By: Heidi Poca
There are many aspects of the natural world that are static and stay in one place – trees, mountains, and lakes just to name a few. Streams and rivers, however, like to travel. This article will cover the four types of river channel forms, as well as basic geomorphology processes that build and change the Earth’s waterways.
There are four types of channel forms:
Straight
Meandering
Braided
Anastomosing.
Simplified diagram of the 4 channel forms. Courtesy, Fluvial Environments.
A single river is often made up of more than one channel form and can have meandering sections leading into braided or anastomosing and back to meandering. There are various environmental and geological attributes that determine the channel form of a river. The main factors that determine channel form are elevation, bank strength, sediment supply, as well as erosion rate, which is also dependent on the water discharge variability (or energy of the water).
Straight channels are the rarest channel and are most often associated with mountain streams. Straight channels are restricted to areas that have high water energy and strong and stable banks. They are also forced to follow specific paths due to static geologic structures such as mountains. Straight channels, often call ‘youthful rivers’, tend to erode their riverbeds faster than they erode their banks. The constant cutting of the channel results in a ‘V-shaped channel or valley, whereas slower-moving meandering rivers tend to have more of a ‘U’ shape cross-section.
Meandering channels are one of the more common channel forms, and it’s the one that most people are familiar with. Meandering streams can be easily identified by their twists and turns snaking across the landscape. For a channel to be labeled as meandering, its sinuosity must be greater than 1.5. Sinuosity is calculated by Channel Length/ Valley length and it indicates how ‘curvy’ a river or stream is. Meandering channels are highly dynamic and undergo quite a bit of movement depending on their location. Meandering channels form in areas of finer-grained sediments such as sandy soils. They also tend to form in areas with gentler slopes. Meandering channels move by high energy water hitting and eroding the outside bank creating ‘cut banks’. The water now carrying the sediment bounces across the river hitting the opposite bank where it will create another cut bank. On the inside bank opposite to cut banks, ‘point bars’ are formed from sediment falling out of suspension. Sediment deposits form point bars because the velocity of water is significantly slower on the inside bend, and the reduced velocity does not have the energy or power to move sediment. As time progresses the meanders or ‘curves’ become more and more exaggerated until the channel creates a loop and runs into itself. When this happens, the water takes the shortest route and the loop becomes cut off, creating an oxbow lake (read more about oxbow lake from one of our previous #TerminologyTuesday posts). Fig. 2 shows the formation of an oxbow lake, as well as sediment being deposited on the inside of each bend.
Time-lapse of a meandering river forming an oxbow lake. Courtesy, The Earth Story
The last two forms are braided and anastomosing. They are often confused with one another due to similar interwoven appearances; however, there are a few unique differences that separate them from one another. Braided channel forms are characterized by having high sediment loads, steep slopes, weak and erodible banks, and variable water discharge levels. Braided rivers are often found in mountains and foothills due to the steep slope and unstable banks. The weak banks provide the river with the necessary sediment that is required to make the ’braided’ channel pattern. The variable water discharge levels come from snowmelt, rain, and runoff from the surrounding mountains. As the water starts to lose energy the sediment falls out of suspension and starts to form ‘braid bars’ small islands that give the form its braided appearance. As braid bars form, water is redirected and creates alternate channels. Although braided rivers appear to have various meandering paths, the path of the actual main channel is relatively straight and unmoving, with all the movement occurring within the main channel itself. Another interesting aspect of braided forms is that braid bars themselves are dynamic. Braid bars are constantly being eroded at the top and having sediment deposited at their base, meaning that over time entire bars move downstream. Overall braided rivers are wide and shallow. When viewed from above in times of high flow, only a few channels may be visible, due to all the shallow islands being underwater, however when floods recede you get to truly appreciate how interconnected all the channels are.
The final channel form is anastomosing. Anastomosing channels usually form in flatter areas with shallower slopes and have stable, erosion-resistant banks. These characteristics result in narrower and deeper channels compared to the wide and shallow channels of braided systems. The large islands within anastomosing channels are also relatively permanent and they are made with alluvium which is fine-grained sediment deposited by running water. To further differentiate anastomosing from braided channels, anastomosing rivers have multiple channels connected together and separated by large permanent islands. Braided rivers have a single channel with multiple threads within it and there are broken up by constantly changing braid bars.
Diagram of four channel forms, depicting channel and alluvial ridge in relation to flood basin.
In conclusion, straight, meandering, braided, and anastomosing are the 4 main channel forms. Each unique form has its own set of specific requirements needed to develop into their respective patterns. Characteristics that tend to have the greatest impact on the construction of channel form are slope, bank strength, sediment type, and water discharge. Table 1 compares these characteristics between the 4 channel forms for a quick and easy understanding of the traits that result in the drastic differences between the channel forms.
Comparison Table for the 4 Channel Forms
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