The sediment budget and dynamics of a delta-canyon-lobe system over the Anthropocene timescale: The Rhone River Delta, Lake Geneva (Switzerland/France)

Deltas are important coastal sediment accumulation zones in both marine and lacustrine settings. However, currents derived from tides, waves, or rivers can transfer that sediment into distal, deep environments, connecting terrestrial and deep marine depozones. The sediment transfer system of the Rhone River in Lake Geneva is composed of a sublacustrine delta, a deeply incised canyon and a distal lobe, which resembles, at a smaller scale, deep-sea fan systems fed by high discharge rivers. From the comparison of two bathymetric datasets, collected in 1891 and 2014, a sediment budget was calculated for eastern Lake Geneva, based on which sediment distribution patterns were defined. During the past 125 years, sediment deposition occurred mostly in three high sedimentation rate areas: the proximal delta front, the canyon-levée system and the distal lobe. Mean sedimentation rates in these areas vary from 0.0246 m yr-1 (distal lobe) to 0.0737 m yr-1 (delta front). Although the delta front–levées–distal lobe complex only comprises 17.0% of the analysed area, it stored 74.9% of the total deposited sediment. Results show that 52.5% of the total sediment stored in this complex was transported toward distal locations through the sublacustrine canyon. Namely, the canyon-levée complex stored 15.9% of the total sediment, while 36.6% was deposited in the distal lobe. The results thus show that in deltaic systems where density currents can occur regularly, a significant proportion of riverine sediment input may be transferred to the canyon-lobe systems leading to important distal sediment accumulation zones