Article

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

A. SILVA, Tiago, et al.

Abstract

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 [...]

Reference

A. SILVA, Tiago, et al. The sediment budget and dynamics of a delta-canyon-lobe system over the Anthropocene timescale: The Rhone River Delta, Lake Geneva (Switzerland/France). Sedimentology, 2019, vol. 66, p. 838-858

DOI : 10.1111/sed.12519

Available at: http://archive-ouverte.unige.ch/unige:107241

Disclaimer: layout of this document may differ from the published version.

1 / 1 Authors post-print, Sedimentology, Vol. XX, XX-XX, 2018 DOI:10.1111/sed.12519; published online: 05.07.2018

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

Tiago A. Silva1,2,*, Stéphanie Girardclos2,3, Laura Stutenbecker4,‡, Marteen Bakker5, Anna Costa6, Fritz Schlunegger4, Stuart N. Lane5, Peter Molnar6, Jean-Luc Loizeau1,2

Abstract 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. Keywords Lake Geneva; Rhone River; sublacustrine delta; sedimentation rates; sediment fluxes; delta front; canyon-levée; distal lobe. 1Department F.-A. Forel for Environmental and Aquatic Sciences, University of Geneva, Geneva, 1205, Switzerland 2Institute for Environmental Sciences, University of Geneva, Geneva ,1205, Switzerland 3Departement of Earth Sciences, University of Geneva, Geneva, 1205, Switzerland; 4Institute for Geological Sciences, University of Bern, Bern, 3012, Switzerland 5Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, 1015, Switzerland 6Institute of Environmental Engineering, ETH Zürich, Zurich, 8093, Switzerland ‡ present adress: Institute of Applied Geosciences, Technische Universität Darmstadt, Darmstadt, 64287, Germany *Corresponding author: [email protected]

1. Introduction by the supply of terrestrial material through underflows that transport and distribute clastic sediments. In marine settings, Sediment accumulation in deltas is a function of sediment underwater deltaic systems fed by high discharge rivers (e.g. supply by rivers. In these environments, the texture, geochem- the Congo and the Yellow Rivers) are also often associated istry (Vogel et al. 2010) and biological characteristics of their with shelf canyons that route terrestrial sediments into deep- deposits reflects changes produced by climatic variability and, sea fan systems, placing these fans among the world’s most where relevant, anthropogenic activities in the associated wa- important sediment depocentres (Talling, 2014). Deltas in tershed (e.g. Anselmetti et al. 2007, Syvitski and Kettner relatively deep lake settings have similar deep sublacustrine 2011, Loizeau et al. 2012,). canyons, such as those first described by Forel (1885; 1888) When underwater deltas are characterized by low gradient for Lake Geneva (Girardclos et al. 2012). delta plains, highly stable channels and high sediment sus- Although canyons are recognized as important sediment rout- pended loads, a delta-fed thalweg and lobe system can develop ing features and distal fans as important sediment depocentres, (e.g. Postma, 1990). These underwater deltas are governed The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 2/22 the quantification of sediment supply and the partitioning of annual discharge of the Rhone is 183 m3 s-1 and the sedi- this flux along a delta-canyon-distal lobe system has not been ment flux varies between less than 0.5 up to almost 5 Mt yr-1 produced at decadal to centennial timescales. Few studies (Loizeau and Dominik, 2000). Most of the sediment input have compared bathymetric datasets from surveys that are from the Rhone River consists of clays to medium-sized silts separated by almost 100 years (e.g. Hickin, 1989; Sabatier et (Kremer et al., 2015). The silty-sandy fraction of this input al., 2006) but were unable to provide that much detail due to accumulates near the river mouth, or at deeper levels on the the small differences and/or the limited quality of the datasets. canyon levées and in the deep lobe area due to underflows In addition, these studies covered areas too small to quantify (Corella et al., 2014, 2016). the partitioning of the supplied sediment between the delta front, the canyon/levée and the distal fan at the same timescale. Thus, there remains a need for a precise budgeting of sedi- ment deposition in similar source to sink systems, which is the scope of this work. This study provides one of the first complete quantitative centennial-scale analyses of a sediment budget in a delta- canyon-distal lobe system. Broadly speaking, this timescale corresponds to the Anthropocene and the Industrial Revolu- tion (Crutzen, 2002). The spatial resolution (50 x 50 m) allows these budgets to be accurately resolved and also to obtain es- timates of associated uncertainties. To this end, we used the oldest complete bathymetric map of Lake Geneva dating from 1891 (data collected in 1886 to 1889) and a recent multi-beam bathymetric map (data collected from December 2012 to De- cember 2013; Sesa, 2014) to determine the thickness of the Figure 1. Location of Lake Geneva in western Switzerland sediment deposits. This was used as basis to calculate a sedi- with its main tributaries and cities. Isobaths between 10 m mentary budget over the last 125 years, while assessing which and 308 m have an equidistance of 50 m. In the eastern zone parts of the system where responsible for sediment storage. of the Grand-Lac, they point to the Rhone River sublacustrine delta and its active canyon. 2. Study area Lake Geneva has a surface area of 580.1 km2 and hosts the largest volumetric freshwater reservoir in Western Europe (89 2.1 Lake Geneva’s lakebed morphology and sedi- km3). The lake is located between the Jura mountain range ment dynamics to the north and the Alps to the south (Fig. 1). The basin un- The oldest complete bathymetric dataset of Lake Geneva (Fig. derlying the lake was formed through glacial sculpting during 2), hereafter called the historical bathymetry dataset, was col- the Pleistocene (Dupuy et al., 2014). lected by Swiss and French engineers (Gosset, Hörnlimann, The lake is divided into two sub-basins: i) the deep basin and Delebecque) between 1886 and 1889 (Fig. S1 in sup- of the “Grand-Lac” to the east between Bouveret and Nyon; plementary material) and subsequently published by Forel and ii) the “Petit-Lac” in the west between Nyon and Geneva. (1892a) and Delebecque (1898). The Grand-Lac, which comprises 86% of the total area of Forel (1885) reported the occurrence of the Rhone River main the lake, is characterized by a large and flat central bottom underwater canyon, which extended for 6 km into the lake, to area at depths greater than 300 m. The Grand-Lac reaches a a water depth of 200 to 230 m, and was between 500 and 800 maximum depth of 309.7 m and has a mean depth of 172 m m wide and 50 to 10 m deep. (CIPEL, 2015). Forel (1888) compared the large underwater canyons in In the eastern part of the lake, between Bouveret and the Lake Geneva and Lake Constance with similar canyons lo- Evian-Lausanne line (Fig. 1), the sublacustrine delta front cated in the vicinity of modern and ancient river mouths in of the Rhone River and the canyon-distal lobe structures are marine settings (for example, the Congo canyon). He sug- the most relevant morphological features of the lakebed. The gested that the Rhone canyon could not result solely from main Rhone canyon extends for 13 km into the deep basin to a erosional processes or morphological inheritance from a for- depth of 300 m. A 300 to 350 m thick sediment sequence com- mer fluvial valley and that the canyon formed through erosion prises the proximal Holocene underwater delta front, whereas by underflows originating from the Rhone River. Forel (1888) the distal canyon-lobe succession is ca 200 m thick (Dupuy et stated that during the summer months, when the Rhone River al., 2014). is fed by cold snow and glacial melt water loaded with sus- The Rhone River is the major tributary to Lake Geneva, con- pended sediments, the density difference between lake and tributing 75% of the total water inflow (Klein, 2016) and 85% river water would lead to plunging of the fluvial waters into of the supplied sediment (Loizeau et al., 2012). The mean the hypolimnion and the export of its suspended sediment The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 3/22

