Braided river
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A braided river channel consists of a network of smaller channels separated by small and often temporary islands called braid bars. Braided streams are common wherever a drastic reduction in stream gradient causes the rapid deposition of the stream's sediment load. Braided channels are also typical of river deltas.
The channels and braid bars are usually highly mobile, with the river layout often changing significantly during flood events. Channels move sideways via differential velocity: On the outside of a curve, deeper, swift water picks up sediment (usually gravel or larger stones), which is re-deposited in slow-moving water on the inside of a bend.
The braided channels may flow within an area defined by relatively stable banks or may occupy an entire valley floor. The Rakaia River in Canterbury, New Zealand has cut a channel 100 metres deep into the surrounding plains.
The dynamic nature and uneven terrain of braided rivers present particular challenges to bridge construction.
Conditions which promote braided channel formation are:
- an abundant supply of sediment
- rapid and frequent variations in water discharge
- erodable banks
The most famous example of a large braided stream in the United States is the Platte River in central and western Nebraska. The sediment of the arid Great Plains is augmented by the presence of the nearby Sand Hills region north of the river.
Extensive braided river systems are found in only a few regions world-wide:
All the above regions contain young, eroding mountains.