Geomorphology 219 (2014) 27–41
Contents lists available at ScienceDirect
Geomorphology
journal homepage: www.elsevier.com/locate/geomorph
The geomorphic evolution and sediment balance of the lower Rhône
River (southern France) over the last 130 years: Hydropower dams
versus other control factors
Mireille Provansal a,⁎, Simon Dufour b, François Sabatier a, Edward J. Anthony a,
Guillaume Raccasi a, Sébastien Robresco a
a
b
Centre Européen de Recherche et d'Enseignement des Géosciences et de l'Environnement, UMR 34, Europôle de l'Arbois, B.P.80, 13545 Aix-en-Provence Cedex 04, France
UMR CNRS 6554 LETG Rennes COSTEL/Université Rennes 2, place recteur Le Moal, 35000 Rennes, France
a r t i c l e
i n f o
Article history:
Received 8 October 2013
Received in revised form 21 April 2014
Accepted 23 April 2014
Available online 15 May 2014
Keywords:
Fluvial geomorphology
Secular sediment balance
River management
Hydropower dams
Source-to-sink sediment transport
Lower Rhône River
a b s t r a c t
The geomorphic evolution of large European fluvial systems since the end of the nineteenth century has been determined, according to recent studies, by changes that have affected the drainage basins, notably climatic changes
at the end of the Little Ice Age (LIA), agricultural practices, and successive phases of channel management involving embankments and dams. However, the respective roles of these various control factors in modifying sediment storage and channel stability are often difficult to identify. The aim of this paper is to determine the main
factors that have driven morphological changes over the past 130 years on the 120-km-long downstream
reach of the Rhône River. A GIS-based analysis of maps and aerial photographs available since 1860, coupled
with chrono-stratigraphic data on the river's alluvial deposits, has enabled quantification of sediment storage
and erosion in different morphological units. Changes measured include riverbed incision, narrowing of the
main channel in relation with accumulation of fine sediments along the channel margins, and reduction of sediment input at the mouth. On the basis of local- and reach-scale sediment balances, we show that these changes
are synchronous with engineering works carried out at the end of the nineteenth century and beginning of the
twentieth century, a period that coincided with a decline in river sediment inputs largely generated by human
activities. The early stabilisation of mountain slopes resulting from a decline in rural agriculture, rural exodus, reforestation and engineered torrent control has had a much greater impact in geomorphically transforming the
Rhône than dams and gravel mining, the major identified causes of river destabilisation in the world. This calls
for caution in the all-too-common and wholesale attribution of declining sediment budgets of European rivers,
especially in the Mediterranean region, to dams.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction: secular changes in river systems and their
downstream impacts
The geomorphic evolution of large rivers in Europe and elsewhere
since the end of the nineteenth century has been the object of several
studies. Changes in natural and/or socioeconomic conditions have
been identified as having had significant but variable geomorphic and
sedimentological impacts. A reduction in the frequency of extreme river
discharges at the end of the LIA (Gregory et al., 2006; Hohensinner
et al., 2008) and agricultural decline in mountain catchments attended
by reforestation have been reported to have led to a decrease in sediment
supply (Bravard, 2002; Wohl, 2006; Hoffmann et al., 2010; Hinderer,
2012). Morphological changes induced in downstream reaches by these
‘external’ control factors have included the disappearance of braided
⁎ Corresponding author at: 12 rue des Saintes-Maries, F-13090, Aix-en-Provence.
Tel.: + 33 442268277, + 33 620554806.
E-mail address: [email protected] (M. Provansal).
http://dx.doi.org/10.1016/j.geomorph.2014.04.033
0169-555X/© 2014 Elsevier B.V. All rights reserved.
channels to the benefit of simple channel configurations, channel-bed
incision, and changes in longitudinal profiles, as well as sediment storage
and afforestation of the riparian zone (Steiger et al., 2003; Rodrigues et al.,
2006; Comiti et al, 2011).
In addition to these external, catchment-scale controls, direct engineering interventions in river channels and on banks, notably through
the construction of embankments, groynes, and walls have exacerbated
channel instability and reinforced sediment storage in the fluvial
margins (Bravard and Peiry, 1993; Kiss et al., 2008; Siché and ArnaudFassetta, in press). Over the last 50–60 years, sediment mining has
further reduced the available bedload fluxes and modified channel-bed
topography, commonly engendering deep artificial pools, as has been
demonstrated by several studies in Western Europe (Petit et al., 1996),
the USA (Kondolf, 1997), and Asia (Luo et al., 2007; Brunier et al., in
press). However, hydropower dams appear to stand out as the overarching cause of fluvial sediment deficit through the interception and storage
of much of the total sediment flux in reservoirs (e.g., Surian and Rinaldi,
2003; Phillips et al., 2005; Chen et al., 2010; Wang et al., 2011; Räsänen
28
M. Provansal et al. / Geomorphology 219 (2014) 27–41
et al., 2012). Downstream of dams, incision of the longitudinal profile
and/or armouring of the bed floor have been attributed to local deficits
in bedload, leading to channel readjustments and reactivation of bank
mobility (Rinaldi, 2003; Petts and Gurnell, 2005; Rollet et al., 2013).
These impacts propagate downstream at rates of 0.1 to 2 km/y, and exceptionally 10 km/y (Bravard and Petit, 2000; Phillips et al., 2005;
Rollet, 2007). The reduction of sediment input from river to coast is sometimes investigated in a source-to-sink approach (Anthony and Julian,
1999; Phillips et al., 2004; Syvitski et al., 2005; Milliman and
Farnsworth, 2011; Wang et al., 2011), with many deltas rendered vulnerable to erosion and sinking as a result of sediment sequestering behind
dams (Ericson et al., 2006; Syvitski et al., 2009; Anthony, 2014).
