Presenter: John Holbrook

So Many Ways to Make a River Bend

John M. Holbrook Texas Christian University

ABSTRACT

The concept of meandering in rivers is old and well known, but so many of the known mechanisms of meandering are novel. The classic model of meander growth by translation and expansion generates upward-fining stories with en echolon accretion sets. This model still holds, but additional mechanisms, and variations on this theme, are now apparent. These mechanisms and variations manifest as differing architecture and lithofacies for deposits on the inner bend that ultimatly impact petroleum production trends.

Bar-bend theory characterizes the growth of river meanders and the processes by which rivers will build bends. Transience in bedload transport demands that the channels will develop flow irregularities that are compensated by erosion at a cutbank. Deposition of a point bar is forced on the inner bend to maintain constant width in single-thread channels as the cutbank expands. This process deposits discrete shingles of bedload sediment as individual and parallel accretion sets on the point bar surface. No rule, however, demands that bar growth is exclusively by expansion or that growth have any constant roll or yaw. The bar growth surface can wobble, and the bar growth direction can translate, rotate, or can toggle inbetween expansion and translation, while still maintaining the constraint of a constant channel width (Figure 1). Each of these growth vectors alters internal bar architecture in separate ways. Wobble tends to form accretion surfaces with non-consistent dip that cross-cut and fragment accretion sets within bars. Protracted translation tends to generate counterpoint bars with concave accretion surfaces bounding muddy accretion sets that are contiguous with more Figure 1: Bar growth and abandonment vectors (From Holbrook and Allen, 2020) sandy convex bars. Toggle and rotation generate alternating and evolving sets that have the depositonal characteristics of expansion and translation, depending on the direction they are growing at the time.

Braided rivers can meander by building bar assemblages. These rivers are subject to width oscillations that foster midchannel bar growth, and are not bound by the constant-width assumption of single-thread meandering channels. They grow bends by accretion of compound midchannel and lateral bars to the inner bank. This forms a bar assemblage instead of a point bar on the inner bend. Point assemblages are amalgamations of bars that comprise complex sandy accretion sets, and are separated by thin and muddy fills of remnant braid channels (Figure 2). Because flow can be partitioned readily between channel anabranches, bars can be as easily attached to the outer cutbank as to the inner point bank. These channels can thus form counterpoint assemblages. Counterpoint assemblages have similar architecture to point assemblages, but backfill cutbank scours. This process means that braided meander bends can not only expand and translate like single-thread rivers, but can as easily contract and locally retreat. Contraction and retreat are much more rare in single- thread rivers, and its extent is highly limited. Meandering and braiding are both means for expending excess stream power in rivers, but are usually applied separately. In the case of the more atypical braided meandering streams, they are applied together. The distinction of braided vs meandering in river channels is thus practical but not definitive.

Figure 2: Surface features and architecture of meander and braid loops (from Holbrook and Allen, 2020)

Supercritical channels can also generate side-attached bars. These bars form lateral-accretion sets building into channels undergoing supercritial flow conditions. The accretion sets are sandy and defined by mixtures of upper-flow-regime structures; including upper-plane-bed, antidune, chute-and-pool, and cyclic-step structures. Supercritical structures dominate early deposition of accretion sets, and give way to drapes of upper-plane bed as flows wane. Incidences of these bars are rarely reported and bar amplitudes are generally small. The degree to which these bars contribute to sinuosity of the channel is still uncertain. Holbrook, J., Allen, S.D., 2020, The case of the braided river that meandered: Bar assemblages as a mechanism for meandering along the pervasively braided Missouri River, USA: GSA Bulletin, https://doi.org/10.1130/B35762.1; 14 figures.