Large (>10 6 cubic meters), highly mobile debris flows represent one of the deadliest yet least understood types of landslides on Earth. These flows often originate when smaller events entrain water and sediment along their channel. The conditions controlling when and where these flows bulk are not well understood, making their hazard unpredictable. Here, we examine this hazard by combining a unique inventory of debris flows from the Wenchuan earthquake with numerical modeling to constrain their magnitude and frequency. We show that large debris flows occur more frequently than expected, on the basis of magnitude-frequency relationships for all debris flows, when high volumes of sediment are deposited in channels. These findings are consistent with other large sediment-generating events globally, such as Mount St. Helens and Mount Pinatubo where multiple large debris flows were triggered following volcanic eruptions that produced several cubic kilometers of sediment.