A second order stream forms part of the classification of stream order relating to drainage basins.
The area which drains water into a river’s mouth or estuary is known as a drainage basin. Stream order relates to the pattern made by streams and rivers within a drainage basin.
The most noted studies of this phenomenon were made by R.E. Horton and A.N. Strahler in the mid 20th century. Strahler was Professor of Geoscience at Columbia University, and Horton had previously done considerable work on the water cycle and the nature of drainage patterns, particularly in New York state.
If seen from the air, or drawn on a map, the stream pattern in a drainage basin resembles the branches of a tree. It is therefore known as dendritic. Not all stream patterns appear to be dendritic, as they can be influenced by outcrops of rock, hills and valleys as well as other features of geomorphology. However the classification of stream order is still applicable.
The line which marks the outer limit of a drainage basin, separating it from a neighbouring basin, is known as the watershed. This is best understood in relation to a ridge of highland which will shed rainwater either to one side or the other. The ridge line along the top of the highland thus becomes the watershed.
According to Horton and Strahler’s classification, any stream which forms the outer point of a branch in the drainage pattern, coming closest to the watershed, as it were like the outermost twig on a branch, is called a First Order stream. When two First Order streams meet and converge, they become a Second Order stream. A Second stream will therefore be larger in volume of water than a First Order stream.
Further down the drainage system, the confluence of two Second Order streams will produce a Third Order stream, and so on down to the river’s mouth which, following the dendritic analogy, is like the base of the main trunk of a tree.
Scientists studying the efficiency of drainage within a drainage basin will calculate and use the bifurcation ratio. This is the ratio between the number of streams in one order of magnitude with that of the next order of magnitude, e.g. the ratio of First Order streams to Second Order streams. If, for example, there are 120 First Order streams in a drainage basin, and 40 Second Order streams, the bifurcation ratio will be 120:40, in other words 3 to 1.
The term bifurcation is something of a misnomer in this context, as it relates to a process of conjunction rather than division.
A Second Order stream is therefore the second in size of the classified order of streams in a drainage basin, according to the classification of Horton and Strahler.