All animals and plants live within the constraints of their physical environment. But the physical form of the flowing water environment is unique. In the sea, there are, of course, vast distances between one ocean and another and these are typically separated by landmasses. And in terrestrial environments, mountain ranges and large waterbodies like lakes, can separate geographic regions. But if you are a whale, fish, jellyfish, seastar, wildebeast, elk, bear or whatever, moving reasonable distances between one location and another is, generally speaking, pretty straightforward. In streams, things are very, very different. And that is one reason why river scientists have come up with a way of classifying stream channels that tells us in a simple way how large a channel is and something about its location in a drainage basin. Streams* occur within drainage basins or catchments that are somewhat tree-like in structure, composed of increasingly smaller branches as one travels upstream from the mouth (the base of the trunk) to the headwaters (the twigs). Source: http://www.solarviews.com/cap/earth/yemen.htm
And so streams are unique in that if an aquatic animal or plant wants to move from one spot to another, it can only travel either upstream (although this is a trifle hard for a plant that cannot move of its own volition) or downstream along a relatively narrow channel. This means that, as the crow flies, an organism might not be far away from a laterally-adjacent stretch of stream – perhaps only 100’s of metres – but to get there, the animal or plant might have to travel downstream and then upstream again, perhaps 10’s or even 100’s of kilometres. Or, in the case of streams on either side of a mountain range, the opportunity to move between them may be extremely limited, and only be possible by those species that can fly.
Actually, there are many different types of drainage patterns in streams. They include the dendritic one above, but also parallel, trellis, rectangular, angular and contorted, to name a few. These patterns are largely influenced by the geology, climate, history of development and slope of the land. And it doesn’t take much imagination to realise that the drainage pattern will have a major influence on how a riverine organism gets around.
Types of drainage basins. Source: http://w3.salemstate.edu/~lhanson/gls210/gls210_streams3.htm
So, the shape of, and spatial relationships of channels within, drainage basins will influence the movement of aquatic organisms. And movement can have a profound effect on the distribution. I must stress here that I am talking about those organisms that cannot hop out of water and crawl or fly between patches of water, but have to stay wet all the time. Those animals with aquatic larval but terrestrial adult phases, like many insects, just make things really complicated. Anyway, for entirely aquatic animals, movements within rivers can be short-term and small-scale, like when looking for food or a cooler, shady spot to sleep. Or they can be long-term, and large-scale, like when eels migrate vast distances down rivers to the sea to breed and then die.
But it is the branching, network-nature of streams and the fact that river scientists need a common language – stream order – to describe the size and location of a channel in a drainage basin, that has been the motivation to come up with general and easily-understood classification of river channels. Stream order is a central plank in stream ecosystem theories like the River Continuum Concept. Various classification systems have been devised over the last hundred years or so. I briefly describe the main ones, their advantages and disadvantages. There are some nice ‘laws’ relating this classification system to stream length and number of streams as well, but they must wait for another post.
Robert Horton (1945) reversed an earlier European classification system, which made the largest, main channel a 1st order stream, and tributaries progressively larger values. Horton instead classified unbranched ‘fingertip’ headwater creeks as 1st order streams, so that these all had the same and smallest value. That made more sense than the previous version, where headwater creeks could have quite different values. 2nd order streams, in Horton’s system, receive flow only from two or more 1st order streams; 3rd order streams receive flow from two or more 2nd order streams and can also receive 1st order streams as well; and so on up the order scale (see below). Horton also came up with a number of laws relating to stream orders, including the ‘bifurcation ratio’ (the ratio of the number of streams in a given order to the number in the next higher order), which are reviewed by Scheidegger (1968).
Horton’s stream order classification. Source: Horton 1945
Arthur Strahler (1952) modified Horton’s scheme slightly, so that only one channel – the main one – ends up with the highest order:
The smallest, or “finger-tip”, channels constitute
the first-order segments. For the most part these
carry wet-weather streams and are normally dry.
A second-order segment is formed by the junction
of any two first-order streams; a third-order segment
is formed by the joining of any two second-order
streams, etc. This method avoids the necessity
of subjective decisions, inherent in Horton’s
method, and assures that there will be only one
stream bearing the highest order number. (Source: Srahler 1952, p. 1120).
Strahler’s stream order classification. Source: http://www.fgmorph.com
The Strahler method has many advantages, including its consistency across drainage basins – and is probably the most widely used. But it does mean that drainage basins of quite different sizes will end up with the same order for the main channel (see below).
Strahler stream order classification for drainage basins of different sizes. Source: http://www.lifeinfreshwater.org.uk/Web%20pages/Rivers/Stream%20Order.htm
R. L. Shreve (1966) compared Horton’s and Strahler’s approaches to stream order and for those interested in the details, I recommend getting hold of his papers. Shreve came up with an alternative stream order classification, which summed the orders of tributaries to give the order of the next channel in the hierarchy: headwater creeks are 1st order channels; the next channel is the sum of the orders of its tributaries; and so on. This is intuitively simple, but can mean that some channels have very large orders. The Shreve method is proabably the next most widely used after the Strahler method.
Shreve stream order classification. Source: http://www.lifeinfreshwater.org.uk/Web%20pages/Rivers/Stream%20Order.htm
There is no real consensus on which stream order classification to use, but the Strahler and Shreve methods are the most widely accepted. The important thing is that you tell people which one you are using. Otherwise it gets a little confusing.
* I, and many others, use ‘stream’ as a generic term for rivers, creeks, brooks, and all the rest of the terms for flowing waters.
References: Horton RE. 1945. Erosional development of streams and their drainage basins; hydrophysical approach to quantitative morphology. Geological Society of America Bulletin 56: 275-370.Scheidegger AE. 1968. Horton’s laws of stream lengths and drainage areas. Water Resources Research 4: 1015-1021. Shreve RL. 1966. Statistical law of stream numbers. The Journal of Geology: 17-37. Strahler AN. 1952. Hypsometric (area-altitude) analysis of erosional topography. Geological Society of America Bulletin 63: 1117-1142. Strahler AN. 1957. Quantitative analysis of watershed geomorphology. Transactions of the American geophysical Union 38: 913-920.