Because of a lack of homogeneous data on sediment fluxes through
the source-to-sink continuum over the long term (secular scale), the determination of sediment balances over these spatial and temporal scales
has rarely been conducted. Similarly, the identification and evaluation
of the role of individual variables impacting a single fluvial reach is a
challenging task, given the multiplicity of these variables and their nonlinear interactions (Surian and Rinaldi, 2003; Rollet et al., 2013). The
theme of the intertwining of natural climatic changes and human impacts on river catchments through agricultural practices is even more
pertinent in the well-watered circum-Mediterranean catchments of
western Europe, where geomorphic changes in the late nineteenth century, and afterwards, simply prolong a long history of such interactions
(Poulos and Collins, 2002; Hooke, 2006; Kettner and Syvitski, 2009).
Meanwhile, our capacity to propose efficient management and restoration strategies to river managers will increasingly depend on a fine understanding of these interactions (Downs and Thorne, 2000; Jungwirth
et al., 2002; Brierley and Fryirs, 2009; Dufour and Piégay, 2009).
The Rhône River catchment covers an area of 98,000 km2 and is the
largest circum-Mediterranean river system in Western Europe. Several
studies have documented marked geomorphic changes, generated by
external factors, that have affected the lower Rhône main stem in southern France over the last century (Arnaud-Fassetta, 2003; Antonelli et al.,
2004; Raccasi, 2008), and the upper Rhône (Bravard, 1986), as well as
its main southern Alpine tributaries, notably the Drôme (Liébault and
Piégay, 2002), the Ain (Rollet, 2007), and the Durance (Miramont
et al., 1998; Chapuis, 2012). Amongst reported changes following fluvial
management dating back to the end of the nineteenth century are channel adjustments that have resulted in heterogeneous fluvial metamorphosis from historical braiding into wandering, the latter state
involving simplification of channel morphology through disappearance
of the braided style, erosion of the bed, and reduction of channel width.
Although the lower Rhône system may be considered convincingly as
one of the best-studied fluvial systems in Europe, a global approach to
these changes and the disentangling of their underlying drivers has
never been implemented.
The aim of this paper is to document the geomorphic changes in the
lower Rhône River that have operated at various time and spatial scales
over the last 130 years in response to various control factors. Achieving
this objective requires an updated quantitative evaluation of the sediment balance and geomorphic evolution over this period, and especially
of the magnitude, the pace and the chronology of changes in the channel
and the margins at this secular scale. Such an evaluation necessarily involves distinguishing the respective impacts of hydrological and socioeconomic changes in the catchment area, and of river engineering
works, mining activities and especially hydropower reservoir dams.
This paper provides the first overall evaluation of the secular evolution
of the lower Rhône River enabling discrimination of human impacts
on the geomorphic evolution of this fluvial system, the second most important in the Mediterranean after the Nile, as well as on discharge and
storage of bedload and fine sediments. In particular, the role played by
dams relative to other control factors is analysed and quantified. The
study also throws light on the specificities of a system characterised
by a Mediterranean bioclimatic context and by mountainous relief in
the upper catchment sector.
2. Study site and environmental setting
2.1. Morphology and sedimentology
The lower reach of the Rhône River runs from the City of Orange to
the Mediterranean Sea in a wide, 120-km-long alluvial plain (Fig. 1).
The main tributaries are the Ardèche River (65 m3/s) on the right
bank and especially the Durance River (190 m3/s) on the left bank, the
coarse load of which has built a large alluvial fan that modifies the longitudinal slope of the Rhône main stem (Bravard et al., 2008). Just north
of the historic city of Arles (Fig. 1), the Rhône splits into two branches in
its deltaic reach (the Camargue delta) to form the Grand Rhône (52 km
long) and the Petit Rhône (70 km long), which convey, respectively, 90%
and 10% of the water discharge. In this study, we will ignore the very
narrow (b 100 m) and strongly engineered Petit Rhône branch, the
channel-width variations of which are insignificant and difficult to measure accurately. Our focus, as far as the deltaic reach is concerned, will be
on the Grand Rhône, largely dominant in the Rhône delta branches
relative to the Petit Rhône, and expressing, with much better spatial
Fig. 1. Location, geological contexts, and relevant features of the five studied reaches in the
lower Rhône River. 1. limestone outcrops; 2. Pleistocene terraces; 3. Modern alluvial plain
and deltaic wetlands; 4. Main cities; 5. Run-of-the-river dams. Reaches: Caderousse
(1), Avignon (2), Vallabrègues (3), Beaucaire (4), Grand Rhône (5a), Mas Thibert (5b).
M. Provansal et al. / Geomorphology 219 (2014) 27–41
29
Fig. 2. Annual discharge of the Rhône River (1840–2007). 1 — Maximum annual discharge; 2 — 10-year mobile windows; 3 — 10-years return discharge.
accuracy, geomorphic change related to external and human-induced
control factors.
The channel width varies between 250 m at the upstream end of the
study area in Caderousse (Fig. 1), and 900 m at the mouth of the Grand
Rhône; and the depth ranges from 5 to 20 m. The longitudinal bed
profile decreases from 0.07 m/km upstream, from Caderousse to
Vallabrègues, to 0.06 m/km at Beaucaire (Fig. 1), and then drastically
in the delta down to 0.003 m/km, contributing to a drop in specific
stream power and increasing sedimentation along the alluvial margins
(Bravard, 2010). The generally large overbank accumulation since the
end of the nineteenth century has varied significantly in terms of location, chronology, thickness, and grain size. These aspects have been
thoroughly documented by Provansal et al. (2010, 2012), and only a
summary description is given here. Deposits are coarse at the base
(sands or gravels inherited from the braiding period) and overlain by
2 to 5 m of fine stratified sandy or clayey silts. In the various former
branches, currents slowed down by dense riparian forests favour finegrained sedimentation. In contrast, incision in the active riverbed ranges
from 3 to more than 10 m, depending on the sector. The present mouth
of the Grand Rhône displays a classic morphology typical of a wavedominated microtidal river delta and is the latest of a series of changing
river mouths and their locations in the course of the last 130 years,
under the strong influence of river engineering (Sabatier et al., 2006).
