Gimp: How To Make Most Of An Image Transparent, Except One Color?

I have an image showing the courses of some rivers. I would like to paste just the rivers onto another image. How can I extract the blue color? Or just make everything else transparent? The blue runs all through the image, so I can't just select one section. The answer may be right in front of me, and I just don't know the right terms. Very new at this. Currently the file is a gif, but I can convert it.
Or maybe there is another program that would be better at this?

Thanks!

 

See if this helps.
https://www.techwalla.com/articles/how-to-remove-all-the-colors-except-one-in-gimp