Quantifying bankfull flow widths of single-threaded rivers from preserved bar clinoforms

Abstract:

The reconstruction of channel geometry from fluvial stratigraphy is central in deciphering the hydrologic and morphodynamic conditions on Earth and other planetary bodies. While significant progress has been made in the quantitative inversion of bankfull flow depth and channel-bed slope from preserved fluvial deposits, we currently lack a robust method to quantify bankfull channel width from fluvial strata. Existing methods rely on proxy measurements such as channel-body dimensions, width-to-depth ratio of modern alluvial rivers, and percentage clay in putative floodplain deposits to infer channel widths—estimates often associated with an order-of-magnitude uncertainty. Here, we leverage the idea that channel bars are macroforms whose sizes scale with the formative channel dimensions to develop a robust, empirical relationship between the bankfull channel width and channel-bar clinoform width—relict stratigraphic surfaces of channel-bar slopes. To objectively measure channel-bar width, we approximate the shape of bar surfaces using a two-parameter sigmoid function and define the bar width as the distance between 95% of the asymptotic values of the sigmoid. Using this definition of bar width, we quantified the widths of bank-attached channel bars and measured bankfull flow widths across 134 bends of 11 single-threaded rivers using high-resolution lidar data, and compiled the same two measurements for an additional 11 rivers from previously published studies. Our data compilation spans three orders of magnitude in channel size, and represents all climate zones. We derive an empirical relationship between the measured bankfull channel width and the estimated channel-bar width, which can be directly applied to the preserved bar clinoforms of ancient strata. While the proposed scaling is sensitive to the obliquity of preserved channel cross section, our analysis indicates that increased sampling of randomly distributed cross-sections will converge on the channel-perpendicular bar width for obliquity angles within 60 degrees of the perpendicular. We also find that the slopes of the preserved bar surfaces could encode information regarding formative flood discharge variability. Our results provide a simple, usable metric to derive paleo-channel width from preserved bar clinoforms.