You could use the "Paths tool" to create the line of your rivers and then stroke the path. There is some stuff on doing rivers in the tutorials as well so you might take a look there.
This looks pretty good but you can't really zoom in on it. I think that's a resolution thing but someone more experienced than I can say better.
Looks to me like you might have a couple river violations - expect a few pointers from the River Police Alternatively, there are some great post on this subject somewhere herein. (I believe the tutorials section contains them).
All in all it's a great first. Starting out with a world map for your first is a big undertaking.
I like the coloring as well.