Abstract Hollows are the concave-out portions of hillslopes not occupied by channels. The topographic convergence in hollows forces colluvial debris to accumulate and causes shallow subsurface runoff to be concentrated during storms. Consequently, hollows are more susceptible to landsliding than side slopes and constitute important mappable source areas of debris flows. Hollows can be extremely subtle topographic features that require recognition in the field; these subtle hollows are commonly tributary to larger hollows, and greatly increase the density of mappable debris flow sources. In a study area in Marin County, California, hollows are spaced 20 to 60 m apart along the slope, resulting in a density of 25 to 35 km of hollow axis per km 2 . Even the subtle hollows can produce debris flows capable of destroying houses, particularly when large trees are carried by a flow. Mitigation measures that focus on draining the main hollow axis may be inadequate because of the destructive ability of debris flows shed from small tributary swales and from side slopes. Road runoff discharged onto hollows can trigger landsliding and gullying, but this problem can be prevented by extending culverts downslope to stream channels. Along the drainage network, from subtle tributary hollows to major hollows, and to first-order channels where many additional hollows enter, the recurrence interval of debris flow events probably systematically decreases as the number of upslope sources increases, perhaps reaching the lowest recurrence interval on second-order channels. Farther downstream, debris flows may occur less frequently. A greater emphasis on hollows as debris flow source areas and as paths for flows from upslope should make a significant contribution toward identifying the hazard to existing structures and toward improved siting of new development.