Figure 2. Lake Geneva bathymetry map printed in 1891. The lake bottom topography is represented as altitude m asl and calculated from the bathymetric soundings performed in 1886-1889 by Swiss and French engineers (Forel, 1892a; Delebecque, 1898). A detail from the map shows the Rhone River sublacustrine delta located in the eastern are of Lake Geneva with the main sublacustrine canyon (Forel, 1892a). away from the coast into the lake. flows would plunge onto the lakebed and propagate following Further studies resulted in an improved knowledge on the role the morphology. Mass movements on the delta slope or in the of currents on shaping the canyon of the Rhone River subla- steep canyon walls were also considered as sources for under- custrine delta (Dussart, 1966). These studies also revealed flows that propagate through the canyon. In the same sense, the extension of the main active canyon and the occurrence Corella et al. (2014, 2016) demonstrated that these mass of a depositional fan (Houbolt and Jonker, 1968). Dominik et movements could form thick deposits at the end of canyon, al. (1983) presented the first analysis of the influence of the which could lead to a rerouting of the canyon. Rhone River plume on the distribution of suspended sediment In 2008, a bathymetric survey focusing on the Rhone River in Lake Geneva and, later, Lambert and Giovanoli (1988) mea- sublacustrine delta area was produced with single-beam and sured for the first time underflows in the Rhone main active multi-beam echosounders (Sastre et al., 2010). A 5 x 5 m canyon. These authors traced five flow events in the canyon Digital Elevation Model (DEM) of the lakebed was produced (with flow velocities >50 cm s-1) originating directly from that allowed the description of features on the lakebed of Lake the Rhone River. The event with the highest flow velocity Geneva that had never been described before. Nine (C1 to (>90 cm s-1) could not be directly related to high Rhone River C9) deeply incised, meandering canyons were identified in the discharges; rather, this event was related to the occurrence of lake platform edge (Fig. 3A). Canyon C8 was linked to the a mass movement in the delta front or canyon wall. present inflow of the Rhone River (Fig. 3A), while canyons Giovanoli (1990) proposed the first model for the deposition C6 and C7 were thought to be active during large flood events and distribution of the Rhone River sediment in the lake. The only. The second most important canyon in terms of length model proposed that sediment was distributed by over-, inter- and depth was canyon C5, linked to the mouth of the Vieux- and underflows. Interflow deposits propagated through the Rhone, a former branch of the Rhone River that was active thermocline, and related deposits would dominate the areas of until 1870/1880. The surface of the delta, when undisturbed the lake where there is no direct influence of the sublacustrine by the canyons, displayed current ripple morphologies (Sastre delta or canyon (Giovanoli, 1990). In summer, underflows et al., 2010) (Fig. 3B and C). Along canyon C8, depositional would be produced by the Rhone River due the relatively high overspilling structures were observed in the south-western densities of the suspended sediment-rich flows. These under- levée and in its distal part, gullies were found on both sides of The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 4/22

Figure 3. A) Eastern Lake Geneva’s lakebed morphology. Canyons C1-C9 (after Sastre et al., 2010) point to varying past Rhone river mouthing. The present main active Rhone river sublacustrine canyon is C8. Canyons C6 and C7 are located near the present Rhone River and might still be active during high discharge events of the Rhone River (Corella et al., 2016). B) Detail of the morphology of canyon C8. The canyon head is deeply incised into the lacustrine delta front, with a depth increase from 10 m to 150 m in less than 5 km distance. The north-eastern canyon wall is steeper than its south-western counterpart and presents many scars produced from slope failures (arrow 1). Undulating morphologies (arrows 2) can be observed at the surface of the canyon bed. These morphologies are similar to the cyclic steps produced by density currents in marine canyons interpreted as linked to possible long-duration hyperpycnal flows (Hughes Clarke et al., 2012; Covault et al. 2017). C) Detail of levées morphology in canyon C8 between 200 and 250 m of depth. The undulating surface of the levées, that can be 0.2-2 m high and 15-100 m long (Girardclos et al., 2012), are interpreted as products of turbidity currents overspilling from the canyon (arrows 1 and 2). Arrows 2 show sediment waves developing on the concave side of the canyon bends indicating possible flow stripping. The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 5/22 the canyon (Fig. 3C, arrows 1 and 2). A final affine transformation was applied to the initial scanned Studies of sediment accumulation rates in Lake Geneva image to scale and stretch the lake map, resulting in an RMS were begun by Krishnaswamy et al. (1971) by applying, for (root mean square) error of ±24.9 m. The RMS error is a mea- the first time, radiometric dating techniques (210Pb and 137Cs sure of the differences between the calculated coordinates and isotopes) to lake sediments. This stimulated further studies those observed, hence representing the uncertainty of the map focusing on the age and rate of deposition of sediment in positioning. This uncertainty is probably due to distortions Lake Geneva and exploring its distribution on the lake basin of the printed map and to the photolithography transfer, but a (e.g. Favarger and Vernet, 1979; Vernet et al., 1975, 1984; possible lack of precision of the initial 1891 map cannot be Loizeau, 1991, 1998; Loizeau et al., 1997, 2012). In addi- excluded. An additional geographical uncertainty of ±9 to 17 tion, researchers correlated patterns of measured magnetic m is due to the thickness of the drawn lines and points. Thus, susceptibility (Loizeau et al., 1997) to produce a rough esti- considering all the above, the estimated location error of the mate of the sediment storage in Lake Geneva since ca. 1950. mapped objects for the historical bathymetry dataset is about Notably, Loizeau et al. (1997) demonstrated that sedimenta- ±42 m. After assessment, by comparison to other georefer- tion rates in eastern Lake Geneva decreased in the mid-1960s enced maps, altitude data were converted into depth values after the construction of hydropower dams and the onset of using a lake level of 372.04 m asl. Finally, after extensive hydro-management practices in the Rhone River watershed. manual cleaning of imperfections of the acquired data (i.e. Loizeau and Dominik (2000) estimated that sedimentation removing isolated pixels, improving line definition), contours rates in Lake Geneva decreased by a factor of two in response and points were vectorized using the automated vectorization to these anthropogenic impacts. tool in the ArcScan toolbar.

