Braided Stream

Braided Stream low sinuosity rivers having a channel and channel-valley ratio of less than 1.5. Braided streams are characterised by a network of constantly shifting low-sinuosity channels around channel sandbars. The main channel is divided into several channels, which meet once, re-divide and again meet each other. Channel bars, which divide the stream into several channels at low flow, are often submerged during high flow. One or more alluvial islands or channel bars may be present in a given channel cross section.

The formation of braids involves deposition of bars of sediment within the channel. A bar is a large-scale bedform that grows and migrates downstream because sediment is transported over the bar and then deposited downstream. As the bar grows, it occupies more of the channel's cross-sectional area that was previously available for water. As a result the stream must erode its banks laterally to enlarge the channel to maintain a capacity to discharge. As the discharge drops from a period of very high flow when much of the bed load was in transportation, previously deposited bars are dissected by erosion and the sediments are redistributed through the system. The channel's pattern in a braided stream constantly changes with fluctuations in discharge. Braided streams occur where gradients are steep, the stream has a high rate of discharge, and the banks are composed of soft sediments, which are easily eroded. The accumulation of debris may also be helped by quick growth of vegetation due to climatic conditions.

The Brahmaputra-Jamuna in Bangladesh is a typical example of one of the world's largest braided rivers. After its entry into Bangladesh, its gradient falls sharply and as a result a heavy sediment load brought in from the source areas in Nepal, China and India are deposited in the channel as braid bars locally known as chars. During the monsoon most of the chars are submerged and all the braided channels become one very wide channel, more than 10 km wide in places. Flooding and riverbank erosion become very prominent during the monsoon. [Kazi Matinuddin Ahmed]