Strahler Stream Order

Post date: Jan 4, 2011 11:07:14 PM

Originally posted January 13, 2010

A new layer has been added to the Riverwatch Map which depicts stream order. Stream order is a useful tool in watershed management because it provides a means of definition between individual tributaries based on their size and strength of flow. On our map, the colours correspond to stream order as follows:

Stream order hierarchy was proposed by a geosciences professor named Arthur Newell Strahler in 1952. First through third order streams are considered to be "headwater streams", and make up approximately 80% of the worlds waterways. Fourth through sixth order streams are considered to be medium tributaries, and anything with an order of seven through twelve is considered to be a river. You will notice that the only waterway in our watershed that falls in the "river" category is the Bonnechere River, which is a seventh order stream.

So how do we define stream order? The principle is actually quite simple, and is best described with the help of the following diagram.

As you can see, a stream that has no other streams flowing into it is considered a first order stream. Stream order numbers increase only when two streams of the same order meet, for example when two first order streams meet, the output is a second order stream. If a stream with a lower number meets one with a higher number, the following segment retains the higher number (eg. A second order stream meets a third order stream, the output is a third order stream). The process of determining stream order is simple enough that it can often be executed by hand, however with larger more complicated catchments or watersheds, mapping software applications can be rather helpful.

Data comparisons made between waterways of the same order has proven to be the most accurate and scientifically correct method when examining overall stream or watershed health. This is because creeks with the same Strahler number often hold similar characteristics such as stream width, flow, amount of sediment, and plant and animal life present. Making accurate comparisons is crucial in understanding the health of our watershed.