This is looking really good, but I have to jump on the river bandwagon. If you put some tapers onto them you'd have a really fantastic map here. Also, it does look like that's a delta, but if so, you may want to think about fanning it out some more, since deltas usually don't just split into 2 forks. Those 2 forks could be the boundaries of your delta, which would then split into a ton of little rivulets (wikipedia's alluvial fan page has a great example, even though a delta and an alluvial fan aren't really the same thing).

Other than that, I really like the style you've got going here.