Scroll bars are inner bank levees along meandering river bends

Abstract

AbstractScroll bars across a 65‐km stretch of the Trinity River in Texas, USA were studied using LiDAR data as well as with a series of 11 trenches spread out across the survey area. We conclude that scroll bars are levees that are deposited along the inner banks of these meandering river bends. Scroll bar crests were found to have similar elevations to those of outer bank levee crests, implying that they are constructional features that create positive topographic relief above the elevation of the floodplain. Trenches reveal that scroll bars are built from reworked suspended sediment, with common ripple‐scale cross stratification, planar laminations and muddy bioturbated layers – characteristics often associated with levee sedimentation in other systems. LiDAR observation of the erosion of scroll bars by bed material transport during flood implies that scroll bar spacing is an imperfect proxy for estimating overall channel migration rates. In addition, interspersed lenses of coarser sediment with dune‐scale cross stratification represent the stratigraphic record of these erosional events and suggest that erosion of the channel‐ward edge of the scroll bar is not uncommon. Preservation of scroll bars is unlikely, given that they are responsible for an average of only the uppermost 12% of the total inner bank relief. We suggest that misidentification of point bar lateral accretion surfaces as scroll bars is common and can lead to issues with reconstructing channel properties due to systematic differences between point bar and scroll bar planform geometries. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.