Stream ordering (Strahler Method) is a method of measuring the relative size of rivers based on their number of tributaries.
When a stream combines with another stream of the same order, a higher order stream would be formed. Referring to the diagram below, the headwater stream (a stream with no tributaries) is classified as a first-order stream. When two first-order streams intersect, a second-order stream would be formed. When two second-order streams meet, they form a third-order stream, and so on.
Stream ordering is useful for determining the relative size of rivers. For example, the largest river in the world, the Amazon River in South America, is classified as a twelfth-order (12th) river (Wheeling Jesuit University/NASA-supported Classroom of the Future, 2004). Locally, Shan Pui River, one of the largest rivers in Hong Kong, is classified as a fifth-order river.