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Abstract 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 , a metric combining ridge height and slope relative to the channel. Our analysis reveals that varies downstream and defines distinct alluvial ridge segments. We identify two characteristic length scales: a longer‐wavelength complex ( 30 km) composed of shorter ridge segments ( 8 km). Segments with 2 correspond to 73% of observed avulsion activity locations ( = 37). Avulsion activity length scales linearly with ; evidence that ridge geometry controls avulsion activity size. These characteristic scales define both the minimum downstream extent of potential impact zones and the spacing between avulsion‐prone reaches , enabling improved hazard assessment.