Thanks for the replies.
Max: Are my rivers having problems concerning their end or their origin? I tried to have most of them spring from mountains...though I'm not sure if I was successful even in that regard.

Waldronate:Thanks for the tips! As I said in the OP, I read 'How to Get Your Rivers in the Right Place', but it didn't help me much. Partly because I have no idea how to make a terrain height map like he did. I lack artistic ability or even a basic grasp of GIMP or PS skills...would you have any idea how to reproduce his effects in Paint.net or GIMP?

Thanks again to both of you!