The role of sediment supply in the adjustment of channel sinuosity across the Amazon Basin

Abstract

Sediment supplies are a fundamental component of alluvial river systems, but the importance of sustained supplies of externally derived sediments for the evolution of meandering planforms remains unclear. Here we demonstrate the importance of sediment supply in enhancing the growth of point bars that influence the rate of sinuosity increase through flow deflections in meander bends. We use an archive of Landsat images of 16 meandering reaches from across the Amazon Basin to show that rivers transporting larger sediment loads increase their sinuosity more rapidly than those carrying smaller loads. Sediment-rich rivers are dominated by downstream-rotating meanders that increase their sinuosity more rapidly than both extensional and upstream-rotating meanders. Downstream-rotating meanders appear to establish larger point bars that expand throughout the meander, in contrast to extensional meanders, which have smaller bars, and upstream rotating meanders, which are characterized by deposition over the bar head. These observations demonstrate that the size and position of point bars within meander bends influences flow routing and thus controls the dominant direction of meander growth. Rivers with low sediment supplies build smaller point bars, which reduces their capacity to increase meander curvature and the resulting sinuosity.

Experimental investigation of the morphodynamic response of riffles and pools to unsteady flow and increased sediment supply

“…In our experiment, where flow was confined entirely within the channel, these effects were not possible, and the fixed width of our experiments prohibited changes in the lateral position and extent of the channel. Higher sediment supply rates have been connected to both increased sinuosity, as well as increased meander migration rates (East et al ., 2015; Ahmed et al, 2019). The fixed width in our experiments required all adjustments to occur within the channel, primarily in the form of reduced channel roughness at the reach‐scale (Figure 9) and locally (Figure 10).…”

Autogenic translation and counter point bar deposition in meandering rivers

“…Although conceptual models of meander expansion often depict perfectly symmetric point bars that result from expansion and no translation or rotation (Ghinassi et al, 2016;Yan et al, 2017), numerical models (e.g., Howard and Knutson, 1984) and measurements in satellite imagery (Sylvester et al, 2019) suggest that bends with 100% expansion are rare. Downstream translation has been discussed in the context of unconfined meandering (e.g., Bridge, 1975;Hooke, 1975;Jackson, 1976;Fustic et al, 2012;Ghinassi et al, 2014;Ahmed et al, 2019), but it is rarely ascribed explicitly to autogenic processes that are inherent to meandering. In studies that focus on downstream translation, it is commonly attributed to reduced erodibility along the outer bank, which inhibits expansion (Smith et al, 2009;Willis and Tang, 2010;Ghinassi et al, 2018).…”

The Influence of Riparian Vegetation on the Sinuosity and Lateral Stability of Meandering Channels

“…Floodplains are developed by two dominant processes: the development of meander cutoffs which fill with sediment and the deposition of fine material on the top of the point bar on the inner bank. There is a relation between sediment supply and meander migration rates (Constantine et al., 2014; Ahmed et al., 2019). Increased sediment supply promotes deposition on point bars, pushing the flow laterally into the outer bank, and promoting lateral migration (Dietrich & Whiting, 1989; Parker et al., 2011).…”