The effect of the Wenchuan earthquake on the fluvial morphology in the Longmen Shan, eastern Tibetan Plateau: Discussion and speculation

Although large earthquakes can have significant impacts on the geomorphology of mountain rivers, the consequences of such impacts remain poorly understood. The Ms 8.0 Wenchuan earthquake of 12 May 2008 created knickpoints (waterfalls) in the river systems of the Longmen Shan, eastern Tibet Plateau, in addition to numerous landslides along rivers. In this study, we use high-resolution remote sensing images and field surveys to examine how the Baisha and Jianjiang River channels responded to the Wenchuan earthquake. The morphological response along the Baisha River was a knickpoint, with a height of 4.5 m, which was studied from 2008 to 2012. However, the river did not show rapid incision via knickpoint migration during the post-earthquake period. In fact, post-seismic flooding destroyed the face of the co-seismic knickpoint. The rapid removal of the knickpoint suggests that the abrupt vertical displacement associated with the earthquake did not exceed the adjustment threshold of the fluvial dynamic equilibrium, and so was unable to drive rapid morphological change at the local scale. Along the Jianjiang River, the average meander migration rate of the Beichuan reach was −2.8 m/yr between 2001 and 2008, but increased to 51.1 m/yr during the five years following the 2008 Wenchuan earthquake. This rapid increase in the meander migration rate is attributed to excess sediment supplied to the river as a consequence of the earthquake. These two case studies indicate that the Wenchuan earthquake caused channel aggradation rather than a significant increase in river incision by locally generating relief across the river.