Planform‐asymmetry and backwater effects on river‐cutoff kinematics and clustering

• Published in: Earth surface processes and landforms Vol. 46; no. 2; pp. 357 - 370
• Main Authors: Ielpi, Alessandro, Lapôtre, Mathieu G.A., Finotello, Alvise, Ghinassi, Massimiliano
• Format: Journal Article
• Language: English
• Published: Bognor Regis Wiley Subscription Services, Inc 01-02-2021
• Subjects: backwater; Backwater effect; Backwaters; Bends; Biogeochemistry; chute; Clustering; Earth surface; Floodplains; Flow structures; fluvial; Fluxes; Great Basin; Imagery; Incidence; Kinematics; Meandering; morphodynamics; neck; Planforms; River engineering; River meanders; Rivers; Skewness; Statistical analysis; Stratigraphy
• ISSN: 0197-9337; 1096-9837
• DOI: 10.1002/esp.5029

River bends occasionally meander to the point of cutoff, whereby a river shortcuts itself and isolates a portion of its course. This fundamental process fingerprints a river's long‐term planform geometry, its stratigraphic record, and biogeochemical fluxes in the floodplain. Although meander cutoffs are common in fast‐migrating channels, timelapse imagery of the Earth surface typically does not offer a long enough baseline for statistically robust analyses of these processes. We seek to bridge this gap by quantifying cutoff kinematics along the Humboldt River (Nevada) – a stream that, from 1994 to 2019, hosted an exceptionally high number of cutoffs (specifically, 174 of the chute type and 53 of the neck type). A coincidence between major floods and cutoff incidence is first suggestive of hydrographic modulation. Moreover, not just higher sinuosity but also upstream planform skewness is associated with higher cutoff incidence and channel widening for a sub‐population of chute cutoffs. We propose a conceptual model to explain our results in terms of channel‐flow structure and then examine the distances between adjacent cutoffs to understand the mechanisms governing their clustering. We find that both local and nonlocal perturbations together trigger the clustering of new cutoffs, over distances capped by the backwater length and over yearly to decadal timescales. Our research suggests that planform geometry and backwater controls might sway the occurrence of cutoff clusters – both local and nonlocal – thereby offering new testable hypotheses to explore the evolution of meandering‐river landscapes that have significant implications for river engineering and stratigraphic modelling. © 2020 John Wiley & Sons, Ltd.
Meander cutoff is fundamental to maintain meandering rivers in their long‐term steady‐state planform. We integrate field, remote‐sensing, and hydrologic data to demonstrate that cutoff incidence in the Humboldt River of Nevada (USA) is controlled by hydrographic modulation and planform geometry. An analysis of spacing between adjacent cutoffs demonstrates that their clustered occurrence is dictated by local and nonlocal effects related to alterations in bed stresses and sediment flux, over distances capped by the backwater‐length scale.