River Avulsion Precursors Encoded in Alluvial Ridge Geometry

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