Field observations of a delta-lip failure generating a turbidity current in a lake

Turbidity currents are one of the largest sediment-moving processes across our planet, connecting terrestrial sources to deep sea sinks. They play an important part in the transport and burial of organic material, transport contaminants such as microplastics and can reach velocities of multiple meters per second, at which point they form a hazard to submarine infrastructure. In lacustrine environments, they influence water quality by delivering nutrients and oxygen along the water column. Understanding what triggers the formation of turbidity currents is therefore key. Recently, some works have shed a light on the role of transient storage of sediment in the triggering of turbidity currents in oceans. In this model, the near-shore sea bed is preconditioned by the delivery of fresh sediment, either by river inflow or alongshore transport, until it is remobilized by wave action, slope failure or otherwise, and a turbidity current forms. However, the role of transient sediment storage in lakes has received little attention. In this contribution, its potential to precondition the bed for turbidity current generation in the Rhône River-fed lacustrine channel in Lake Geneva will be investigated.A combination of repeat event-wise gridded boat-towed Acoustic Doppler Current Profiler (ADCP) measurements, years-long infrared timelapse camera imagery and months-long moored ADCP measurements were used to study the occurrence of turbidity currents and coinciding changes in the bed depth near the Rhône River mouth at Lake Geneva.The boat-towed ADCP measurements revealed an aggradation over three months and a subsequent degradation over two weeks of up to 11 m near the river mouth. The structure of the bed changes pointed out that they were caused by a progradation and transgression of the delta lip. Timelapse images of the Rhône River plume showed an abrupt plume retreat symptomatic of a sudden slope failure, rather than a gradual erosion of the bed. In fjord systems, such failures have been proven to cause strong turbidity current events. Indeed, in this work timelapse images of a similar abrupt plume retreat were linked to a strong turbidity current event in the Lake Geneva lacustrine channel, establishing a link between transient storage of sediment followed by slope failures, and strong turbidity current generation in lakes.