Debris flows always occur in small drainage basins in mountainous areas, i.e., in basins almost in size of less than 50 km2 in China. Nearly 6000 basins out of the collection of the Database of Debris Flow in China are taken into account in the context of the country and several particularly chosen regions where debris flow is frequently consequential to reveal the relationship between drainage area and occurrence frequency. Statistics of these basins indicates a distribution in the general exponential form, i.e., the generalized Gamma distribution, which takes Weibull, Gamma, and exponential distribution as its particularities.The Weibull distribution is perhaps the most likelihood that, in a natural way, defines a hazard rate for the weighting probability of the basin area; and, of more significance, from which results a failure mechanism of the debris-flow system.Finally, observational data of debris-flow-surge discharge of Jiangjia Gully is mustered as surrogate variables to confirm the general magnitude-frequency relation of debris flow. Distribution in the same exponential form as the drainage area has been found, which implies a correlation between debris flow and the spatial features of a valley.
