Abstract: The longitudinal profile of a bedrock channel in an equilibrium state appears as a smooth curve. However, the longitudinal profile of a river in nature often exhibits a transient state characterized by knickpoints, which are caused by the influence of earth's various internal and external dynamic effects. Therefore, the unbalanced channel profile and characteristics of knickpoints can indicate the disturbance of external environment. The drainage system of Minjiang River basin on the eastern margin orogenic belts of Tibetan Plateau is generally characterized by a transient state with widely developed knickpoints. In this study, we extract the spatial distribution of knickpoints based on a digital elevation model. After analyzing causes of the knickpoints, it was found that the formation of these
knickpoints are affected by lithology, glacial action, and tectonics. The knickpoints controlled by lithology were mainly located at the boundary between granite and other strata. High-altitude knickpoints located in the region with low terrain relief are mainly controlled by glacier. The slope-break knickpoints distributed in layers at low altitudes in the basin could be mainly formed by multiple episodes of regional tectonic uplift. In the longitudinal profile of lower part of Minjiang River, the stepped vertical-step knickpoints situated between steep slopes and gentle slopes were mainly caused by seismic landslides. The results help better understand regional geomorphic evolution and its influencing factors. Also, the results suggest that it is necessary to take into account glaciers and landslides in the analysis of the knickpoints in the river longitudinal profile as these factors have important impacts on longitudinal profiles of modern rivers. In addition, the results provide a new perspective for interpreting geomorphological evolution in an orogenic belt.

Key words: longitudinal profile, knickpoint, Minjiang River, seismic landslides, eastern margin of Tibetan Plateau