Large woody debris (LWD) could increase the risk of flooding in the event of heavy rainfall, and fast estimation of the accumulated LWD is crucial for river basin management. We develop a method to automatically detect LWD floating in reservoirs using airborne images, and estimated the total volume of LWD accumulated in a reservoir during a severe flood event in northern Kyushu, Japan, early July 2017. The LWD was successfully extracted from the images based on colour and brightness by using ImageJ. The total volume of the accumulated LWD estimated from images was similar to the LWD volume actually removed from the reservoir by the end of October. Most LWD pieces were concentrated in the upstream-side of the reservoir, and a longitudinal variation in density and length of LWD pieces was also evident.