Classifying Stream Orders
Stream order is a method for classifying the relative location of a Reach within the larger river system.
This system shown here is the Strahler (1957) modified Horton (1945) method for Stream order, which assigns each headwater perennial Stream an order of 1, and then at the confluence of two 1st-order Streams assigns the downstream Reach an order of 2. In this method, the confluence of two 2nd-order Streams results in a downstream Reach of order 3, and so on, as illustrated by this picture.
Image courtesy of FISRWG