Much of the accumulation at the mouth concerns shallow depths
down to − 20 m. Sands are dominant down to a depth of − 8 m,
below which silts dominate (Maillet et al., 2011).
2.2. River liquid and solid discharge
The mean annual discharge of the Rhône measured at the Beaucaire
gauging station (1920–2007) is 1850 m3/s (100-year return interval (RI)
flood N 10,300 m3/s). Following a long period of frequent large floods
during the LIA until 1850–1860 (Pichard, 1995; HistRhône Databank,
2013), high discharge ranging from 7000 to 8000 m3/s (i.e., close to the
10-year RI N 8000 m3/s) prevailed between 1880 and 1940–1945, followed by irregular discharge up to 1990, a period characterised by low flow
and moderate floods. In contrast, several relatively important (10-year
RI) floods have occurred over the last 15 years (Fig. 2).
Total suspended sediment (TSS) has been systematically measured
in Arles since the end of the nineteenth century. An automatic gauging
station, SORA, installed in Arles, has been in operation over the last
10 years. Total suspended sediment has undergone a reduction of
about 60%, from about 18.8 Mm3/y at the end of the nineteenth century
(Pardé, 1925) to 6.1 Mm3/y at present (Pont et al., 2002; Eyrolle et al.,
2010), with an intermediate value of 9.1 Mm3/y in 1956–1958
calculated from archive data and corresponding to the transition between the pre- and post-dam periods (IRS, 2000). The overall reduction
in suspended load has, thus, been of the order of 12 Mm3/y over the last
130 years. The present bedload discharge has been estimated at between 0.4 and 1.4 Mm3/y from dredging in Arles (Dugas, 1989), but
also from a comparison of transverse profiles in the delta (Antonelli,
2002; Antonelli et al., 2004; Maillet et al., 2007) and from empirical
calculations between Beaucaire and Arles (IRS, 2000). This represents
a massive reduction in bedload from about 10–15 Mm3/y at the beginning of the twentieth century (Pardé, 1925; IRS, 2000). Pebbles, gravels
and sand compose the bedload from Orange, at the upstream limit of
the study sector, to kilometric point (PK)1 292, downstream of Arles;
whereas sand is present throughout the deltaic reach (ArnaudFassetta et al., 2003).
In addition to these efforts at establishing partial and/or grain-size
based secular sediment budgets in the Rhône River, Sabatier et al.
(2006) and Maillet et al. (2007) have attempted to derive sediment
budgets of the delta lobe off the mouth of the Grand Rhône 3 km offshore and down to a depth of 20 m. In agreement with the significant
decline in river sediment input highlighted in the foregoing paragraph,
these authors identified, from bathymetric chart differencing, a decrease in lobe sedimentation by a factor of 4 between 1841 and 1974.
The morphology of the mouth of the Rhône and the dynamics of lobe
sedimentation have been described in detail by Maillet et al. (2007,
2011) and Sabatier et al. (2009), respectively.
3. Secular changes in factors controlling morphogenesis of the
Rhône catchment
Several geomorphic, stratigraphic, and GIS-based studies have documented the metamorphosis of the Rhône River and tributaries in the
course of the last 130 years. This secular transformation is attributed
to a succession of, and interactions amongst, multiple control factors
at the catchment and local scales. The main factors and their impacts
are briefly described here.
3.1. Climatic and socioeconomic factors
At the end of the nineteenth century and in the course of the first half
of the twentieth, changes affecting the Rhône catchment led to a significant reduction in sediment flux in the tributaries and the Rhône main
stem. This reduction is jointly attributed to a natural decrease in the frequency of major floods at the end of the LIA (Pichard, 1995; HistRhône
1
PK (for kilometric point).
30
M. Provansal et al. / Geomorphology 219 (2014) 27–41
Databank, 2013), rural exodus associated with a decline in mountain agriculture, catchment reforestation, and engineered stabilisation of feeder torrents (Sclafert, 1959; Miramont et al., 1998; Bravard, 2002). The
Mediterranean–Alpine tributaries of the Rhône were important sediment sources, especially the large Durance River basin (14,000 km2).
A French government programme aimed at reforesting the Rhône
catchment and controlling torrents (known as Restoration des Terrains
en Montagne or RTM) has resulted in a considerable reduction in
sediment delivery to the Rhône. Engineered torrent control was
progressively implemented during the second half of the nineteenth century in the southern Alps (Jorda, 1985; Miramont et al., 1998), and from
1940 to 1960 in the central and northern Alps (Liébault and Piégay,
2002; Piégay et al., 2004). On the Durance River, TSS, estimated at
2.5 Mm3/y at the end of the nineteenth century, is presently 1.3 Mm3/y
(SOGREAH et al., 2001), whereas the gravel supply decreased from 0.3
to 0.02 Mm3/y over the twentieth century (Chapuis, 2012). The active
channel width in this tributary was reduced by 50% between 1898 and
1936. At the same time, the decrease in the frequency of major floods
and the embankments built at the end of the nineteenth century resulted
in a slow-down in bedload transfer and in storage of suspended load
under the stabilising effect of riparian forest growth in the margins
(Miramont et al., 1998; Piégay et al., 2004; Warner, 2006).
