Looking good so far.

One method I discovered that helped me make some nice rivers was, if you're drawing them roughly by hand with a brush, set the jitter on the brush to a moderate level--not so the brush strokes completely scatter on you, but enough to make the line you draw look a little rough and jaggedy. Then, let your stroke meander around in a rivery way. You'll also need a way to vary the width, having it get wider toward the mouth, but it looks like you've already got that part figured out.