3.1.2 Multi-beam bathymetric dataset 3. Datasets and Methods The recent dataset (hereafter called multi-beam bathymetry 3.1 Bathymetric datasets dataset) was produced with a multi-beam echosounder in a In this study, the results of two bathymetric surveys of the joint survey by the University of Geneva and the University lake, conducted in the 19th and 21st Centuries, were compared of Bern. Data were acquired between December 2012 and within a GIS environment (ArcMap 10.1®). This data was November 2013 using a Kongsberg EM2040 echosounder used to map and quantify how the sediment supplied by the operated at 300 kHz, providing a beam width of 1° x 1°. The Rhone River was routed and partitioned in Lake Geneva over position of the boat was determined using a Leica GX1230 the past 125 years. Positional and elevation errors for the RTK-GNSS receiver, a Trimble SPS361 GPS compass and a comparison of the datasets are discussed in detail in the last Kongsberg Seatex MRU5+ motion sensor. Sound velocity at paragraph (Error estimation) of this section. the water surface was continuously measured throughout the survey with a Valeport Mini SVP probe. The survey resulted 3.1.1 Historical bathymetric dataset in a raster grid dataset with a 2 x 2 m cell size for the whole In the present study, a photolithographic reduction at a 1:50 Lake Geneva for depths greater than 4-5 m. The data was 000 scale, is used. The data, edited in 1891 by the Swiss Fed- projected onto the Swiss Grid coordinate system (CH1903+). eral Office of Topography and printed by Kümmerly Frères in The grid depth data was calculated using the conventional Bern, had been presented by Forel (Fig. 2). The map displays lake level of 372.04 m asl as zero-metre depth. the original bathymetric soundings points and the inferred contours as 10 m equidistance isohypses. In areas of more 3.2 Digital elevation models complex bathymetry, the equidistance is reduced to 5 m. Only The height points and contour lines from the historical bathy- 1176 points, out of the original 11955, are labelled with their metric dataset were used to construct a Triangulated Irregular altitude. The map data is expressed relative to the ‘Repère Network (TIN). The irregular distribution of the nodes allows de la Pierre du Niton’ (RPN), which serves as the altitudinal the TIN to accurately represent areas where the topography reference level in Switzerland, and was defined as 376.86 m is complex (Lane, 1998). However, TINs may also produce above sea level (asl) in 1891 (Federal Office of Topography, artificial flat areas resulting from the distribution of points. 2017). In 1902, the RPN was corrected to 373.60 m asl, and Because of this limitation, cross-validation of the produced thus a value of 3.26 m was subtracted from the altitudinal val- TIN was performed. Contour lines were computed from the ues of the historical map data to correct them into the current TIN and compared with the original contour lines from the altitudinal system. historical bathymetric map. Erroneously flat areas were man- A printed version of the 1891 map (Fig. 2) was scanned at a ually corrected by adjusting the TIN nodes so that they better resolution of 400 dpi, representing a pixel size of 2.15 m. The reproduced the original geometry. Finally, the corrected TIN resulting TIFF file was loaded into a ArcGIS and georefer- was converted into a Digital elevation model (DEM) with a enced. Points of known coordinates, given by the Swiss grid 50 x 50 m cell size (Fig. 4A). of the map, were used to make a first approximate georeferenc- As the multi-beam bathymetric dataset produced a 2 x 2 m ing. Notable lakeshore points, such as harbours that still exist cell size raster, a ‘nearest point’ resampling to a 50 x 50 m today, were added later to refine and improve the positioning. cell size was necessary to compare both datasets (Fig 4A and The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 6/22

B). Although a considerable loss of information resulted from estimate of this error can be obtained by using the modern the increased cell size, this step was necessary to produce multi-beam bathymetric map data to estimate the likely sub- comparable datasets. grid scale error due to interpolation to the 50 x 50 m DEM The thickness of sediment deposited between the two bathy- (σi; Eq. 3). metric surveys was calculated by subtracting the multi-beam The errors associated with the multi-beam bathymetric map bathymetry DEM from the historical bathymetry DEM. This are much better constrained. The standard deviation of errors difference was then transformed to an annual sedimentation due to the positioning of the boat (σB; Eq. 3), linked to the rate through division by the time span between the surveys DGPS, are of the order of ±0.03 m and ±0.05 m for the hori- (125 years). zontal and vertical positioning, respectively. If the same logic as used above is applied, these errors are clearly small and 3.3 Volume and mass of deposited sediments again much smaller than the likely subgrid scale error due to The volume of sediment in each cell was calculated using interpolation to the 50 x 50 m DEM (σi), an error that also the thickness of the sediment cover and the cell size of the applies to the multi-beam bathymetric data as it was used. raster as basis. The density of in situ dry sediment (ρinsitu The error of the bathymetric measurements linked to the multi- (dry)) was calculated from the porosity (θ, Equations 1 and beam echosounder (σB) was calculated to be smaller than 0.05 -3 2) with a particle density of 2600 kg m (ρsed) and a density m and between ±0.2 and ±0.3 m for shallow and deep flat -3 of pore water of 1000 kg m (ρwater). The mean bulk density areas, respectively (Sesa, 2014). The maximum value of ±0.3 of sediment sequences (ρinsitu (bulk)) was calculated from m was thus used, which likely results in an overestimation of measured density using a Multi-Sensor Core Logger (MSCL) the error. in sediment intervals from five cores (Fig. 5 and Table S1, in On this basis, the following errors were propagated here: (i) supplementary materials). the error in the historical vertical soundings, σH ; (ii) the error in the multi-beam echosounder vertical soundings, σB; and ρinsitu(bulk) − ρsed θ = (1) (iii) the error in the interpolation to a 50 x 50 m resolution (σi) ρwater − ρsed that has to be applied twice, as both datasets are interpolated to this resolution. Given these constraints, the level of detec- tion for change at the 95% level (LoD95) can be calculated ρinsitu(dry) = (1 − θ)ρsed (2) from Eq. 3 (Lane et al., 2003):