The second type consists of run-of-the-river dams, 19 of which were
constructed between 1947 and 1986, most achieved by 1975, on the
Rhône main stem by the Compagnie Nationale du Rhône (CNR),
which still runs these dams. Caderousse (in operation since 1975),
Avignon (since 1973), and Vallabrègue (since 1970) are the last downstream dams of this type on the lower Rhône. A diversion canal bypasses
the natural channel and directs the flow to a hydropower plant. In the
bypassed channel, the usual managed reduced discharges (50 to 400
m3/s) are insufficient to transport the coarser sediments, and this has
resulted in local armouring of the bed (IRS, 2000). However, given the
limited flow capacity (2000 to 2500 m3/s) of this canal, much of the
water and sediments (both suspended and bedload) are conveyed
through the bypassed channel during large floods. Between Lyon and
Arles, about 11 Mm3 of gravel bedload have been eroded over the
last 60 years (0.18 Mm3/y) in this bypassed channel (Bravard and
Clemens, 2008). However, this load is lower than that prior to river regulation (0.4 Mm3/y, IRS, 2000). Notably hydropower dams do not
modify the liquid discharge. The CNR dams on the Rhône let through
flow at discharges exceeding 2500 m3/s, whereas the EDF dams erected
on the Alpine catchments (especially the Serre-Ponçon Dam on the
Durance River) only affect low to moderate floods but not major floods
(SMAVD, 2003).
3.2. Engineering practices to improve fluvial navigation
3.4. Gravel mining
Following the 100-year flood of 1876, continuous embankments
were built in 1860 all along the alluvial margins of the Rhône to protect
inhabitants. Between 1870 and 1920, secondary channels were cut off
and a series of floodable embankments, compartments2 and groynes
were built in the main channel from the Swiss border to the sea in
order to improve navigation. These engineering works progressively
generated a change in channel morphology from braiding to a more
simple single-thread sinuous pattern associated with erosion of the
bed and reduction of width as fine sediments became increasingly
trapped along the channel by the riparian forest (Bravard et al., 1999;
Arnaud-Fassetta, 2003; Piégay et al., 2008). Since the end of the nineteenth century, an estimated 20 to 200 Mm3 of sediments have thus
been stored in the margins of the Rhône between Lyon (Fig. 1) and
Arles (Bravard and Clemens, 2008).
The amount of dredged gravel after World War II, mainly between
1970 and 1990, has been estimated at about 84 Mm3 (0.5.6 Mm3/y)
for the entire Rhône basin (IRS, 2000; Bravard and Clemens, 2008). Between Lyon and Arles, 13.5 Mm3 (0.83 Mm3/y) were extracted over this
period in the Rhône main-stem channel, 2.2 Mm3 (0.3 Mm3/y) of which
concerned the lower Rhône downstream of Caderousse. In the tributaries, mining activities were five times more important, mainly in the
southern part of the catchment: 60 Mm3 (2 Mm3/y) between 1960
and 1990 in the Durance, 3 Mm3 (0.06 Mm3/y) in the Cèze and the
Gardon (SOGREAH et al., 2001). These extractions resulted in an overall
decrease in available bedload flux (Petit et al., 1996) and contributed to
bed incision and channel compartmenting, a process that restricts sediment through-flow (Warner, 2006).
To summarise, the sediment supply in the Rhône River has been
drastically reduced over the last 130 years by a combination of several
factors, starting with climatic and socioeconomic changes. The sediment
transport system has been particularly affected over the last 50–
60 years by dams and mining. The respective roles of these factors,
especially dams, are a core part of our study. We need to emphasise,
however, that the volume estimates quoted above, and derived from
the grey and the published literature, incorporate large uncertainties.
3.3. Hydropower dams
During the second half of the twentieth century, two types of hydropower dams were constructed in the Rhône catchment (Fig. 3). The first
type consists of the reservoir dams of Genissiat on the Rhône main stem
upstream, Vouglans on the Ain River, and mainly Roselend, Tignes, and
Serre-Ponçon on the Alpine tributaries, constructed between 1950 and
1970 by Electricité de France (EDF). Since 1950–1960, the total amount
of sediment retained by the main Alpine tributary dams has been estimated at about 200 Mm3, i.e., 4 Mm3/y (EDF, unpublished data), of
which 25 Mm3 (0.5 Mm3/y) is behind the large Serre-Ponçon Dam
and smaller dams on the Durance River. Channel incision downstream
of these reservoir dams has been attributed to a local deficit in bedload,
generating channel readjustment and armouring (Rollet et al., 2013). In
several dams on the Ain, Isère, and Durance tributaries, fine-grained deposits are periodically evacuated downstream through artificially generated flash flows. The EDF claims (unpublished data) that their dams
thus let through suspended sediments. The last artificial flash flows in
May–June 2008 reportedly led to the evacuation of about 4 Mm3 from
the Isère and the Durance Rivers, of which 30 to 50% was stored upstream of Caderousse or Beaucaire (Eyrolle et al., 2012).
2
Compartment: the usual technical French word is (casier), corresponding to crossed
longitudinal and transverse dykes, which divide the borders of the main channel into several compartments. The system is different from the simple transverse dykes in the secondary barred arms.
4. Materials and methods
To analyse the impact of the various control factors that have influenced the lower Rhône system and to obtain an accurate evaluation of
the river's sediment balance over the past 130 years, morphological
changes and their time frame were determined based on GIS analysis
of maps and aerial photographs. Fine-tuning of the chronological framework has been further obtained from sediment cores and their isotope
analysis.
4.1. Reach delimitation
Using geological criteria and dams to delimit the morphosedimentary
reach units analysed in this study, we identified five reaches covering a
total length of 97 km (Fig. 1, Table 1). The first three contiguous reaches,
Caderousse (reach 1), Avignon (reach 2), and Vallabrègues (reach 3),
correspond to a variably wide alluvial plain bounded upstream and
downstream by gorges cut in outcrops of hard Cretaceous limestone.
