The Upper Missouri River watershed (for me, defined as the portion of the river from its headwaters to its confluence with the Big Sioux). This portion of the river was a convenient shape to fit into my atlas and onto a poster. I find the drainage patterns fascinating: zones of north-south alongside zones of east-west.
In making this, I learned how many towns in the West are not on rivers, but on railroads instead. By the time much of this area was settled, the railroad was a better option for transportation. I probably knew this already, but I never really thought about it until I realized how many places I was leaving off the map because they weren’t on the major rivers in the area.
In making this map I also caught an error in the National Atlas, which is one of my main data providers in the US government — they had the Marias River marked as the Marais River. I’ve been relying on them a lot up to this point, so hopefully they don’t have too many other errors that I’ve propagated.
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