I know it might sound silly, but in my experience with rivers, getting out the ole pencil brush in PhotoShop and reducing the size to a single pixel can yield a good start to a river. I usually magnify the image to about 300% at that point and then just start to draw something of a random line. You can widen it manually as you go if you so desire.

I'm not going to pretend that the rivers are any good, because mine could sure use some work. If I find a more effective method I'll let you know. Maybe Ascension has some better methods. I always struggled with rivers, so you are certainly not alone.

BTW - I love you too ;-)