Journal Article

River Avulsion Precursors Encoded in Alluvial Ridge Geometry

Gearon, J. H., Edmonds, D. A.

2025

Overview

River avulsions generate catastrophic floods that threaten communities, ecosystems, and infrastructure worldwide. Alluvial ridges—elevated regions of near‐channel topography—are thought to precede avulsions, yet their spatial patterns and relationship to avulsion impact remain poorly understood. We analyzed pre‐event topographic cross‐sections from 14 rivers to quantify avulsion potential (Λ)$({\\Lambda })$ , a metric combining ridge height and slope relative to the channel. Our analysis reveals that Λ${\\Lambda }$varies downstream and defines distinct alluvial ridge segments. We identify two characteristic length scales: a longer‐wavelength complex (Lλ‾≈$\\overline{{L}_{\\lambda }}\\approx $  30 km) composed of shorter ridge segments (LC‾≈$\\overline{{L}_{C}}\\approx $  8 km). Segments with Λ≥${\\Lambda }\\ge $2 correspond to 73% of observed avulsion activity locations (n$n$  = 37). Avulsion activity length LA$\\left({L}_{A}\\right)$scales linearly with LC${L}_{C}$ ; evidence that ridge geometry controls avulsion activity size. These characteristic scales define both the minimum downstream extent of potential impact zones LC$\\left({L}_{C}\\right)$and the spacing between avulsion‐prone reaches Lλ$\\left({L}_{\\lambda }\\right)$ , enabling improved hazard assessment. Plain Language Summary River avulsions are sudden shifts in a river's course, often causing devastating floods. Predicting avulsions is crucial for mitigating flood risk, but our current understanding of their precursors is limited. This study examines alluvial ridges—elevated areas near river channels—as potential indicators of avulsion risk. By analyzing a suite of river topography data from globally distributed examples, we found that a metric called avulsion potential, which combines measurements of relative alluvial ridge height and slope, reveals distinct high‐risk zones along rivers. These zones correlate strongly with future avulsion activity locations. Moreover, the size of these ridges appears to control the extent of future avulsion activity. This finding could improve flood hazard assessments, particularly in data‐scarce regions. Key Points Alluvial ridges are spatially discontinuous landforms that occur in distinct segments along avulsing rivers Alluvial ridges exhibit a nested hierarchy of scales, from segments to complexes The length of the avulsion scales positively with alluvial ridge length

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Alluvial channels / alluvial ridges / Avulsion / Downstream / Environmental risk / Flood hazards / Flood predictions / Flood risk / Floods / fluvial geomorphology / fluvial sedimentology / Hazard assessment / Height / Precursors / remote sensing / Ridges / river avulsion / River channels / Rivers / Segments / spatial statistics / Topography / Wavelength