The river channel type characterized by multiple channels and numerous gravel bars is known as a braided river or braided channel. The flow is divided and recombined repeatedly around islands and mid-channel bars of accumulated sediment. The appearance of these rivers from above is often likened to the interwoven strands of a braid, which gives the formation its name. The channels and bars constantly shift and reform, particularly during periods of high water flow.
The Defining Physical Features of Braided Channels
Braided channels are defined by a distinct morphology. The river’s flow is distributed across a network of multiple, smaller, and shallower channels that split and rejoin. This multi-threaded pattern is created by the presence of mid-channel bars, which are accumulations of coarse sediment like gravel and sand.
These sediment accumulations, often termed braid bars, are unstable and ephemeral, meaning they are frequently submerged and reshaped during high-flow events. The river system exhibits a high width-to-depth ratio, meaning the channel is significantly wider relative to its depth. The constant deposition and re-erosion of material cause the entire channel configuration to be highly mobile, with the banks and bars shifting laterally and longitudinally over time.
Hydrological and Geological Conditions for Formation
The formation of a braided river requires a specific combination of hydrological and geological prerequisites. A high sediment load, particularly coarse-grained material like gravel, is a primary requirement for braiding to occur. This excess sediment overwhelms the river’s transport capacity, causing the material to be deposited within the channel rather than carried downstream.
A relatively steep gradient or slope is also necessary, as this provides the energy for the river to move the large volume of coarse sediment. Even with this energy, localized decreases in water velocity lead to the formation of small deposits, which then grow into bars that split the flow. The river’s banks are non-cohesive and easily erodible, which allows the channel to widen and shift laterally.
Significant variability in water discharge is another condition that promotes channel braiding. Fluctuations, such as seasonal snowmelt or episodic flash floods, are responsible for moving large quantities of sediment and reshaping the channel during high-flow periods. When the flow subsequently decreases, the deposited bars become exposed and stabilized, forcing the reduced water volume to flow through the remaining, narrower channels. This cyclical process of high-energy erosion and low-energy deposition maintains the dynamic, multi-channel pattern.
Common Geographic Settings and Examples
Braided rivers are found in diverse environments globally, but they are most common in areas with an abundant supply of coarse sediment and a flow regime that is highly variable. One typical setting is in proglacial areas, where melting glaciers provide enormous volumes of poorly sorted sediment and highly fluctuating meltwater flow. The rivers flowing out of the Southern Alps in New Zealand, such as the Waimakariri River, are classic examples of this glacial-fed braided system.
They also frequently occur in the piedmont zones near young, eroding mountain ranges, where the steep slopes transition to flatter plains. In these areas, the high energy from the mountains mobilizes sediment, which is then rapidly deposited as the slope decreases, leading to bar formation. The Brahmaputra River in India and Bangladesh demonstrates this phenomenon as it descends from the Himalayas. Braided channels can also be found in arid or semi-arid regions where sparse vegetation and episodic, high-intensity rainfall events produce large sediment loads and highly variable discharge.