The Strahler Stream Order is a simple hydrology algorithm used to define stream size based on a hierarchy of its tributaries.

The streams range from one at the headwaters (which is a "1") to the most powerful which is the Amazon River which is a "12." The Ohio River is an "8" and the Mississippi River is a "10." 80 percent of the streams and rivers on the planet are first or second order.

To qualify as a stream it must be perennial. When two first-order streams come together, they form a second-order stream. When two second-order streams come together, they form a third-order stream. Streams of lower order joining a higher order stream do not change the order of the higher stream. Thus, if a first-order stream joins a second-order stream, it remains a second-order stream. It is not until a second-order stream combines with another second-order stream that it becomes a third-order stream.

Alan Needle Strahler first proposed the heirarchy in 1952 in an article “Dynamic basis of geomorphology,” in the Geological Society of America Bulletin. It is often referenced in professional descriptions of rivers as Strahler 1952.

External link

• CET article