Powell's watershed map lays out administration zones based on the major flows of water. http://www.npr.org/programs/atc/features/2003/aug/water/part1.html http

Powell's watershed map lays out administration zones based on the major flows of water. http://www.npr.org/programs/atc/features/2003/aug/water/part1.html http://www.npr.org/programs/atc/features/2003/aug/water/part... It gets my vote, watersheds are the strongest constant we have next to continents. Technology, language, rivers, and coastlines all shift.