The black is very likely the water planet/sphere seeping through the skin of the main planet, but without the water shader, as that's masked out.
I think the erosion river setup is best used on one planet, just use the mask to add a water shader to the flattened river.
Or use the erosion shader to carve the rivers (then don't check the flattening), and use the uncarved, slightly lowered terrain on a sphere as water surface. You need to mask the river area then, and maybe clamp/color adjust the mask, or use a opacity mask (default shader) to take out all surface except river.