Braided Rivers

Braided Rivers exhibit numerous channels that split off and rejoin each other to give a braided appearance. They typically carry fairly coarse-grained sediment down a fairly steep gradient. Consequently, braided rivers usually exist near mountainous regions, especially those with glaciers or in arid to semi-arid areas where vegetation is sparse or where water discharge tends to be highly variable. Deposits of Braided Rivers tend to be coarse-grained and contain abundant amalgamated channels. Braided river systems are characterized by braiding parameters greater than 1, low sinuosity, steep slopes, abundant sediment supply, high and variable discharge and high width/depth ratios (Miall,1982). As flow regimes in a river vary braiding is developed by the systematic sorting as the stream deposits load sizes it is no longer able to carry. This bedload deposition will result in a number of different types of bars and bedforms, which inturn impede and divert the flow. Banks which are easily eroded will facilitate this process (Miall,1982).

Click here to see Braided River core examples from the Cretaceous of British Columbia (from University of British Columbia, Canada)  

 


Modern Examples

Brahamaputra River

Large scale braided channel systems can produce channel and bar complexes of first and second order magnitude. The Brahmaputra River, just above its confulence with the River Ganges, is a multi channel river system with a contunuum between braided, meandering and anastomosed rivers (Bristow, 1987). The primary control on the channel patterns is the channel discharge. The river changes from being more braided in the north to more sinuous in the south. This change in character may be due to a gradient reduction from 0.000077 to 0.00005, or it may be due to peak discharge reductions as flood waters are captured by the Atrai Gur flood basin.

The first order channel of the river encompassed the entire river which has an average width of 10 km and a maximum depth of 45 meters. The channel is realtively stable and migrates within a channel belt as much as 20 km wide. First order channel migration averages about 70 m per year. The second order channels have widths of 5 km and depths of up to 40 m, and they migrate up to 1km/year. Most of the channel movement is through lateral migration, however some channel switching does occur. Third order channels have widths on the order of hundreds of meters. and they generally occur within second order channels. These channels are responsible for bar dissection and erosion.(Bristow, 1987).

Five types of bars have been recognized in the Brahmaputra with lateral, medial and tributary bars (see Channel Confluences) being the dominant types. The bars scale with the channels and the bar length from crossover to crossover is approximately 3 to 4 times channel width. Bar heigth varies from half to full channel depth. Bars are stable over several floods, but dissection can produce complex bar forms. Bar deposition occurs by lateral accretion (57%), downstream accretion (29%) and upstream accretion (14%). Bar deposition accounts for 53 persent of the sediment deposition with other types of deposition being lateral acretion to the bank (19%), channel abandonment (15%), and new mid-channel bars (13%). Bars can coalesce to form bar assembledges as much as 20 km long. This is usually accompanied by anastomosed channels and the bar complexed remain stable over tens of years.(Bristow, 1987). In its higher reaches in Tibet, braiding also occurs - see photo below.
On October 13, 2000, the Expedition 3 crew of the International Space Station, high over Tibet, took this interesting photo of the Brahmaputra River. Flowing east and known as the Tsangpo this 15 kilometer stretch is situated about 35 km south of the ancient Tibetan capital of Lhasa where the river flow becomes intricately braided as it works and reworks its way through extensive deposits of erosional material. This pattern is indicative of a combination heavy sediment discharge from tributaries and reduction of the river's flow from either a change in gradient or perhaps even climate conditions over the watershed.
Photo ID: h4w48z | Photographer: NASAVE NASA Visable Earth | Credit Line: Courtesy NASA, Visible Earth  

Studies by Sapozhnikov and Foufoula-Georgiou, (1996) on 3 braided river systems (the Aichilik and Hulahula in Alaska and the Brahmaputra in Bangladesh), have shown that braided river systems that the spatial structure of the whole braided river and its structure at the level of channels appear to have some kind of similarity i.e. they exhibit anisotropic scaling (self-affinity) with fractal exponents vx =0.72–0.74 and vy = 0.51–0.52, the x axis being oriented along the river and the y axis in the perpendicular direction. The fact that despite large differences in scales (0.5–15 km in braid plain width), slopes and types of bed material (gravel to sand), the analyzed braided rivers show similar spatial scaling might indicate the presence of universal features in the underlying mechanisms responsible for the formation of the spatial structure of braided rivers.

A more detailed review of Braided River Systems can be found here.



Kicking Horse River, Yoho National Park, British Columbia

Resurrection River, Kenai Peninsula, Alaska

Resurrection River, Kenai Peninsula, Alaska

At-Bashi River, Tien Shan Mtns., Kyrgyzstan

Braided stream.  Tien Shan Mtns, Kyrgyzstan

Son-Kul River, Tien Shan Mtns., Kyrgyzstan

High Energy Outwash Sediments

An analysis of high energy Pleistocene outwash sediments by Dawson and Bryant (1987) identified three facies within the depositional sequence. The coarsest grain facies sonsist of gravelly lithofacies which are interpreted as sheets deposited by laterally migrating bar complexes in the active channel belt off a low sinuosity river. The second facies comprises thin, discontinuous sheets of sandy lithofacies which were deposited as floodplain or bar top sediments in a shallow flow regime. The third facies type consists of sands and fine gravels deposited as channel fills.