The Capbreton submarine canyon is a striking feature of the south-east of the Bay of Biscay. This canyon forms a deep incision through the continental shelf and slope, and displays remarkable structures linked to its present-day hydrosedimentary activity. Its head has been disconnected from the Adour River since 1310 CE, but remains close enough to the coast to be supplied with sediment by longshore drift. Gravity processes in the canyon body are abundantly described and documented, but activity in the head and the fan of the canyon is poorly constrained. Furthermore, many questions remain regarding the details of processes affecting the head, the body and the fan of the Capbreton canyon. In this work, we address the paucity of documentation concerning (1) the temporal evolution of sediment transfer between the head and the deep reaches of the canyon, and (2) the interaction between gravity processes and the morphology of the canyon floor, including both shaping and feedback mechanisms.

This study is based on the analysis and comparison of eight multibeam bathymetric surveys acquired in the upper part of the Capbreton canyon between 1998 and 2018, in depths ranging 10–320 m below sea level. This rare time series exposes the morphological evolution of this outstanding dynamic area over the last 20 years. Our work shows that much of the changes are located in the canyon floor and head. Following a period characterized by a unique flat floor thalweg, the canyon was affected by an incision with low lateral terraces which resulted in a narrow axial thalweg. The deepening of the narrow thalweg was induced by retrogressive erosion according to the presence of upstream-migrating knickpoints, while low elevation residual terraces formed as the canyon reached a local equilibrium profile.

The flat thalweg observed in 1998 is likely a result of a partial filling of the canyon thalweg by a substantial emptying of the canyon head and significant mass transfer to the proximal part of the canyon. A flat floor thalweg was not observed again in the remaining of our time series (since 2010), suggesting a possible quieter working mode of the canyon. We also propose the first accurate volume quantification of sediment displacement on the canyon floor. Our findings underline the alternation of filling and erosive periods in the canyon axis and an unexpected continuous sediment deposition in the canyon head during the last 20 years.