Fig. 1

The location of the study area is marked with a red dot. (a) Satellite image (Google Earth Pro version 7.3.6 https://www.google.com/earth) of the DYG Gully watershed, with red lines indicating the water divide. (b) A photograph taken by investigators within the gully, clearly showing the steep slopes on both sides (taken by Lin Tian). (c) A boulder transported by the debris flow during the August 18 event (taken by Lin Tian).