what am I even looking at here? It looks like a world-gen map with a custom shape landmass? I didn't know you could even do that.
Yeah, it's a custom worldgen. It's...
I beleive that is world smith. 
Never used advanced world gen without going , toooo many numbers hbgffgbi.
I'm not sure what world smith is. Anyway, it's...
Its perfect world.
No, not that either. Actually, it's the resurrection of an old project of mine. I first posted it a couple of years ago,
here. I lay out the world with a text file, then turn that into elevations and so forth with a python parser. In this case, I'm also using a python script I wrote to do the geometry of the masterwork symbol.
But it's not giving me enough rivers!
I just crank up rainfall as high as it'll go and set start and end rivers to 800. It gives me a ton of waterways to work with.
In the picture above, RIVERS_MIN was set to 800, I think. However, I did disable periodic erosion, because it'll mess up my patterns of biomes that I use to generate very unusual embark sites. Without it, you wouldn't have the opportunity for a single embark to have mountain, forest, plains, desert, and glacier, and I enjoy embarks like that.
I don't think it's a factor, but I use a rainfall of 100 for the forests. But I still see very little in the way of rivers.
I think in previous versions of Dwarf Fortress I just set the RIVERS_MIN parameter high and it worked straightaway. It just seems like maybe that's changed, and now there's more to it. I'd like to have an embark with a minor river, at least; but they're few and far between.