The three reaches correspond to segments of the lower Rhône separated
M. Provansal et al. / Geomorphology 219 (2014) 27–41
31
Fig. 3. Location of the main hydropower dams and the Marcoule nuclear plant in the Rhône River catchment. 1. Run-of-the-river dams (CNR); 2. Alpine reservoir dams (EDF); 3. Main
cities; 4. Marcoule nuclear plant.
from each other by CNR hydropower dams. The rocky gorge segments are
devoid of alluvial deposits and are not further considered in this study,
nor the large alluvial fan of the Durance River confluence downstream
of the Avignon reach (about 8 km long). Downstream of the Vallabrègues
dam, the Beaucaire-Arles (4) and Grand-Rhône (5) reaches correspond,
respectively, to the wide coastal alluvial plain downstream of the limestone hinterland and the delta. Within reach 5, the focus is essentially
on the Mas Thibert section (identified as reach 5a), for which a large corpus of unpublished data exists. We also propose a sediment balance for
the active lobe off the mouth of the Grand Rhône, enabling an overall
source-to-sink view of the lower Rhône.
4.2. Evolution of the channel and fluvial margins
The two-dimensional changes in the channel and the riverbanks between the nonsubmersible embankments built in 1860 were quantified
using topographic maps and orthophotographs covering the period
1860 to 2008 (Table 2). The GIS processing of maps and aerial
Table 1
Characterisation of the studied reaches.
Reaches
PK
Length (km)
Bed Slope
Date of dam building
Cores
Isotopic data
1. Caderousse
2. Avignon
3. Vallabrègues
4. Beaucaire–Arles
5. Grand Rhône
5a. Mas Thibert subreach
210–220
230–249
260–269
272–279
279–334
302–308
10
19
9
7
52
6
0.07%
0.07%
0.07%
0.06%
0.003%
1975
1973
1970
no
no
no
2
2
Drills (PANDA)
6
2
4
1
1
3
11
1
8
32
M. Provansal et al. / Geomorphology 219 (2014) 27–41
Table 2
Two- and three-dimensional (topographic and depth) data.
Reaches
Planimetric data (2D)
Topographic data (3D)
CNR dam construction
1. Caderousse
2. Avignon
3. Vallabrègues
4. Beaucaire–Arles
5. Grand Rhône
5.1 Mas Thibert subreach
Rhône mouth
1860, 1910, 1955, 1973, 2006
1860, 1905, 1953, 1973, 2004
1905, 1947, 1962, 1971, 2006
1876, 1905, 1950, 2008
1855, 1944, 1999
1883, 1907, 1950, 1970, 2006
1902, 1975, 2008
1902, 1973, 2004
1876, 1970, 2006
1876, 1950, 2008
1908, 1965, 1999
1869, 1907, 1999, 2006
1872, 1975, 2006
1975
1973
1970
Two-dimensional data are based on archival sources: (i) the Ponts et Chaussees Atlas (1860 to 1876), the Branciard Map (1905 to 1910); (ii) aerial photographs from 1944 to 2006 provided by the Institut Géographique National (IGN).
Three-dimensional data are based on archival sources: (i) topographic and depth maps (Ponts and Chaussees, 1869 to 1876), Service Spécial du Rhône (1902), and CNR three-dimensional
maps from 1976 to 2008; (ii) IGN orthophotographs, photogrammetric analysis, and Lidar surveys (2004–06 to 2008). The CNR acquired heterogeneous and discontinuous bathymetric
data only in the channel in 1950 and 2008 in reach 4, and in 1965 and 1999 in reach 5 (Antonelli et al., 2004).
photographs have been based on standard methods using ArcGis 9.3
software (Gurnell et al., 2003). All local and historical geodetic systems
were converted to the common and official French coordinate system
(Lambert 93). The horizontal error margin of the maps following their
georeferencing and digitisation is estimated at ± 10 m for the nineteenth century maps and ± 5 m for the more recent ones (Raccasi,
2008). Artificial structures and the active channel limits were digitised
from all the documents. The active channel width corresponds to the
channel and the mobile bars visible during low-flow conditions, islands
and vegetated bars being excluded. In order to compare the geomorphic
evolution between the different reaches and periods, the changes in
percentage of the initial surface for each period constitutes a good criterion for evaluating variations in width of the channel. Given that the
bank morphology is very commonly high (N2 m) and vertical, the comparison of surface area between two dates is not affected by moderate
variations in flow discharge as all the maps and aerial photographs
depict conditions during low discharges.
The three-dimensional diachronic evolution of the channel bed and
channel margins was quantified by differencing successive digital terrain models (DTM) constructed from topographic and depth surveys
(Table 2). Depth profiles in the channel were measured every 10 or 20
m in 1876 and 1902. Between 1950–1970 and 2008, the high density
of points on profiles spaced every 100 m (CNR surveys) has enabled
the drawing of depth contours. Vertical errors associated with depth
measurements are estimated to be between ±0.6 and ±0.2 m for the
older maps (1876–1902) and ± 0.1 m for the more recent CNRderived depths (Raccasi, 2008). Horizontal errors are similar to those
for the two-dimensional evolution, i.e., around 10 m for earlier maps
(1876–1902) and 5 m for the more recent ones.
In order to have a complete picture of source-to-sink sediment budget
changes, accumulation at the mouth of the Rhône River was quantified
from bathymetric surveys carried out by the Service Hydrographique et
Océanographique de la Marine (SHOM)3 in 1872 and 1975 and by the
Coast Guard Office in 2006, complementing the previous work conducted
by Sabatier et al. (2006) and Maillet et al. (2007). Only the 1872 data required digitisation, the rest being available in digital format and directly
integrated in a GIS. The nineteenth century SHOM data have a vertical
error range of between + 0.7 and − 0.2 m and ±10 m for position
(Sabatier et al., 2006); the twentieth century data are more precise,
with ±0.2/0.1 m for the vertical error range and ±10/0.3 m for position
in 1975 and 2006, respectively.
