A method of classifying the relationship between segments of flowing water bodies.

Stream order, also known as a Strahler or Horton–Strahler number, is a method of classifying the relationship between segments of flowing water bodies such as streams and rivers. A first order segment has no other segments flowing into it. When two first orders come together they form a second order. Two second orders become a third order, and so on. A lower order segment joining a higher order does not change the higher order classification - a one flowing into a two does not create a three, it's still a two.

This is actually a mathematical concept that can be used to describe any branching network.

See also the Wikipedia entry for Strahler number, which includes graphic examples.