An investigation on 152 gullies along the Daheba River in the Tongde sedimentary basin was performed. Debris flows develop in gullies with an excess topography Z_E, which represents the sediment availability, above a critical threshold value. Debris-flows in the Daheba watershed are supply-unlimited, i.e sediment is abundantly available from the steep erodible gully banks. Debris flows consist of a head and a body. The body propagates faster than the head and constantly supplies it with sediment. The body and head propagate in an intermittent way through the transient storage of sediment on the riverbed and its subsequent remobilization. Although the main sediment supply is provided by bank collapse, debris-flow events also incise the gully bed. The growth and incision of debris-flow gullies in supply-unlimited watersheds is mainly controlled by the frequency of occurrence of debris flows, which is closely related to Z_E. With growth of the gully drainage area, Z_E and the debris-flow frequency initially increase, until they reach maximum values in gullies with a drainage area of intermediate size, which are assumed to be the morphologically most active gullies. With further growth of the gully drainage area, Z_E and the debris-flow frequency decrease, which opposes the development of debris flows and leads to a more stable gully morphology. The observations indicate and explain the upstream migrating incision of the Daheba watershed. The lack of available sediment in the mountain reach is supposed to limit the further upstream migration of the reach of most active debris flows.