. And I`m not sure could be a stream placed at all.

On the contrary, what we have in another engines?
In CE3 you dont need to waste your time for placing two volumes, you dont need waste your time for transforming terrain. it is done by pressing several buttons after you have placed spline. And all options for settings are in your hands, f.e. stream settings, foam settings.

In Torque 3d possiblities are very close, except manual transforming terrain. BUT you can move every knot of spline in 3 dimensions and river material will follow it. For my level it is extremely essential.

That I`m talking about. I would have in UE4 rivertool like in CE3 but with knot following river materal as in Torque 3d.