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 ZE, 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 ZE. With growth of the gully drainage area, ZE 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, ZE 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.

Anthropogenic impacts and climate change modify instream flow, altering ecosystem services and impacting on aquatic ecosystems. Alpine rivers and streams on the Qinghai-Tibet Plateau (QTP), are especially vulnerable to disturbance due to a limited taxonomic complexity. The effects of variations in flow have been studied using specific taxa, however, the flow-biota relationships of assemblages are poorly understood. A multi-metric habitat suitability model (MM-HSM) was developed, using biological integrity measures of macroinvertebrate assemblages to substitute for habitat suitability indices (HSI) derived from individual taxa. The MM-HSM was trained using macroinvertebrate data from three representative alpine rivers (the Yarlung Tsangpo, the Nujiang, and the Bai Rivers) on the QTP, and was verified using data from the Lanmucuo River. The model produced reliable predictions using the training dataset (R2 = 0.587) and the verification dataset (R2 = 0.489), and was robust to inter-basin differences and changes in dataset size. By coupling the MM-HSM with hydrodynamic simulations, the relationship between weighted usable area (WUA) and flow variations (0.11–1.99 m3/s) for macroinvertebrates was established, and a unimodal response pattern (optimal flow Q = 1.21 m3/s) was observed for macroinvertebrate assemblages from the Lanmucuo River. This was in contrast to the skewed unimodal or monotonically increasing relationships observed for individual indicator taxa, supporting our hypothesis that biological integrity varies with changing flow and conforms to the intermediate disturbance hypothesis. The MM-HSM provides a novel framework to quantify species-environment relationships, which may be used for integrated river basin management.