STRUCTURE AND PROCESS IN A BRAIDED RIVER

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Abstract
The Donjek is a proglacial braided river with a coarse heterogeneous bedload ranging from −7 to +8 φ. The three reaches studied comprise, in downstream order, a zig‐zag reach formed by the interaction of the river and tributary fans, a straight reach, and a meandering reach, all with internal braids. The straight to meandering junction is unusual in that slope and discharge both increase.The dominant bed‐forms are longitudinal gravel bars, which migrate only during flood. Their internal structure is poorly defined horizontal bedding, which suggests that gravel deposition takes place on the upper bar surfaces, rather than on foreset slopes at the downstream margins. At lower stage sand accumulates in wedge‐shaped units lateral to the bars, with internal high‐angle cross‐stratification and ripple cross‐lamination; other sedimentary structures are rare. A simple facies model can be constructed on the basis of two trends: a proximal‐distal trend, and an active‐stable trend. Both result in the increased abundance of fine‐grained sediment.Random directional measurements give good estimates of the river trend from small‐scale structures (mainly ripples) and from channels, the latter giving the closest approximation. The variability of ripple orientation always exceeds that of channels. Both increase in the meandering reach, although the divergence of the vector mean from the river trend is not significantly greater than in the straight reach.