Once soundings and elevation points were digitised, DTMs were
constructed from triangulated irregular networks (TINs) in ArcGis. The
diachronic evolution of the channel bed, channel margins, and delta
lobe were quantified by differencing successive DTMs.
The temporal resolutions are not the same for the available two- and
three-dimensional data. Three time slices can be distinguished for the
former, each corresponding to specific engineering control factors:
(i) 1880–1920, works on the channel to improve navigation along the
3
SHOM Service hydrographique et océanographique de la marine.
entire Rhône River; (ii) 1945–1970, construction of reservoir dams in
the Alps and most of the run-of-the-river dams on the upper and middle
Rhône; and (iii) 1970–1975, construction of the three downstream runof-the-river dams on the lower Rhône. This third period concerns only
the bypassed sectors in reaches 1, 2, and 3, as well as reaches 4 and 5
where (natural) fluvial processes persist. The three-dimensional data
cannot be individualised for the first two periods. Nevertheless we put
forward the hypothesis that the two- and three-dimensional sediment
balances follow the same trends in the five studied reaches. Given the
heterogeneity of the data sets concerning the five reaches, two periods
of analysis are defined herewith: before and after the construction of
the EDF and CNR dams around 1950/1970–1980, hereafter referred to
as BD and AD, respectively.
4.3. Alluvial chronostratigraphy
The fine sediments (silt and sand) overlying gravel bars and banks
from the formerly braided channel (a pattern that prevailed until the
end of the nineteenth century) were analysed from drill cores at 25
locations using a PANDA penetrometer and from ten 2–5 m-long
cores extracted with a semiautomatic drill core (Fig. 4, Table 1). Sampling locations were based on diachronic GIS results (Raccasi, 2008),
field observations and geomorphic classification (Antonelli, 2002;
Arnaud-Fassetta et al., 2003), and recent geophysical surveys (electric
resistivity tomography — ERT) in the Avignon and Beaucaire–Arles alluvial plain (Vella et al., 2013). Grain size measurements were made by
laser diffractometry using a Beckmann Coulter LS 13 320. Textural analysis was used to describe the major flood periods in conjunction with
flood histories. These data are simply evoked here to enhance the diachronic GIS analysis but will not be discussed in detail. Details on the
geomorphic location and stratigraphic and sedimentological descriptions of the drill and core data are reported, respectively, in Provansal
et al. (2010), Provansal et al. (2012), and Ferrand et al. (2012) for the
10 cores. The five core sites selected in reaches 1, 4, 5, and 5a (Fig. 4)
are representative of the various environments that could influence
the sedimentation rate and the grain size of the deposits: sandy–silty
high banks (cores CA 1, PI, and MT), silty low banks (FO), and compartments (BA).
Artificial radionuclides stored in riverbank deposits offer the possibility of evaluating sedimentation rates, notably 210Pbxs and 137Cs
isotopes, as shown by Walling and He (1997), Piégay et al. (2008),
Detriché et al. (2010), amongst others. The history of nuclear releases
from the Marcoule nuclear plant (Fig. 3), the main source of artificial radioactivity in the Rhône catchment, provides a reliable chronology over
the last 100 years (Eyrolle et al., 2005; Ferrand et al., 2012). The 137Cs
flux arrived with liquid releases from Marcoule in the lower Rhone
from 1961 to the early 1990s. The 210Pbxs activity profile is widely
used to date deposits younger than 100 years. The five selected cores
highlight the generally large accumulation of fine deposits over the
last century (2 to 5 m), the thickest deposits being in reach 4, as well
as their large chronological variability.
M. Provansal et al. / Geomorphology 219 (2014) 27–41
33
Fig. 4. Chronostratigraphy of the studied reaches. (A) Location of cores: 1. Modern alluvial plain with channel; 2. Substratum (limestone and Pleistocene deposits); 3. Reaches; 4. Cores
(symbols correspond to locations of the cores, five of which are described in the text). (B) Stratigraphy, grain size, and radionuclide activity at the six selected sites (adapted from Provansal
et al., 2012). Grain size (in %): 1. Clay (b2 μm); 2. Silt (2–63 μm); 3. Sand (63–600 μm). Radionuclide activity (in Bq/kg, with error bars): 6. 137Cs; 7. 210Pbxs.
5. Results
Given the disparities in the available data, the active-channel width
(two-dimensional changes) and sediment balance in the channel and
its margins (three-dimensional changes) are analysed separately for
the five reaches, followed by the sediment balance produced for the
active lobe of the Rhône.
5.1. Two-dimensional evolution of the width of the active channel
5.1.1. Reach 1 (Caderousse), river PK 210–220
The former braided channel disappears in part between 1860
and 1910 in conjunction with navigation engineering works (bank
stabilisation and channel narrowing). The main secondary branch became an abandoned channel by 1955. The total active channel width reduction has been 71 m since 1860. The width (Fig. 5) initially decreased
rapidly (− 0.34%/y, 1860–1910), and then slowly (− 0.11%/y, 1910–
1955). Since 1975, following the construction of the hydropower
dams, the width of the bypassed channel (5 km long, managed
discharge 10 m3/s) has been fairly stable (−0.05%/y, 1973–2006).
5.1.2. Reach 2 (Avignon), river PK 230–249
In 1860 the fluvial system was characterised by an anabranching
style with forested islands and several connected branches. The total reduction in active channel width has been 94 m since 1860. Narrowing of
the active channel (Fig. 5) was initially rapid (−0.43%/y from 1860 to
1905) in conjunction with navigation works, and then slowed down
(− 0.12%/y from 1905 to 1953). Since dam construction (1973), the
width of the bypassed channel (13 km long, managed discharge 400
m3/s) has remained stable (−0.05%/y from 1973 to 2004).
