Records of riverbome turbidity currents and indications of slope failures in the Rhone delta of Lake Geneva

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Abstract
The sublacustrine topography of the Rhone delta in Lake Geneva is characterized by a channel that can be traced about 9 km beyond the mouth of the river. Two instrumented moorings were deployed from June to August 1985 in the channel axis at depths of 150 and 201 m to detect underflows of dense, sediment‐laden river water. Each array consisted of three current meters and a thermistor chain. Data were recorded every 15 min (currents) or 30 min (temperatures). During 78 d of effective measurement, 31 well‐defined downslope events were recorded, five with speeds >50 cm s−1. Major events correlate with discharge peaks of the Rhone, and current activity is characterized by single pulses lasting a few hours. Some flow events were accompanied by temperature increases of up to 3°C, suggesting two different current origins: the first is intrusion of sediment‐laden, warm river water into the cold bottom water of the lake as a dense turbidity underflow, and the second is turbidity‐current flow triggered by sliding or slumping of delta deposits. Damage caused to instruments and breakage of mooring links suggests sliding of large amounts of deltaic sediments or avalanchelike turbidity currents.

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