A second order stream is a term are used primarily in physical geography and hydrology. It describes a watercourse which is fed by two other streams. Second order streams are typically shallow, fast flowing and found in upland areas.
Arthur Newell Strahler introduced the idea of stream order in 1952. At the time Srahler was a professor of geosciences at Columbia University. He reasoned that streams could be classified by the number of tributaries that fed into them.
Strahler defined stream order, which is sometimes called the Strahler number, in the following manner. A stream flowing from its source to first confluence is of first order. The confluence of two first order streams gives rise to a second order stream. The confluence of second order stream gives rise to a third order stream and so on. The stream order does not increase when a stream merges with a lower order stream. The stream order only rises by one when the stream merges with an equal order stream. The method is only applied to perennial watercourses that carry water throughout the year.
Using this notation Strahler found that he had an unambiguous way in which to classify rivers. His description is particularly useful for identifying the erosive properties of waterways. Streams of order 1 to 3 tend to be fast flowing through upland areas and are scouring. Typical features being “V” shaped valleys, rapids, waterfalls and scour pools. Streams that have an order of seven or more are defined as rivers. These transfer sediment downstream and are typified by slow moving meandering watercourses that traverse “U” shaped valleys. High order rivers are typified by large meandering flood plains. Among very large rivers in the lower reaches the Ohio is an eight order river, the Mississippi a tenth order river and the Amazon a twelfth order river. Worldwide nearly 80% of all classified stream segments fall into the headwater 1 to 3 category.
Bio-geographers have also found that the “order” system of classification gives an indication of the number of type of life forms that can be found in a river. Their research is based upon the River Continuum Hypothesis. The River Continuum Hypothesis divides a river into three zones similar to those used by the physical geographers. In the headwaters, which have stream orders from 1 to 3, the stream is usually very narrow, lined with thick vegetation. The steep sides prohibit photosynthesis and organic materials fall into the stream. In their natural state the headwater areas are often forested. Headwater streams are very important to river ecology because they transport nutrients and pollutants downstream into the river basin.
In short, a seocnd order stream is a part if a draingage systeml lying close to the source. Second order streams typically flow in deep gullies through upland areas. Their banks are often forested and the streams are often full of organic material such as peat and twigs.
Further reading
Macroinvertebrate diversity in headwater streams: a review
