Grain size and sediment storage explain why postfire debris flows are absent in some mountainous landscapes

Postfire debris flow occurrence varies widely in mountainous landscapes, even under similar rainfall conditions. To understand why, we investigated two burned landscapes that experienced intense postfire rainfall: one in southern California with over 100 postfire debris flows and one in northern California with no postfire debris flows. Debris flows can initiate from bed failures that are predicted to occur when Shields stress [Formula: see text] exceeds a threshold. We characterized [Formula: see text] at the two sites, investigated bed failure, and conducted numerical experiments to identify controlling factors. Postfire increases in clear-water runoff raised [Formula: see text] by up to threefold, whereas postfire reductions in channel grain size raised [Formula: see text] by two orders of magnitude. No increase in runoff was required to initiate debris flows at the southern California site, yet debris flows were unlikely to initiate at the northern California site even for considerable increases in runoff. These results provide a mechanistic explanation for discrepancies in debris flow occurrence in mountainous landscapes.