Evolution and deposits of a gravelly braid bar, Sagavanirktok River, Alaska

Abstract

The evolution, migration and deposits of a gravelly braid bar in the Sagavanirktok River, northern Alaska, are described in unprecedented detail using annual aerial photographs, ground‐penetrating radar (GPR) profiles, trenches and cores. Compound braid bars in the Sagavanirktok River form by chute cut‐off of point bars and by growth of mid‐channel unit bars. Subsequent growth is primarily by accretion of unit bars onto their lateral and downstream margins. The upstream ends of braid bars may be sites of erosion or unit bar deposition. Compound braid bar deposits vary in thickness laterally and are thickest in medial sections and near cut banks. Compound bar deposits are typically composed of three to seven sets of simple large‐scale inclined strata, each simple set formed by a unit bar. The simple large‐scale strata contain medium‐scale cross‐strata (from dune migration) and planar strata (from migration of bedload sheets). The upstream and medial parts of compound braid bar deposits show very little vertical variation in grain size, but downstream and lateral margins tend to fine upwards. The deposits are mostly poorly sorted sands and gravels, although sands tend to be deposited at the top of the braid bar, and open‐framework gravels preferentially occur near the top and base of the braid bar. The patterns of braid bar growth and migration, and the nature of the deposits, described from the Sagavanirktok River are generally similar to other sandy and gravelly braided rivers, and consistent with the theoretical braid bar model of Bridge (1993).

Grain sorting in the morphological active layer of a braided river physical model

“…In other words, there is nearly equal probability of encountering any texture value at any relative elevation within the morphological active layer. This is consistent with some observations suggesting very little general vertical sorting trends within gravelly braided alluvium (Lunt and Bridge, 2004;Guerit et al, 2014). In morphological approaches to computing bedload transport in braided rivers (Ashmore and Church, 1998) it is implicit that transport involves the entire morphological active layer.…”

“…In sedimentological analyses, descriptions of braided river gravels have tended to emphasize the sedimentary structure and sedimentological detail with little direct analysis of grain size sorting except for limited vertical sections, trenches or cores using direct physical grain measurement or indirect grain size methods such as imagebased automatic sizing (Storz-Peretz and Laronne, 2013a). Analyses have typically focused on facies patterns and sedimentary structure, but several have mentioned that there is little vertical trend in grain sizes in braided river gravels (e.g., Bluck, 1979;Sambrook Smith, 2000;Heinz et al, 2003;Lunt and Bridge, 2004;Guerit et al, 2014;Marren, 2005;StorzPeretz and Laronne, 2013b). Similarly, physical models of aggraded braided gravel alluvium show patches and threads of distinct facies but no clear trend in grain size (e.g., Moreton et al, 2002).…”

Image analysis for measuring the size stratification in sand–gravel laboratory experiments

“…Sorting processes at local scale are also seen in bars, where coarser sediment tends to be found on bar heads and fines on bar tails (e.g. Leopold and Wolman, 1957;Ashworth and Ferguson, 1986;Lunt and Bridge, 2004). Lateral sorting can be observed in meander bends where the shallower inner bends are finer than the deeper outer bends (e.g.…”

From the Geomorphic Process to Basin Architecture: Anatomy of the Infill of an Alluvial-Lacustrine System in Southern Spain

“…6, Table 1). These lithofacies comprise the following: 1) matrix-supported boulders carried by debris flows [42] (lithofacies Gms), primarily found in the feeder channel of the transverse fan in the detrital catchment; 2) massive or normal-graded clast-supported granules and cobbles, usually having weak clast imbrication (Gmm), corresponding to lag or braid-bar head deposits in the talweg of the channels [43,44] (Figs. 9, 17); 3) imbricated graded clast-supported gravels with horizontal or low-angle cross stratification (Gh) corresponding to the initial upper-flow regime phase of sheet flows in the medial to distal parts of the subaerial fans [40,45] (Fig.…”