Evaluating controls on scroll bar spacing and morphology: comparing chute channel and main channel scroll bar development

Abstract

Abstract Scroll bars are curvilinear ridges of sediment deposited as meandering channels migrate laterally. It is unclear exactly which variables control the spacing, height, and curvature of scroll ridges, and in turn how these properties record channel migration histories. This study tests the relationship between channel and scroll bar geometries in river systems with laterally migrating chute channels that deposit their own scroll packages. LiDAR- and Shuttle Radar Topography Mission-derived digital elevation models (DEMs) are used to calculate the spacing, relief, wavelength, and curvature of scroll bars in eight different chute-channel-dominated river systems with a range of channel dimensions, slopes, and water and sediment discharge regimes. Seven of the eight rivers have statistically indistinguishable chute channel (CC) and main channel (MC) scroll spacings, even though the chute channels have smaller channel dimensions. Additionally, there is no correlation between scroll spacing and relief for main or chute channels. For LiDAR-derived DEMs, the ratio between median scroll spacing and median channel width is low and falls within a narrow range for MC scrolls (from 0.13 to 0.22; mean 0.16), but is higher with a wider range for CC scrolls (from 0.48 to 1.33; mean 0.8). CC scrolls are straighter overall than MC scrolls, but there is no correlation between curvature and scroll spacing. Overall, these results show that (1) chute channel scrolls are often indistinguishable from main channel scrolls based on spacing and relief, and (2) MC and CC scrolls have different relationships with formative channel widths, which suggests that other variables, such as lateral migration rate and flood inundation depth, are key controls on scroll bar geometries.