I think your map is very interesting and looks nice. It has a very distinct style. Rather than lighten rivers perhaps the real issue is the choice of where to use black lines and where to make a coastline with blank water like your seas. I think what may be troubling to some is how the rivers get large at their mouths and why aren't they coasts or fjords at that point instead of black river lines. The same goes for the lakes, why black? Just a thought. I had a similar issue with the rivers in my Linlandia map. I re did them with a smaller ink nib size in GIMP and it worked out reasonably well. Just some thoughts.