You could try using two seperate heightfields instead of the single terrain PFS, then you'd be able to reverse the displacement settings on the right hand side terrain to match the 'bubbled' look of the left... ?
As it is just now, the right bank is being stretched away from the camera in the same direction as the left, not like a river's cut. You could probably still keep the left hand bank by using 'create heightfield from shader', and selecting that portion of terrain, I think.
