Braided river
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A braided river is one of a number of channel types and has a channel that consists of a network of small channels separated by small and often temporary islands called braid bars or, in British usage, aits or eyots. Braided streams occur in rivers with high slope and/or large sediment load (Schumm and Kahn 1972). Braided channels are also typical of environments that dramatically decrease channel depth, and consequently channel velocity, such as river deltas, alluvial fans and peneplains.
Braided rivers, as distinct from meandering rivers, occur when a threshold level of sediment load or slope is reached. Geologically speaking an increase in sediment load will over time increase the slope of the river, so these two conditions can be considered synonymous and consequently a variation of slope can model a variation in sediment load. A threshold slope was experimentally determined to be 0.016 (ft/ft) for a 0.15 CFS stream with poorly sorted coarse sand (Schumm and Kahn 1972). Any slope over this threshold created a braided stream, and under the threshold created a meandering stream or for very low slopes a straight channel. So the main controlling factor on river development is the amount of sediment that the river carries, once a given system crosses a threshold value for sediment load it will convert from a meandering system to a braided system. Also important to channel development is the proportion of suspended load sediment to bed load. An increase in suspended sediment allowed for the deposition of fine erosion resistant material on the inside of a curve which accentuated the curve and in some instances caused a river to shift from a braided to a meandering profile (Schumm and Kahn 1972).
The channels and braid bars are usually highly mobile, with the river layout often changing significantly during flood events (Hickin and Sichingabula 1988). 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.
Conditions which promote braided channel formation are:
- an abundant supply of sediment
- high stream gradient
- rapid and frequent variations in water discharge
- erodible 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 Sandhills 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.
[edit] Notes
Gray D, Harding JS; (2007) Braided river ecology: a literature review of physical habitats and aquatic invertebrate communities. Science for Conservation 279. p 50. Published by Department of Conservation, New Zealand. [1]
Schumm, S and Kahn H (1972). “Experimental Study of Channel Patterns.” Bulletin of the Geological Society of America (83) pp. 1755-1770.
Hickin, E and Sichingabula, H (1988). “The geomorphic impact of the catastrophic October 1984 flood on the planform of the Squamish River, southwestern British Columbia.” Canadian Journal of Earth Science (25) pp. 1078-1087.
