Abstract Reconstruction of active channel geometry from fluvial strata is critical to constrain the water and sediment fluxes in ancient terrestrial landscapes. Robust methods—grounded in extensive field observations, numerical simulations, and physical experiments—exist for estimating the bankfull flow depth and channel-bed slope from preserved deposits; however, we lack similar tools to quantify bankfull channel widths. We combined high-resolution lidar data from 134 meander bends across 11 rivers that span over two orders of magnitude in size to develop a robust, empirical relation between the bankfull channel width and channel-bar clinoform width (relict stratigraphic surfaces of bank-attached channel bars). We parameterized the bar cross-sectional shape using a two-parameter sigmoid, defining bar width as the cross-stream distance between 95% of the asymptotes of the fit sigmoid. We combined this objective definition of the bar width with Bayesian linear regression analysis to show that the measured bankfull flow width is 2.34 ± 0.13 times the channel-bar width. We validated our model using field measurements of channel-bar and bankfull flow widths of meandering rivers that span all climate zones (R2 = 0.79) and concurrent measurements of channel-bar clinoform width and mud-plug width in fluvial strata (R2 = 0.80). We also show that the transverse bed slopes of bars are inversely correlated with bend curvature, consistent with theory. Results provide a simple, usable metric to derive paleochannel width from preserved bar clinoforms.
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Geometric and Spectral Analysis of Relict Channel Planforms in Central Baja California, Mexico: A Novel Approach to Paleo-Sea Level Reconstruction and Testing Hypotheses for Genetic Divergence
Genetic divergence along the central Baja California Peninsula, Mexico, has been hypothesized to reflect a Pliocene cross-peninsular seaway that previously isolated northern and southern populations of terrestrial plants and animals. One way to test this hypothesis is through quantitative analysis of relict channels preserved on low-relief paleo-surfaces. Recognition of tidal channels on relict landscapes offers a powerful tool for reconstructing past sea level in tectonically active arid coastal regions where crustal uplift results in relative sea-level fall and preservation of ancient channel networks. This method requires reliable criteria to distinguish fluvial versus tidal channels, which is challenging due to the overlap of standard metrics for the two channel types, and possible inheritance or overprinting of geometries. We improve the utility of existing metrics and explore the potential for identifying paleo-sea-level indicators by analyzing modern and ancient channels to identify unique patterns in planform geometry and to evaluate their applicability for classifying tidal versus fluvial origins. Preliminary measurements of geographically diverse modern systems reveal distinct, quantifiable differences between the two channel types in along-channel curvature, width, and wavelet spectra. Modern tidal channels display a pronounced and systematic down-channel increase in channel width and decrease in curvature. In contrast, modern fluvial channels do not display spatial patterns in channel width and curvature along their lengths. These patterns provide diagnostic criteria that can be paired with wavelet analysis of meander belts to classify the paleoenvironment of ancient channels based on their planform geometry. We apply this approach to evaluate the origin of channels preserved on relict landscapes in the San Ignacio trough in the central Baja California peninsula, a former low-relief embayment of the Pacific Ocean. Early results reveal the presence of ancient tidal channel networks at elevations of ~ 50-300 m above modern sea level on surfaces that are independently dated to be ca. 4-5 Ma. These findings provide evidence for post 4-Ma uplift in the mid-peninsular region and an ancient tidal environment that may have isolated northern and southern terrestrial populations.
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- Award ID(s):
- 1925535
- NSF-PAR ID:
- 10345198
- Date Published:
- Journal Name:
- American Geophysical Union
- Page Range / eLocation ID:
- B55L-1345
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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