2 2 2 0.5 3.4 Error estimation LoD95 = 1.96[σH + σB + 2σi ] (3) Error estimation is essential in order to distinguish between real elevation differences produced by aggradation/erosion The values of σi and σB were taken to be spatially uniform. of sediment over the past 125 years and artefacts due to un- The values of σi were calculated from the standard deviation certainties in position or altimetry in both the historical and of altitudes found in each grid cell within the 50 x 50 m DEM. multi-beam bathymetric datasets. Using the results of Equation 1, it is also possible to calculate Different types of errors dominate the two bathymetric maps the total sediment volume uncertainty (Vu, Equation 4). For presented in this study. For the historical bathymetric map, any grid cell, the volume of change is RdZ, where R is the grid Forel (1892b) reported three different sources of errors: (i) the resolution (50 m). Equation 4 gives the volumetric uncertainty exactitude of the measurements taken by the sounding equip- (Vu) at the 95% confidence level as: ment; (ii) errors in the reading and positioning of the sounding 2 0.5 points in the bathymetric map; and (iii) errors produced by Vu = R [∑LoD95] (4) the leeway of the boat or the steel wire during soundings. A fourth source of error, related to the use of a 50 x 50 m 4. Results resolution DEM and the subgrid resolution variability in ele- vation, should additionally be considered. The first source of 4.1 Lake Geneva bathymetric comparison and its error was assessed by using two different types of sounding limitations equipment. Measurements in the central plain of the lake (at The comparison of the historical and multi-beam bathymetry 309 m of depth) with both pieces of equipment placed side DEMs allowed the quantification of the thickness of the de- by side (Forel, 1892b) resulted in a maximum error of 0.3 posited and eroded sediment over the last 125 years (Fig. 6A). m in the historical bathymetry data (σH ; Eq. 3). The sec- Positive values in lakebed variation represent sediment de- ond and third errors lead to planimetric and hence to vertical position and negative values indicate erosion. Although the uncertainties, respectively, where the topography is locally complete lake map shows a complex pattern of aggradation steep. However, the uncertainty in lateral boat positioning is and erosion (Fig. 6A), a modal value of +0.29 ± 4.35 m substantially lower than the eventual DEM resolution used, 50 indicates that most of the lakebed surface was aggradational m x 50 m, making the fourth error source dominant. The latter in the past 125 years. is likely to be spatially variable as a function of topographic The overall analysis shows that in lake areas where vertical complexity, but it is also not known for the Forel data. An changes appear relatively small (i.e. thickness values between The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 7/22

Figure 4. Bathymetric raster maps of Lake Geneva produced from the historical (A) and recent multi-beam (B) data. A) Bathymetric map interpolated with data collected between 1886/89. Only the active canyon of the Rhone delta (C8) is represented; B) Bathymetric map interpolated with data collected in 2013/14 with a multi-beam echosounder. The full bottom coverage of this acquisition method reveals a total of nine canyons in the Rhone delta. At this scale, only canyons C8 and C5 are clearly visible.

-0.5 and ±0.5 m), the resulting complex aggradation and ero- than 200 m are scattered throughout the lake and do not follow sion pattern reveals the limits of the applied method. Although any apparent regular pattern. Three arguments point to the the work conducted at the end of the 19th Century was very observed ‘background’ erosion as resulting from differences precise, the density of the measurements per surface unit is in measurements of the height of the lakebed: (i) the small ca. 10 000 times lower than the multi-beam bathymetry (20 magnitude of background erosion, (with a maximum value soundings per km2 and 250’000 points per km2, respectively). of -0.042 m yr-1); (ii) the great depths at which they occur; The present comparison thus leads to the identification of ero- and (iii) the apparent disassociation from sedimentological sional areas that are artefacts due to the different resolutions processes that could produce such a chaotic pattern of erosion of the original bathymetric datasets. Erosional areas have a and deposition. mean value of -2.5 m and a maximum value of -67.25 m (Fig. The same explanation may apply to the high values of sed- 6A). Highest erosion rates are found on the lake slopes, in iment erosion values in the canyons of Lake Geneva (Fig. the canyon beds and on the delta front. These might result 6B). In the 1889 map, the inactive canyons (C1 to C4, C6 to from positioning errors during acquisition of historical data C7, C9; Fig. 3A) were not detected and only the C8 (active) or, in the case of marginal slopes, from actual changes such and C5 canyons are clearly drawn (Fig. 4A). As the inactive as mass movements. Erosional areas found at depths greater canyons formed before the end of the 19th Century (Kremer et The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 8/22

Figure 5. A) Location and bathymetry map of eastern Lake Geneva with long sediment cores locations. B) long sediment cores TS-K1 to K6 with core picture, lithology profile and density curve. The density measurements (retrieved with MSCL-S core logger) were used to calculate a mean density (red slashed line) to the sediment sequence deposited in the last 125 years, that were assigned to each of the sedimentation areas (Area I to IV) using the closest core location(s). al., 2015), their absence in the historical map emphasizes the 4.2 Sediment thickness and sedimentation rates in problem of the limited resolution of the historical DEM. This eastern Lake Geneva is also true in canyons C8 (active) and C5, but in these chan- In contrast to the low sediment accumulation areas recorded nels underflows may be partly responsible for the observed over the western half of the lake, eastern Lake Geneva (black erosion (Girardclos et al, 2012). contoured surface, Fig. 6A) shows a more consistent thickness map, with a mean value of +2.31 ± 0.18 m (Table 1). A The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 9/22

Figure 6. A) Lake Geneva sediment thickness map showing the complex spatial distribution of sediment deposited in the last 125 years (1886/89-2012/13). The map shows a chaotic pattern of aggradation and erosion in most parts of the lake where sedimentation is low. Eastern Lake Geneva (black outline) points to a much clearer pattern of sedimentation. B) Area of sediment deposit thickness analysed in this study. Most of the study area aggraded but artefacts of erosional areas appear on the floor of inactive (black arrows, see text). detailed analysis is thus restricted to this part of the basin. width of ca 500 m (Area II - Fig. 9). This high sedimentation The map of sediment thickness (Fig. 6B) in eastern Lake area covers 8.18 km2 and presents a mean deposit thickness Geneva was converted into a sedimentation rate (SR) map of +4.94 ± 0.08 m (Table 1). Thickness values decrease (Fig. 7). The analysed area shows a mean SR of 0.0185 ± gradually away from the C8 canyon axis on both sides. Such 0.0014 m yr-1 (Table 1). enhanced levée sedimentation is only mapped along the C8 canyon, confirming that the other canyons were relatively 4.2.1 Rhone delta front (Area I) inactive during the past 125 years (Sastre et al., 2010). ± Maximum sediment aggradation (+52.8 3.35 m) occurred Mean SR in Area II is +0.0259 ± 0.0006 m yr-1, with a ± near the Rhone river mouth with a mean value of +9.22 0.16 maximum of +0.17 ± 0.01 m yr-1 (Table 1, Fig. 9). The m (Area I; Table 1). This 6.17 km2 zone has a tongue-shaped maximum SR in this area is found where the comparison morphology (Fig. 8) which develops between the C8 and C5 of the historical and multi-beam bathymetric maps shows a canyons and extends 4 km lakeward (Area I; Fig. 8). difference in the position of the active canyon wall (Fig. 9, ± -1 Mean SR in Area I is +0.0737 0.0013 m yr , with a maxi- red arrow). mum of +0.42 ± 0.03 m yr-1 (Table 1). Sedimentation rates decrease away from the mouth of the Rhone River mouth (Fig. 4.2.3 Rhone distal lobe (Area III) 8) and areas surrounding inactive canyons experience much lower SRs or even net erosion. A third area of high sediment accumulation (Area III; Fig. 10) indicates mean deposit thickness of +5.06 ± 0.02 m and a 4.2.2 Rhone canyon and levées (Area II) maximum accumulation thickness of +9.8 ± 8.2 m (Table 1) at Between water depths of 190 and 250 m, thick sediment the terminal part of the C8 canyon where water depth is greater sequences (i.e. maximum of +21.9 ± 1.24 m) accumulated than 250 m. This area has an elongated lobe-shaped morphol- along the levées of the Rhone main active canyon (C8) with a ogy, interpreted as a lacustrine form of classical deepsea fans The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 10/22

Figure 7. Map of mean sedimentation rate (SR) during the last 125 years (1886/89-2012/13) in eastern Lake Geneva. Three main areas of high sedimentation (Areas I to III) are outlined with black, red and grey colours, respectively. Highest sedimentation rates are found in Area I, close to the Rhone River mouth. Another important area of sedimentation appears at the end of the main active canyon C8 (Area III).

