Shear-thickening suspensions—materials that abruptly become more viscous or jam under stress—are widespread in nature and industry. Yet their dynamics in gravity-driven flows remain poorly understood. Here, we show that such suspensions spread by forming a sharp, vertical front—a liquid dam—that advances at a constant speed, independent of released volume, flow height, and slope. This counterintuitive behavior arises from a jammed, frictional front that localizes dissipation and decouples flow dynamics from geometry. A simple gravito-rheological model predicts the observed constant-speed regime, with a velocity scale set by the suspension rheology. The same scaling governs spreading and wave propagation across diverse flow configurations, revealing how shear thickening can critically shape gravity-driven flows in both natural and industrial settings.

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• Flow instability
• Rheology
• Suspensions
• Shear thickening