Let's move on to rivers.
Load "RiversLayer.png" into GIMP.
Create a white layer and merge the Rivers layer with it.
Apply Gaussian Blur with 2px two times.
Now the "background" of the layer is probably a bit darker than "pure white", and we don't want that. Pick the background color with the "color picker", copy the HTML value of the color, and go to Layer->Transparency->Color to Alpha. Click on the color, and enter the HTML value. Now you should again have a layer that only has the rivers in it and nothing else.
Duplicate the layer and merge the two copies.
Create a Drop Shadow with -2/-2/4/Black/80%
Apply the following to the layer with the river:
Filters->Noise->Spread with 3/3
Filters->Artistic->Softglow with 10/0.75/0.85
Copy this layer twice and merge the three copies.
Now you are finished with the rivers. Save the image as "RiversColor.png"
But don't close the image yet! Instead go to File->Open As Layer and import the other Color images you have created. Move the layers around until you have the following order, from top to bottom:
An empty, white layer.
It starts to look interesting, doesn't it? Save the whole thing as "BackgroundColor.png" - it will be useful in future steps.