[GIMP] How to add an outfading outline for contrast to rivers and texts?

[KathaBlanca]

Hi guys,

my question is simple: I created a oldschool fashioned map. Here are my layers:

Outermost Border
Text (Captation, City Names e.g.)
Rivers
Overworld (Mountains, Forests e.g.)
Texture (Paper-sytle)
Landmass (dark tin color)
monochromatic Background (light tin)

A want to draw some black lines in the river Layer, that represent, as the name says, rivers.

My Problem is: The black lines look ugly on top of the Overworld-Layer (see map for clearness). I tried to select the line, expand the border 3 or 4 px and delete the structures on the Overworld layer, but it looks bad as well.

Is it possible, to add a border around the river-Line, which has the job to "fade out" the Overworld Layer? I want the river and later the captions to be in contrast to the respective background.

Thanks in advance!