Watersheds and milksheds are best classified as functional regions.

They are defined by what they do rather than by uniform culture or formal political boundaries:

  • A watershed is an area where all water drains to a common outlet, forming a single drainage system.
  • A milkshed is the area from which milk can be produced and delivered fresh to a particular market before spoiling.

Because each is organized around a specific function (water flow to one outlet, milk supply to a market), they fit the geographic concept of a functional region.