I’m not sure the best way, but I think Sculpting or Displacement is the way to go. If you have an image of the rivers then Displacement is probably the best route. You can also paint the displacement which is a little work to setup for eevee users. Sculpting is probably the most common way to do it in blender. In the future, when you create terrain Always map out water first. Like lakes and rivers. It is easier to build land around these using face sets(Sculpting) or vertex groups(Everywhere).