Table 1. Thickness of sediment (m) deposited/eroded for total surface and each of the Areas (I to IV) defined in eastern Lake Geneva. Minimum, maximum and mean Sedimentation Rates (SR; m yr-1) were also calculated for each area. Both errors and standard errors for the means are indicated for a 95% level of confidence. Area Sediment deposit thickness Sedimentation rate (SR) (km2) (%) (m) (m/yr) Min Max Mean Min Max Mean Area I 6.17 4 -16.1 (± 18.03) 52.8(± 335) 9.22 (± 0.16) -0.12 (± 0.14) 0.42 (± 0.03) 0.0737 (± 0.0013) Area II 8.18 5 -24.9 (± 7.45) 21.9 (± 1.24) 4.94 (± 0.08) -0.19 (± 0.06) 0.17 (± 0.01) 0.0259 (± 0.0006) Area III 12.39 8 -1.60 (± 3.22) 9.8 (± 8.22) 5.06 (± 0.02) -0.01 (± 0.03) 0.08 (± 0.06) 0.0246 (± 0.0001) Area IV 131.62 83 -27.7 (±15.45) 26.1 (± 6.97) 1.11 (± 0.01) -0.22 (± 0.12) 0.21 (± 0.06) 0.0090 (± 0.0008) Total 158.36 100 -27.7 (± 15.45) 52.8 (± 3.35) 2.31 (± 0.18) -0.22 (± 0.12) 0.42 (± 0.03) 0.0185 (± 0.0014)

Figure 8. Detail map of SR near the Rhone River delta front (Area I, black contour line). The mean SR in this area is +0.0737 m yr-1, with a maximum of +0.42 m yr-1 (Table 2). Values decrease away from the Rhone River mouth. Red arrows are pointing to erosion artefacts.

(for example, the Rhone River neofan in the Mediterranean short-axis for a total surface area of 12.39 km2. In this lobe, Sea – Bonnel et al., 2005), with a 6 km long-axis and a 2.5 km thickness values decrease away from the pathway of an older The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 11/22

Figure 9. Detail map of SR along the Rhone canyon levées (Area II, red contour line). Mean SR in this area is +0.0259 cm yr-1, with a maximum of +0.17 m yr-1 (Table 2). The maximum SR is found in a part of the canyon where the comparison of historical and current bathymetric maps shows a difference in the position of the active canyon wall (red arrow). Sedimentation rates decrease distally and with increasing distance to the canyon axis.

C8 canyon end (Fig. 10, black dashed line 1) found 270 m to LoD95, are mainly artefacts due to the absence of the ancient the north of the present termination of the canyon (Corella et canyons in the historical dataset. The results for this area al., 2014, 2016). were validated by comparing the same area with the recent The sediment deposit in the northern part of the distal lobe is (post-1960) mass accumulation rate map proposed by Loizeau thicker than its southern part. The axis of the thickest part of et al. (2012), based on a collection of dated sediment cores. the deposit is found 730 m away (Fig. 10, red dashed line 3) Using results from Loizeau et al. (2012), it was calculated from the axis of the present-day end of the canyon (Fig. 10, that 87.7 Mt of sediment was deposited in Area IV in the last black dashed line 1). 125 years, which is in reasonable agreement with the results In the Rhone sublacustrine distal lobe (Area III), the mean SR obtained in the present study for Area IV (62.4 ± 7.7 Mt). is +0.0246 ± 0.0001 m yr-1, with a maximum of +0.08 ± 0.06 Overall, these results reflect the sediment flux to each zone m yr-1 (Table 1). Sedimentation rates in this area follow a and point to the relative importance of the various sedimentary decreasing trend towards the west (i.e. away from the canyon environments in the routing system of the sublacustrine Rhone apex, Fig. 10). delta. Highest yields occurred in the delta front (Area I) with 72.2 ± 0.9 kt yr-1 km-2, followed by the distal lobe (Area 4.3 Sediment budget in eastern Lake Geneva III) with 58.7 ± 0.7 kt yr-1 km-2 and, lastly, the canyon and The comparison of the historical and multi-beam bathyme- levées (Area II) with 38.8 ± 0.5 kt yr-1 km-2 (Table 2). The tries suggests that 22.37 ± 0.14 x 107 m3 of material were remaining relatively low sedimentation Area IV has a mean deposited within the study area over the last 125 years, corre- yield of 3.8 ± 0.1 kt yr-1 km-2. sponding to 248.7 ± 18.4 Mt of sediment (Table 2). Sediment volume and mass were calculated for the high sedi- 5. Discussion mentation Areas I, II and III (i.e. delta front, canyon-levées, distal lobe), representing the sublacustrine sediment routing 5.1 Sedimentological processes at the Rhone delta system of the Rhone delta, as well as for the remaining Area front IV (‘rest’; Table 2; Fig. 6B). Although the three high sedi- The Rhone delta front (Area I) stored 55.7 ± 6.9 Mt of sedi- mentation areas account for only 17% (26.74 km2) of the total ment (22.4% of the total), over the last 125 years. Sediment analysed area, they stored 74.9% (186.3 ± 16.7 Mt) of the accumulation resulted from the deposition of bedload sedi- sediment deposited (Table 2); the large remaining part of the ment when the Rhone River decreases its velocity upon en- studied area (131.62 km2, 83% of the study area) accounted tering the lake, inducing the rapid decrease of sedimentation only for 25.1% (62.4 ± 7.7 Mt) of the total sediment mass rates lakeward and producing the lobe-shaped distribution of (Table 2). The calculation of the sediment budget for Area sediment near the Rhone mouth (Fig. 8). The coarser frac- IV disregarded values of erosion that were greater than -5 tion of the suspended load will also be deposited from the m. In this area the highest erosion rates, which are above the settling of the dilute surface river plume (Hizzett et al., 2018). The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 12/22

Figure 10. Detailed map of SR in the distal lobe at the end of the main active Rhone canyon (Area III, grey contour line). Mean SR in this area is +0.0246 m yr-1, with a maximum of +0.08 cm yr-1 (Table 2). A former canyon apex (dashed line 1) existed until ca. 2000 AD (Corella et al. 2014; 2016) northwards of the current canyon apex (dashed line 2). The north part of the lobe (dashed line 3) is interpreted as linked to the former apex (dashed line 1). The previously unknown south lobe (dashed line 4) might be linked to an even earlier phase of the canyon.

