A basic way of defining channel pattern is to say that it is, “a term to describe how a river looks from above.” (Leopold ‘A view of a river’ pp56.) However for this essay I feel that it is more appropriate to use the view of Knighton and Nanson who describe the term as “one of the means whereby a natural river can adjust to its channel form to imposed flow and sediment.” Leopold and Wolman (1957) classified channel pattern into three types; straight, meandering and braided. As time has progressed several other patterns have been defined. These include ‘wandering channels’ (Carson, 1984) and ‘Anastomosing channels’ (Smith and Putnam,1980.)
This essay will describe the characteristics of each of the 5 major types of channel pattern and will further attempt to answer the question of whether river channel patterns are distinctive in that they are easily classified or whether they remain in a continuous transitional state whereby one type progresses into the next. Most of the literature that I have read concerning channel patterns have classified that there are 5 major types; straight, meandering, braided, anastomosing and wandering. There is however some reference to schumm and his 14 types of channel pattern that are catergorised by the type of load moved through the channel. The diagram below shows the major characteristics of this.
Meandering rivers are sinuous in shape and maintain a single in curves having a definite geometric shape. Leopold states that meandering river channels are the most common type of pattern found particularly in the U.K. A river is considered to be meandering generally when sinuosity is greater than 1.5. Meandering channels vary in form but there are some main characteristics which occur in a large number of this type of pattern. These include the observation that meander wavelength is commonly about 10 times the channel width and about 5 times the radius of curvature, Natural meanders rarely have a perfectly symmetric and regular form largely because of variations in channel bed material. In rivers with coarse bed material meander forms are often highly distorted. It has been said that meandering channels are the result of natural tendency of liquids in flow motion.
The absence of long straight reaches and the presence of sinuous flow in straight reaches and the presence of sinuous flow in straight reaches is regarded as evidence of an inherent tendency in natural streams to meander irrespective of scale or boundary material. Knighton points out that although in many diagrams meanders are seen to be symmetrical, in reality there is no guarantee that the feature will be particularly regular or that regularity will be maintained over a long river distance.
However it is not uncommon. “It can be said that meanders are neither completely regular nor purely random and can be regarded as a compromise between the two”. The form of meandering channels is dependent on the environmental conditions in which a particular river is found. An example of a meandering river is the Arrowod in Canada and also the Milk river in Montana. Meandering channels are also characterized by the following: – Riffle-pool-riffle sequence – Oblique riffles – Point bar growth lateral accretation – Erosion at apex.
Some rivers migrate. Kinoshita (1901) identified 2 basic types of river meander: meanders of low amplitude which migrate consistently downstream and maintain their symmetric form; and meanders of large amplitude in whch differential shifting at points around the bends leads to complex plan form growth. Braided reaches are multi-channel forms in which the channels are separated by bars or islands. They are contained within a dominant pair of floodplain banks. Some examples are the Brahmaputra in Bangladesh, the Sunwapta in Canada and the Rakaia in New Zealand. “The characteristic feature of the braided pattern is the repeated division and joining of channels, and the associated divergence and convergence of flow, which contributes to a high rate of fluvial activity relative to other river types.”
Hierarchies of channels have been recognized in braided rivers reflecting variations in the level of channel dominance, but the proposed ordering schemes may be difficult to apply consistently. Distinctive toographic levels can be often identified across the channeled area, ranging from the most active channels to elevated, abandoned areas. Generally speaking braided rivers are not geometric. Braided rivers tend to have high slopes with high discharges. You can generally catorgorise them into x and y shape forms. Braid streams are characterized by frequent shifts in the channel position. An example is the Kosi River in India which, between 1736 and 1964 migrated westward over a distance of112k , reworking its own deposits and maintaining a braided appearance in the process.
Straight channels are sinuous but apparently random in the occurrence of bends. River channels are seldom straight except for short distances. This type of pattern is rare and there are very few that are bigger than 10 channel widths. They have minimal erosion and deposition, with vertical banks, no flow deflection. A straight river channel is simply a number of straight reaches connected by bends. Anastomosing channels are characterized by multiple channels separated by islands excised from the floodplain. Anastomosing channels consist of distributaries which branch and rejoin and have a superficial resemblance to braided patterns.
Braided channels, however are single channel forms in which the flow is diverted around obstructions in the channel itself, whereas anastomising patterns consist of discrete, interconnected channels which are separated by bedrock or by stable alluvium. Anastomosing channels are essentially erosional in nature since material between channels is too resistant to be transported except by exceptional flows.
This type of pattern is characterized by low gradients, very small stream powers and cohesive banks which produce laterally stable channels of low width: depth ratio. Anastomosing channel development has been regarded as symptomatic of aggradational regime which is imposed on the river through either basin subsidence or a rising base-level downstream (Smith, 1983). Anastomosing rivers are generally agreed to have stable banks with individual channels showing little tendency to migrate (Knighton and Nanson, 1993).
Wandering channels are another type of channel pattern they show meandering and braiding tendencies. Some meandering rivers tend to migrate, Kinoshita (1961) identified two basic types of river meander: meanders of low amplitude which migrate consistently downstream and maintain their symmetric form; and meanders of large amplitude in which differential shifting at points around the bends leads to complex plan growth.
Wandering channels can also take on braid characteristics as braid streams are characterized by frequent shifts in channel position. An example is the 1736-1964 Kosi river in India migrated westward over a distance of 112km, reworking its own deposits and maintaing a braided appearance in the process. Wandering channels have wide shallow channels with large expanses of bare gravel and low braid intensity. They also have well defined pool-riffle sequence with the erosional banks not exclusive to outer bends.
There is no universally accepted classification of channel pattern. The classic sub-division of Leopold and Wolman (1957) into straight, meandering and braided has provided a useful starting point but, with the wide variety of channel types now recognized, it is proving to be a limited interpretation (knighton, 1984; Ferguson, 1987). Specifically it gives no explicit recognition to anastomosed channels or wandering channels. It gives the impression that river channels can be easily defined when it has been stated by many that this not the case. River channel patterns merge into one another they don’t just change. A number of channel patterns show a lot of features of other channel patterns within them. River patterns are a continuum of one another.