We explored why bridge foundations erode over time and why floods alone do not explain this risk. Using many simulated river flows linked to a model that includes erosion and sediment refilling, we found that scour is highly variable, persistent, and only weakly linked to flood extremes. Sediment coming from upstream strongly alters erosion patterns. These results show that common design methods can misjudge risk, highlighting the need to account for uncertainty in safer bridge design.