Table 2. Surface, dry in situ density, sediment volume, sediment mass and sediment flux for each depositional area (Area I to IV) and the entire studied surface. Although high sedimentation areas (I to III) comprise only 17% of the total analysed area, they stored 74.9% of the total sediment mass. Both errors and standard errors for the means are indicated for a 95% level of confidence. HSR = high sedimetation rates Area Density Sediment volume Sediment mass Sediment flux Specific sediment flux (km2) (%) (g cm-3) (*107 m3) (%) (Mt) (%) (kt yr-1) (kt yr-1 km-2) Area I (delta front) 6.17 4 0.98 (±0.12) 5.68 (±0.07) 25.4 55.7 (±6.9) 22.4 445 (±5) 72.2 (±0.9) Area II (levées) 8.18 5 0.98 (± 0.12) 4.05 (± 0.05) 18.1 39.7 (±4.9) 15.9 318 (±4) 38.8 (±0.5) Area III (lobe) 12.39 8 1.45 (± 0.23) 6.27 (± 0.02) 28.0 90.9 (±14.4) 36.6 727 (±12) 58.7 (±0.7) Area IV (rest) 131.62 83 0.98 (±0.12) 6.37 (±0.11) 28.5 62.4 (±7.7) 25.1 499 (±6) 3.8 (±0.1) Area of HSR (I+II+III) 26.74 17 - 16.00 (±0.09) 71.5 186.3 (±16.7) 74.9 1490 (±13) - Total 158.36 100 - 22.37 (±0.14) 100.0 248.7 (±18.4) 100.0 1989 (±15) 12.6 (±0.1)

The development of the delta front lobe towards the north part of the hemipelagic sedimentation that occurs through the of the river mouth, rather than perpendicular to the coast, is entire lake basin. interpreted as due to the deviation of the river plume by the Some of the sediment deposited in the delta front would also Coriolis force (Dominik et al., 1983). Finer suspended parti- be exported, on occasions, to distal zones of the lake basin cles may also be dispersed in the lake by either overflows or due to density flows resulting from mass moments (Fig. 3B interflows that propagate at the thermocline of the lake body. and 8). Such turbid plumes have been measured in the lake for 7 to 30 km and at depths from 15-30 m (Dussart, 1948; Dominik 5.2 Sedimentological processes in the active Rhone et al., 1983). This process occurs when the lake waters are canyon/levée area stratified, which they normally are between the spring and The levées of the main active Rhone canyon (C8, Fig. 9) autumn months (March to October) (Savoye et al., 2014). The stored 15.9 % of the sediment during the past 125 years (39.7 presence of this plume, originating directly from the Rhone ± 4.9 Mt). Sediment aggradation in this area is mainly due River, has been demonstrated by the isotopic signature of the to the deposition of overspill sediments transported by un- water column (Halder et al., 2016). Sturm and Mater (1978) derflows. There are two main overspill mechanisms: 1) flow described the same mechanisms of deposition of sediment stripping, which occurs in sharp bends in the canyon channel in Lake Brienz, another peri-Alpine lake that receives sedi- driven by centrifugal forces (Komar, 1973; Piper and Nor- ment from two Alpine rivers. The dispersion of terrigenous mark, 1983; Hay, 1987) or; 2) flow spilling, which occurs sediment coming from the Rhone by over- or inter-flows is a when the flow thickness is greater than the bankfull depth The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 13/22

(Hiscott et al., 1997). ca. 2000 AD, filled the previous channel course in the deep Sediment waves and gullies in both canyon levées (Fig. 3C, Rhone fan (Corella et al., 2016) inducing the formation of arrows 1 and 2) are a morphological indication that over- a new channel mouth (Fig. 10, black dashed line 2) that is spilling occurs in the C8 canyon (for example, Damuth, 1979; currently active. As the mapped northern lobe (Fig. 10, red Normark et al., 1980, 2002; Carter et al., 1990; Savoye et dashed line 3) corresponds to the same location as ‘the pre- al., 1993; Nakajima et al., 1998; Migeon et al., 2000, 2004; 2000 AD event’ channel mouth (Fig. 10, black dashed line Wynn et al., 2000; Covault et al., 2017). Both processes of 1), it is likely that this channel termination must have been overspilling seem to occur, as suggested by: (i) increased sedi- previously (relatively) stable. An additional but smaller lobe, mentation rates on both sides of the canyon (flow spilling, Fig. present in the south-east part of the fan (Fig. 10, red dashed 9); (ii) increased sedimentation rates in the concave bend of line 4), points to a possible, and previously unknown, position canyon meanders (flow stripping, Fig. 9) and; (iii) sediment of the canyon end during the past 125 years. waves and small gullies exiting the levées at canyon bends (flow stripping, Fig. 3C, arrows 1 and 2). Flow spilling can only occur when the thickness of the tur- 5.4 Sediment budget, spatial distribution and fluxes bidity current is greater than the depth of the channel (Pirmez in eastern Lake Geneva and Flood, 1995; Hiscott et al., 1997). This implies that The comparison of the bathymetric datasets (Fig. 4A and B) flows propagating through the Rhone canyon would have to reveals that 248.7 ± 18.4 Mt of sediment was deposited in the be thicker than the canyon depth. Levee sedimentation along analysed area in the last 125 years (Table 2), which translates -1 the canyon appears after a significant break in the canyon bed to a mean sediment flux of 1.99 ± 0.15 Mt yr and a mean -1 -2 slope, from 4% to 1% (Fig. S3, supplementary materials), specific sediment flux of 12.6 ± 0.1 kt yr km . The calcu- and a decrease in the canyon depth from 30 m, between 50 to lated sediment flux lies in the same order of magnitude of the 190 m water depth, to 20 m, between 190 m to 250 m water mean sediment suspended load in the Rhone River (2.05 Mt -1 depth (Fig. S3). Accordingly, under the assumption that the yr ; computed by Loizeau and Dominik, 2000), supporting canyon bed depth was stable over the past 125 years, only the results of sediment budgets calculated in this study for the density flows thicker than 20 m would be able to shape the eastern part of Lake Geneva. distal levée deposits. This is in agreement with thicknesses of In the analysed area, 74.9% (186.3 ± 16.7 Mt) of the sediment density flows in other lakes, where reported values vary from was deposited in the three high sedimentation areas (Table 2). 16 m to 50 m (for example, Lambert et al., 1976; Lambert Similar sediment routing patterns have been reported for other and Hsu, 1979; Pharo and Carmarck, 1979; Weirich, 1984, large peri-alpine lakes (for example, Lake Brienz, Sturm and 1986a, b; Best et al., 2005; De Cesare, 2006; Gilbert et al., Matter, 1978; Lake Constance, Wessels et al., 2010, 2015) 2006; Girardclos et al., 2007). In marine settings (for example, and marine environments (for example, the Mediterranean Gaoping and Var canyons and Cariaco basin) thicknesses of Rhone River delta-canyon-fan system, Droz and Bellaiche. density flows are larger and reported values range between 30 1985; Bonnel et al., 2005; Fanget et al, 2014). and 300 m (Thunnell et al., 1999; Khripounoff et al., 2009; In previous studies, related sedimentation areas have roughly Liu et al., 2012). been delimited based on morphological considerations (for example, Giovanoli, 1990; Loizeau 1991) and extrapolations of sediment fluxes, which, in turn have been determined on 5.3 Sedimentological processes in the Rhone dis- the basis of sedimentological data collected from dated short- tal lobe sediment cores (Loizeau et al., 2012). The interpretation of The sediment deposited in the Rhone distal lobe (Fig. 10 and these results was limited because radiometric dating methods Table 2 – 90.9 ± 14.4 Mt, 36.6% of the total) is interpreted as only allowed extrapolations to the past 70 years (137Cs activ- resulting from sediment transported by underflows along the ity detection in sediments starts in 1954 and 210Pb dating is sublacustrine canyon, bypassing proximal, shallower areas. not efficient in high sedimentation rates) and coring in high The sediment transported could originate directly from the sedimentation areas is very difficult due to the presence of Rhone River (with plume settling or supply by hyperpycnal coarser-grained sediments. Stratigraphic correlation between flows), or from landslide-triggered flows. Alternatively, the the sediment cores in high sedimentation areas is generally material could also derive from the resuspension of sediment unachievable because of the stochastic nature of sedimentolog- through the erosion of the canyon axis or delta-front by under- ical processes that generate the sediment sequence (Loizeau et flows. al., 1997). In this sense, the results of the present study present When the canyon topographic relief vanishes, these flows be- a detailed sediment budget for this part of the lake for the past come unconstrained leading to the formation of a lobe-shaped 125 years. The details of these results also allowed, for the deposit, where the thickest sediment sequences are deposited first time, the quantification of the sediment distribution on close to the end of the canyon. the delta front-canyon-lobe system (Fig. 11) for the same Variations in deposit thickness are related to shifts in the ter- timescale (Jobe et al., 2018), revealing the great efficiency minal position of the canyon (Girardclos et al., 2012; Corella of this system in exporting large quantities of sediment into et al., 2014; 2016). For example, a large mass flow, dated to distal parts of the lake basin through the active Rhone canyon The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 14/22

