Abstract
Many of the world's alluvial rivers are characterised by single or multiple channels that are often sinuous and that migrate to produce a mosaicked floodplain landscape of truncated scroll (or point) bars. Surprisingly little is known about the morphology and geometry of scroll bars despite increasing interest from hydrocarbon geoscientists working with ancient large meandering river deposits. This paper uses remote sensing imagery, LiDAR data-sets of meandering scroll bar topography, and global coverage elevation data to quantify scroll bar geometry, anatomy, relief, and spacing. The analysis focuses on preserved scroll bars in the Mississippi River (USA) floodplain but also compares attributes to 19 rivers of different scale and depositional environments from around the world.Analysis of 10 large scroll bars (median area = 25 km2) on the Mississippi shows that the point bar deposits can be categorised into three different geomorphological units of increasing scale: individual ‘scrolls’, ‘depositional packages’, and ‘point bar complexes’. Scroll heights and curvatures are greatest near the modern channel and at the terminating boundaries of different depositional packages, confirming the importance of the formative main channel on subsequent scroll bar relief and shape. Fourier analysis shows a periodic variation in signal (scroll bar height) with an average period (spacing) of 167 m (range 150–190 m) for the Mississippi point bars. For other rivers, a strong relationship exists between the period of scroll bars and the adjacent primary channel width for a range of rivers from 55 to 2042 mis ~50% of the main channel width. The strength of this correlation over nearly two orders of magnitude of channel size indicates a scale independence of scroll bar spacing and suggests a strong link between channel migration and scroll bar construction with apparent regularities despite different flow regimes. This investigation of meandering river dynamics and floodplain patterns shows that it is possible to develop a suite of metrics that describe scroll bar morphology and geometry that can be valuable to geoscientists predicting the heterogeneity of subsurface meandering deposits.
Highlights
Alluvial floodplains, and in particular those associated with the world's largest rivers, have a complex relief (Rozo et al, 2012; Lewin and Ashworth, 2014a; Latrubesse, 2015) that is produced by recurring erosional, and depositional events (Day et al, 2008)
Within each meander bend we identified a hierarchy of forms that characterise larger river meander deposits: idividual scrolls (S), depositional packages (DP) and point bar complexes (PBCs) as defined in Table 1
Data were collected from light detection and ranging (LiDAR)-derived bare-earth DEMs provided by the U.S Geological Survey (USGS)
Summary
In particular those associated with the world's largest rivers, have a complex relief (Rozo et al, 2012; Lewin and Ashworth, 2014a; Latrubesse, 2015) that is produced by recurring erosional, and depositional events (Day et al, 2008). Because many river floodplains are inaccessible, and in the world's largest rivers are often densely vegetated (e.g., Trigg et al, 2012, 2014; O'Loughlin et al, 2013), it is especially challenging to characterise and quantify the morphology, geometry, and topography of floodplain relief. Such metrics are important for defining habitat structure, flood hazard mapping, and isolating zones for preferential accumulation of fine-grained sediment, organics, and contaminants. Point bars are usually produced by river lateral migration causing concomitant outer-bend bank erosion and corresponding innerbend deposition to produce a series of undulating ‘ridges’ and ‘swales’ or scrolls (Hickin and Nanson, 1975; Hooke and Harvey, 1983; see Fig. 1)
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