Abstract:

The stream-power incision model shows that a bedrock channel longitudinal profile is characterized by a smooth, concave-up shape at the steady state, and its characteristics reflect the influences from external forces, such as tectonics, climate, and rock resistance. However, most of the natural rivers present a transient state characterized by knickpoints on longitudinal profiles, which can also infer the influences from external forces. Widespread knickpoints at high altitudes on river longitudinal profiles along the Zoulang Nan Shan (mountain), which is a part of northern Qilian Mountains, provide a particular case for studies on the factor affecting the disequilibrium profile. The analysis of the knickpoints indicates that the formation of the knickpoint at high altitudes is not influenced by lithology, climate and/or tectonics. By comparing the plaeo-glaicial evidences, we proposed that the high-altitude knickpoint reflects the boundary between residual glacier valleys and fluvial channels. The result suggests that we should pay more attention to the inheritance landform by ancient glaciation when analyzing the knickpoint located at high altitudes. This study would greatly increase the knowledge about the geomorphic evolution on high mountain ranges along orogenic belts.

Key words: river longitudinal profile, steepness index, knickpoint, Zoulang Nan Shan, glaciation