(C8). From the total mass of sediment deposited in the high sed- imentation areas (Table 2), a total of 52.5% (130.6 ± 15.2 Mt) was stored in the canyon-levée complex and in the dis- tal lobe. This highlights the Rhone canyon as a preferred conduit for sediment transfer to distal parts of the lake. This percentage also emphasizes the role of density currents in sediment transport in these systems. Other rivers in Alpine lakes (for example, the Breggia and Greggio river deltas in Lake Como, Italy) that have no canyons appear to deposit virtually all their sediment in their delta fronts and prodeltas (Fanneti and Vezzoli, 2007). Deltas in marine settings can also deposit most of their sediment in the delta front/prodelta area. Hickin (1989) reported that the Squamish delta front/prodelta area, which accounted for only 30% of the basin area, stored 98.4% of the sediment supplied to the estuary. Mean sediment accumulation rates of 0.20 ± 0.05 m yr-1 in the Squamish delta front/prodelta areas are in the same order of magnitude as the sediment accumulation rate encountered for the Rhone lacustrine delta front (Table 1). Sediment load in Area IV (6.24 ± 7.7 Mt - Table 2 and Fig. 11), accounts for 25.1% of the sediment mass stored in the study area. Sediment in Area IV is composed of fine clastic sediments and autochthonous particles (biogenic carbonates, diatom frustules, etc.) (Loizeau et al., 2012). The contribution of autochthonous particles to the total sediment budget in the lake is marginal, corresponding to an estimated flux of about 0.3 kt yr-1 km-2 (data from Loizeau et al., 2012) compared to the mean specific flux of 12.6 ± 0.1 kt yr-1 km-2 (Table 2) over the entire study area. Thus, the sediment flux in Area Figure 11. Diagram representing the Rhone River catchment IV is mainly composed of fine-grained particles supplied by and its sediment routing system in Lake Geneva. The three rivers and dispersed in suspension as “hemipelagic” material. high sedimentation areas are represented (delta front, Although the calculated sediment flux in this area is one or- canyon/ levées and distal lobe) with their percentage of the der of magnitude smaller than that calculated for the high accumulated sediment mass (black numbers) and their sedimentation rate areas, the significant percentage of stored respective specific sediment flux in kt yr-1 km-2 (italic bold sediment reflects the large area of Area IV (131.62 km2, 83% grey numbers). of the study area). However, this is only a minimum value * The lake bed value represents background of the studied because in Area IV, the areas where erosion is less than -5 m area and doesn’t include the whole Grand-Lac basin. have been taken into account (Figs. 6A, 6B and Fig.7) but re- lated erosional estimates could be artefacts from comparisons between the datasets. river mouth (Fig. 7). In marine settings where the head of the canyon is connected or close to a river mouth (the Zaïre, 5.5 Rhone delta front-canyon-lobe system in Lake Magdalena, Bengal and Gaoping rivers - Weber et al., 1997; Geneva – a small scale analogue for an oceanic Babonneau et al. 2002, Khripounoff et al., 2003; Hsu et al., sediment routing system? 2008; Carter et al., 2012; Cooper et al., 2013), hyperpycnal This study shows that the Rhone distal lobe is similar to the flows can form when river discharge has suspended sediment depositional lobes found in lower fan systems, while overspill concentrations greater than 36 kg m-3, while in lacustrine and levée deposits correspond to the structures recognized in settings the suspended sediment concentrations can be much middle fan reaches (Shanmugam and Moiola, 1988; Pickering smaller (Mulder et al., 2003). Thus, hyperpycnal flows should and Hiscott, 2015). Upper fan environments in marine set- occur more frequently in the Rhone sublacustrine delta than tings are characterized by canyons with steep and high walls in marine settings, possibly increasing their impact on the that do not allow the overspill of the density flows within. sediment export into distal areas. In Lake Geneva, Lambert This corresponds to the canyon head area in Lake Geneva, and Giovanoli (1988) observed 16 major turbidity events in where the walls of the canyon are high, and no significant the Rhone canyon, in 78 days. This observation reinforces the sediment accumulation is observed on the levées close to the importance of river-induced turbidity currents with unknown The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 15/22 initial processes (i.e. possibly hyperpycnal flow, plume set- area but have stored 74.9% (186.3 ± 16.7 Mt) of the total tling, proximal sublacustrine landslide) in lacustrine settings. sediment input, demonstrating their high sediment storage ca- In marine settings, Mulder et al. (2001) calculated the relation- pacity and the importance of sediment supply. Of this 74.9%, ships between water fluxes and the occurrence of hyperpycnal a total of 52.5% (130.6 ± 15.2 Mt) of the sediment has been flows for the Var River over a period of 49 years. They found stored in the canyon-levée and distal lobe areas. Sediment that only 9 to ten events presented water fluxes large enough deposited in these areas has mainly been transported by un- to trigger these flows, corresponding to a recurrence frequency derflows through the canyon, suggesting that canyons are of ca. 5 years. In a related study, in the Gaoping River system, preferred sediment conduits and that they can bypass high Carter et al. (2012) and Liu et al. (2012, 2013) reported that quantities of sediment to the distal parts of marine/lacustrine underflows have been linked to peaks in river discharge during basins. The pattern of sediment accumulation shows that the typhoons. Rhone River sublacustrine routing system is comparable to Some morphological features of the Rhone canyon (for ex- marine deep-sea fans fed by canyons linked to major rivers ample, canyon sinuosity and periodic bedforms in the canyon (i.e. Zaïre, Magdalena, Bengal and Gaoping rivers). In both bed; Fig 3A and B), are also observed in marine canyons (for marine and lacustrine settings, the geomorphological elements example, Mulder et al., 2003; Hughes Clarke et al., 2012; (for example, delta fronts, incised canyons, distal depositional Covault et al., 2017) and point to the action of underflows in lobes) and the type of sedimentary processes responsible for these systems. the sediment distribution (overflows, interflows and under- The Rhone system in Lake Geneva is different to most marine flows) are similar. This suggests that the results from Lake environments in the very close proximity of the river mouth Geneva may be employed as a proxy for a qualitative com- and delta front to the canyon head (Fig. 3A and 3B), where parison of sediment partition in similar lacustrine and marine the absence of a lacustrine shelf results in the juxtaposition of systems. these two environments. This can lead to a situation where However, the Rhone River canyon head in Lake Geneva is the residence time of the delta front sediments is short, and directly connected to delta front deposits, thus increasing the where the transport of sediment through the canyon to the efficiency of sediment transport by underflows to the distal levée and distal lobe areas occurs over short timescales. This lobe. geomorphic condition can also favour the triggering of tur- bidity currents from river plume settling processes, such as Acknowledgments observed at Squamish delta (Hizzett et al., 2018). This relatively direct connectivity between the delta front and This work was funded by the Swiss National Science Founda- the distal lobe environment might be rare in marine/oceanic tion (Research grant Sinergia CRSII2_147689 and R’Equip- settings. Nevertheless, at places where canyon heads are close 146889). We warmly thank our colleagues Frédéric Arlaud, to river inflows and where the delta front regularly collapses Philippe Arpagaus, Flavio Anselmetti, Benjamin Bellwald, (Clare et al. 2016; Hizzett et al., 2018), export of sediment Michael Hilbe, and Walter Wildi who coproduced the Decem- from the marine shelf to a distal fan/lobe could also occur over ber 2013 Lake Geneva multi-beam bathymetry, and particu- short timescales. This transfer of material might then either be larly its owner the Département Général de l’Environnement accomplished through hyperpycnal flows, river plume settling from the State of Vaud (Switzerland) for the data courtesy. or slope failures (Talling et al., 2015). We would also like to thank our colleagues at the University of Geneva: Anna Rauch, for the help producing the base for 6. Summary and Conclusions Fig.11 and Sébastien Castelltort for the fruitful discussion on deltas. We would like to thank the reviewers of this paper, A GIS-based comparison of bathymetric datasets was success- Peter Talling, Jim Best, George Postma and the editor Christo- fully applied for the first time at this timescale, to establish pher R. Fielding for the insightful comments that helped to an accurate and complete sediment budget and a complete increase the quality of the paper. overview of the sediment partition in a lacustrine delta front- canyon/levée-distal lobe system during the past 125 years. In Nomenclature total, 248.7 ± 18.4 Mt of sediment (mean sediment specific flux of 12.6 ± 0.1 kt yr-1 km-2) has been deposited in eastern θ ; porosity of sediment deposit -3 Lake Geneva over the past 125 years. Highest sediment fluxes ρinsitu(bulk) ; in situ density of bulk sediment (kg m ) -3 occurred in the delta front area and the distal lobe, where ρinsitu(dry) ; in situ density of dry sediment (kg m ) -1 -2 -3 corresponding yields are 72.2 ± 0.9 kt yr km and 58.7 ± ρwater ; density of pore water (1000 kg m ) -1 -2 -3 0.7 kt yr km , respectively, followed by the canyon-levée ρsed ; density of sediment particles (2600 kg m ) -1 -2 with yields of 38.8 ± 0.5 kt yr km . The remaining area, σH ; maximum altimetric error in historical bathymetric data lake basin background deposition, showed a sediment yield of σB ; altimetric error in the echosounder multi-beam bathymet- 3.8 ± 0.1 kt yr-1 km-2, which is thus one order of magnitude ric data smaller. σi ; subgrid scale error due to interpolation of DEM to 50 *50 High sedimentation areas comprise only 17% of the analysed m cells The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 16/22

