EVALUATING CONTROLS ON SCROLL BAR SPACING AND CURVATURE: EVIDENCE FROM SUPERIMPOSED CHANNEL SCALES WITHIN THE LOWER MISSISSIPPI RIVER VALLEY, USA

Scroll bars are curvilinear ridges of alluvium deposited on the inner bank of laterally migrating, sinuous channels. Perturbations to the fluvial system, such as extreme discharge events, can cause episodes of rapid lateral channel migration and subsequent scroll bar formation; thus, scroll bars record a history of channel response to system perturbations. However, questions remain about the relationship between channel migration and scroll bar properties, including how channel dimensions are related to scroll spacing and whether there is a predictable relationship between scroll spacing and curvature. This study uses detailed mapping of lidar-derived DEMs to analyze the spacing and curvature of Holocene scroll bars in the Lower Mississippi River Valley, Louisiana, USA at two distinct scales. Direct comparisons are made between scrolls associated with the main-channel point bars (MPBs) and scrolls deposited when smaller-scale bar-top channels (BTCs) migrate laterally across the bar top, depositing a new set of scrolls.

Preliminary results show that the median spacing between MPB scrolls ranges from 65-190m, which is remarkably similar to the median spacing of BTC scrolls, which ranges from 35-150m. These scroll bar spacings are statistically indistinguishable, even though the formative channel widths differ substantially (from 458-930m for the main channels (median = 661m) and 62-298m for the bar-top channels (median = 124.5m)). This suggests that channel width is not a dominant control on scroll bar spacing in this system.

BTC scrolls are straighter than MPB scrolls for all bars in this study (radius of curvature/channel width ranges from 3.5 to >100 in BTCs and 1.47 to 7 in MPBs). BTC scrolls also show a weak negative correlation between median curvature and median scroll spacing, but MPB scrolls have no correlation between these variables. This suggests that the larger-scale channel has a more complex relationship between curvature and scroll spacing than the BTCs, possibly because of more variable bank substrates. Overall, this work suggests that system perturbations could have identifiable impacts on scroll bar spacing even when formative channel widths differ, but the relationship between curvature and spacing in these deposits may be scale-dependent.