Human‐initiated autocyclic delta failures

ABSTRACT River regulations have resulted in changes in the hydrology and particle budgets of fluvial systems. Since the 19th century, many rivers have been significantly modified to control flood hazards, to gain land from swamp areas for agricultural purposes, and to stabilize river‐levels and lake‐levels to facilitate navigation. These dramatic changes of the river courses have impacted the sediment budgets and grain‐size dissemination along them as well as the sediment distribution at the delta mouths in the downstream lakes, which could lead to slope instabilities. Deposits of such catastrophic lacustrine mass movements caused by delta collapses have been, for instance, observed in Lake Brienz (Switzerland), where relatively thick (0.5 to 1.3 m) and voluminous (>1 million m 3 ) megaturbidites are stacked in the deep basin witnessing these processes. This study uses sediment cores and seismic data to reconstruct the megaturbidites' history in Lake Brienz. Data reveal that mass‐movement deposits, originating from the Aare Delta, one of the two main inflows, have mean ages of 1853, 1905, 1942 and 1996 ce and that they were unprecedented in, at least, half a millennium. The fact that the numbers of floods and earthquakes have not changed radically over this time period implies that human impact is the most likely explanation for these failure events. Therefore, the recurrent delta collapses are attributed to the focused sediment accumulation at the front of the channelized inflow in the proximal delta region, caused by the modification of the Aare River through its straightening and channelization during the late 19th century. These findings indicate that river regulation can affect delta sedimentation, leading to autocyclic delta collapses. Those collapses, in turn, can potentially generate tsunami waves, representing an additional natural hazard for shoreline communities.