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Supplementary information

Table S1. Mean density of the sediment cores used to attribute mean density to each of the analyszed Areas I to IV. Dryinsitu densities were calculated taking into account porosity of the cores and used to calculate deposited sediment weights (see Methods).

Area Core Core mean density Area mean density Dryinsitu density (kg m-3) (kg m-3) (kg m-3) TS-K3 1590 Area I 1600 980 TS-K4 1610 Area II TS-K6 1600 1600 980 Area III TS-K1 1890 1890 1450 Area IV TS-K2 1600 1600 980

Figure S1. Historical bathymetric survey of Lake Geneva documented in 1886-1889. A) Picture of the positioning system and the surveying boat. At a maximum distance of 1400 m, the position of the boat was determined from its angular position and the level staff on the boat. Further away the position of the boat was determined by triangulating its position. B) Image of the bathymetric device composed of a winch with 0.8 mm diameter steel cable connected to measuring device. Spherical lead weights were attached to the end of the cable to ensure a straight descent to the bottom of the lake. The device’s lever and gears allowed a single man to operate it. The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 21/22

Figure S2. Morphology map of Lake Geneva’s lakebed. This map was produced from the multibeam bathymetric data collected from 2013 to 2014. The panels in the eastern part of the lake (A,B,C) refer to the panels A, B, and C presented in Fig. 3 of the article. The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 22/22

Figure S3.Top panel: Slope percentage (or percent rise) map of the eastern part of Lake Geneva. The lake lateral slopes are very steep, with slope percentages that can vary from 20 to 332.45%. The delta front of the Rhone sublacsutrine delta also show slope percentages that can reach up to 150%. The canyon walls are also highlighted in this map with steep slopes, with higher values in canyon head and in the concave side of first three canyon bends. Up to a depth of 300 m, the lakebed presents lineations of steep slopes. This highlights the presence of sediment waves bedforms. Bottom panel: Topographic profile of the Rhone active canyon (from east to west). The location of the C8 canyon axis is marked by the red line in the slope percentage map in the top panel. A slope percentage of 4% is observed between 50 and 190 m of depth. The steepness in this area of the canyon is much higher than the steepness from 190 to 250 m, which only reaches 1%. The profile of the canyon has a vertical exaggeration (VE) of 20 times.