If you are in Gimp I would "select" the rivers one by one holding down the shift key so they are all highlighted (Unless you already have the rivers on separate layer). Create a new layer called rivers. The shape of the rivers should still be selected so now you just want to put some color in them. What I do for a quick river is 'Filters/Render/Clouds/Plasma' (go for as even a distribution as you can get) then use 'Colors/Colorize' and change it to whatever blueish color you like.

You may then have to fiddle with the layers mode to use 'overlay', 'merge', 'multiply' whatever.

You could skip the rendering of the plasma but that would mean you just drag blue to the selected rivers. It wouldn't have much character.

edit: I'm doing this from memory so hopefully I didn't skip anything important. Sorry if I loose you there but I'm not even sure if you are using Gimp. Oh, and don't feel bad if you can't follow a tutorial I have yet to be able to finish one either.