Strahler number is a term used in the field of hydrology to describe a stream network. It is the highest order of streams in a drainage basin. The Strahler number is calculated by determining the order of each stream in the network and then assigning a value to it. Synonyms for the term Strahler number include Horton-Strahler number and river order, both resembling the use of a numerical system to organize stream channels. Other closely related synonyms for Strahler number include Stream Order, Hierarchical level of Stream, and Stream Hierarchy. Regardless of the term used, Strahler number is an essential piece of information used to describe the drainage basin and water flow.