5.1.3. Reach 3 (Vallabregues), river PK 260–269
The braided morphology, still persistent in 1905, disappeared
progressively up to 1962. The total active channel-width reduction
has been 370 m since 1905. As in the previous reaches, navigation
works initially led to rapid narrowing of the active channel (Fig. 5)
(−0.71%/y, 1905–1947), followed by a decrease in the rate of narrowing
(− 0.12%/y, 1947–1962). Since the Vallabrègues Dam was completed
(1970), the width of the bypassed channel (6.5 km long, managed discharge 10 m3/s) has remained stable (−0.05%/y, 1971–2006).
5.1.4. Reach 4 (Beaucaire–Arles), river PK 272–279
The main channel was characterised by several secondary branches
and mobile bars in 1876 and evolved to a sinuous single-thread channel
in 1906 (Provansal et al., 2010). The total active channel-width reduction in this reach has been 230 m since 1876. In 1950, all the banks
and former branches were forested (Fig. 6A). Active channel narrowing
(Fig. 5) was rapid between 1876 and 1905 (− 0.85%/y) as a result of
navigation works carried out between 1876 and 1920, and slowed
down between 1905 and 1950 (− 0.06%/y). Narrowing remained an
active process throughout the second half of the twentieth century
(−0.26%/y between 1950 and 2008).
5.1.5. Reach 5 (Grand Rhône downstream of Arles), river PK 280 to 332
At the end of the nineteenth century, this reach was braided with
mobile sandy bars (Antonelli, 2002), and it has undergone a width
34
M. Provansal et al. / Geomorphology 219 (2014) 27–41
Fig. 5. Secular evolution of the active channel width, expressed as a percentage of the initial surface for each period.
reduction of 133 m since 1855, the year the bars started disappearing. A
drastic reduction in the active channel width (Fig. 5) occurred between
1855 and 1944 (−0.24%/y), followed by a slow-down and stabilisation
after 1944 (−0.03%/y).
5.1.6. Subreach 5a (Mas Thibert), river PK 302–308
In 1869, this subreach was predominantly braided with several mobile sandy bars. These bars disappeared progressively starting in 1876,
attended by an overall channel-width reduction of 200 m since. Navigation works in 1905 led to narrowing of the channel and its change in
planform from straight to moderately sinuous. Changes have been
minor since 1905 (Fig. 6B). The reduction in channel width (Fig. 5), initially rapid (−0.93%/y, 1883–1907), slowed (− 0.52%/y, 1907–1950),
and finally stabilised in 1970–2006.
5.2. Three-dimensional evolution and sediment balance
The GIS analysis enables assessment of the three-dimensional evolution of the channel and the margins (Fig. 7). The volumes of fine-grained
sediments stored in the margins on the top of the inherited gravel bars
and those of the coarse bedload eroded on the channel floor are
analysed separately for the two periods BD and AD. The values are
expressed in Mm 3 to yield the overall sediment balance, and in
m3/y/km in order to compare sediment budgets of reaches at different
spatial and temporal scales.
5.2.1. Reach 1 (Caderousse)
In the channel, BD corresponds to a period of mild erosion
(−0.37 Mm3, − 500 m3/y/km), the process ceasing after 1975. In the
fluvial margins, fine-grained deposition attained a positive balance
of + 10 Mm3 (+ 13,200 m3/y/km) in 1902–1975, then + 0.01 Mm3
(+0.6 m3/y/km) in 1975–2008 for the AD period. Core CA1 extracted
on a high bank close to the channel shows 3.5 m of sandy silts overlying
an inherited pebble bar. Radioisotope dating indicates deposition of
these sandy silts over the last 100 years, excepting the last decimetre,
which accumulated after 1960 (Fig. 4).
5.2.2. Reach 2 (Avignon)
In the channel (Fig. 8A), erosion, prevalent during the BD period
(−1.9 Mm3, −1420 m3/y/km), slowed down between 1973 and 2004
(−0.37 Mm3, −700 m3/y/km). In contrast, the BD balance of fine sediments trapped along the fluvial margins in 1902–1976 was very largely
positive at + 30 Mm3 (+ 22,500 m3/y/km), but only slightly so in the
1976–2004 AD period with 0.22 Mm3 (+416 m3/y/km). Six drill cores
(undated) located close to the channel revealed a 0.3–1 m-thick layer
of fine sediments at the upstream end of the reach (PK 232–233) and
another layer more than 5 m thick at the downstream end (PK 239).
5.2.3. Reach 3 (Vallabrègues)
In the main channel, BD erosion, initially strong (−7.8 Mm3, −9000
m3/y/km), decreased in the 1970–2006 AD period to − 0.77 Mm3
(− 3030 m3/y/km). In the margins, the fine sediment balance was of
the order of +3 Mm3 (+5830 m3/y/km) in the 1876–1970 BD period,
then + 0.75 Mm3 (+ 3630 m3/y/km) in 1970–2006. Two (undated)
cores confirm the variable thickness of these fine deposits: 1.5 m at
the top of a former gravel bar to over 6.5 m in a former channel.
5.2.4. Reach 4 (Beaucaire–Arles)
In the channel, erosion occurred to the tune of −3.3 Mm3 (−6300
3
m /y/km) during the BD period, increasing strongly after 1950
(− 9.4 Mm3, − 18,800 m3/y/km). The margins had a positive balance
of + 6.7 Mm3 (+ 12,900 m3/y/km) for the BD period of 1876–1950
that nearly doubled to + 12.8 Mm3 (+ 31,500 m3/y/km) in 1950–
2008 (Fig. 8B). The sediment succession in two cores (PI and FO,
Fig. 4) shows accumulation of 4 to 5 m of fine sediments since the end
of the nineteenth century, deposition being greater on the higher bank
(PI). Radioisotope dating shows an increase in the rate of deposition
since 1960 (1.7 to 5 m in cores FO and PI respectively) and thus confirms
the results of the GIS analysis.
