Hi IceDealer,
So I have been looking into this a bit and it is definitely engine related. It has to do with the way that the “adaptive tessellation” works in the engine and how it tries to determine where to add polygons to a mesh and when. In general it does a good job of this and it is basically never noticed by the user, but in cases like my grass material which is incredibly varies with each individual grass blade it has trouble with this and causes errors. I have a fix for you, that is not ideal, but it does work.
If you open the “master_terrain_blend” material and off to the left in the properties panel you will scroll down to where you see “Adaptive Tessellation” and turn this feature off. Then just make sure not to set your tessellation density on the black material instances any higher than 1. You will not get as much detail in your heightmap, but it will fix this issue for you as well as the wobbling of the terrain. The reason this is bad though, is this means there will be an even number of polygons across the entire terrain no matter how far from the camera. So the terrain will be fully tessellated at all times, but if you keep Tessellation Density at 1 or below it shouldn’t affect performance for you. I hope this helps and basically there isn’t much I can do about the way the engine handles the adaptive tessellation unfortunately, but this is at least some kind of solution for you 
As for the river foam, try adjusting your “Depth Shoreline” and “Depth Shoreline Falloff” in the material instance “river_flow_b”
You can also try turning on and off the “foam_depth_based_opacity” to see what that does for you. “foam fade” “foam intensity” and “foam contrast” may also help
Hi SaOK,
I have only seen this happen before when I have not built my level yet? Does building your level fix the issue for you? Has lightmaps been disabled on the assets? Also what version of the engine are you running? Thanks