M. Provansal et al. / Geomorphology 219 (2014) 27–41
35
Fig. 6. Two-dimensional geomorphic evolution of the channel and fluvial margins during the BD and AD periods in reaches 4 and 5a. (A) Reach 4 (Beaucaire–Arles); (B) Subreach 5a (Mas
Thibert). 1. Channel; 2. Afforested banks; 3. Mobile silt-sandy or gravelly banks and bars; 4. Alluvial plain (farmland); 5. Built-up area; 6. Unfloodable embankments (erected in 1860); 7.
Submersible longitudinal embankments (erected between 1870 and 1880); 9. Groynes. Compartments consist of submersibles embankments associated with groynes.
5.2.5. Reach 5 (Grand Rhône)
Because of the absence of data on bank sedimentation, determining
the balance of fine sediments in this reach has not been possible.
Instead, core BA extracted in a compartment located south of Arles
shows more than 3.8 m of fine sediments, the top 1.6 m having been deposited after 1960 (Fig. 4). Channel erosion volumes covering the twentieth century, and measured in a riffle and pool sequence (river PK 292–
296 and 306–314 respectively; Antonelli et al., 2004), were extrapolated to the set of pools and riffles in the entire reach. Antonelli et al.
(2004) showed that the channel bed was relatively stable over the riffle
(−150 m3/y/km). In the pool, erosion was estimated at −15,000 m3/y/
km before 1965, and −6000 m3/y/km from 1965 to 1999. Extrapolation
of these results indicates negative sediment balances of −20–26 Mm3
corresponding roughly to the BD period before 1960–1965 and
between − 8 and − 14.4 Mm3 for the AD period. The riffles appear to
be relatively stable, and we consider that this extrapolation yields an acceptable sediment balance.
5.2.6. Mas Thibert subreach (5a)
In the channel, the strong erosion during the BD period
(− 5.1 Mm3, − 14,700 m3/y/km) decreased after 1965 (−0.42 Mm3,
−5800 m3/y/km), in good correlation with the sediment balance
estimates of the entire Grand Rhône reach. The balance in the fluvial margins was +0.77 Mm3 (+1575 m3/y/km) in 1907–1965, but almost even
at +0.01 (+175 m3/y/km) in 1965–1999. Core MT, which has a length of
3 m, recorded only atmospheric global fallout before 1960 (Fig. 4).
5.2.7. Delta lobe
Deposition on the delta lobe off the mouth of the Grand Rhône
amounted to about + 3.4 M m 3 /y between 1872 and 1975 and
about + 1.2 Mm 3 /y between 1975 and 2006. These values are of
the same order of magnitude as those proposed by Sabatier et al.
(2006, 2009) for the period 1895–1975. Our results clearly confirm
the drastic reduction in sediment input from the Rhône to the sea
by a factor of 2.8 before and after dam construction.
5.3. Synthesis on channel-width changes and sediment balance
In all the studied reaches, the highest rates of channel narrowing occurred at the end of the nineteenth century and beginning of the twentieth century. This narrowing was concomitant with management
works designed to improve navigation. Beginning within a period ranging from 1870 to 1910, the rate of narrowing was very rapid and then
decreased between 1910 and 1950, becoming negligible after. After
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M. Provansal et al. / Geomorphology 219 (2014) 27–41
Fig. 7. Erosion/accumulation and global sediment balances within each reach for the BD and AD periods. (A) Accumulation (in Mm3); (B) Erosion (in Mm3); (C) Relative accumulation
(in Mm3/y/km); (D) Relative erosion (in Mm3/y/km).
the construction of the CNR dams in the 1970s, channel narrowing
slowed down in reaches 4 and 5 and was insignificant or nil in reaches
1, 2, and 3.
The fine-grained sediment balance within the channel margins
throughout the studied reaches (Fig. 7A) is approximately +64.3 Mm3
(+0.1 to + 0.7 Mm3/y), corresponding to + 50.5.7 Mm3 (+0.5
to +0.8 Mm3/y) for the BD period and +13.7 Mm3 (+0.4 Mm3/y), for
the AD period. This balance is almost certainly underestimated because
of the incomplete data on distal bank sedimentation in reaches 1 to
3 and the absence of detailed data in reach 5, except subreach 5a.
Nevertheless these results propose orders of magnitude never published
before for the lower Rhone. By extrapolation, we estimate a balance of
about +198 Mm3 (+1.6 Mm3/y) of fine sediments stored over the
same period in the entire 300-km Lyon to the Mediterranean Sea Rhône
reach (upstream of Lyon, the glacial geomorphic heritage modifies alluvial storage; Bravard et al., 2008). This corresponds to the upper value proposed by CNR (+20 to +200 Mm3; Bravard and Clemens, 2008).
Regarding the channel itself, the sediment balance concerns mainly
gravels and pebbles (D50 = 70 mm in reaches 1 to 3 and 15 mm in
Arles) down to the Terrin riffle in reach 5 (river PK 292–296) (Fig. 1),
and coarse to medium sands downstream of this riffle to the mouth
(Arnaud-Fassetta et al., 2003). The total balance over the study period
(Fig. 7B) is between −51.4 and −68.4 Mm3, depending on the values
used for the Grand Rhône reach, i.e., − 33.39 to − 37.39 Mm3 for the
BD period, and −18.4 to −24.9 Mm3 for the AD period. In consequence,
gravel mining over the last 20 years in the lower Rhône (2.2 Mm3) has
had little impact on this balance (IRS, 2000).
At the reach scale, the sediment balance is highly variable. The BD
period in reaches 1 and 2 was characterised by a net positive balance
(respectively, + 9.63 and + 28.1 Mm3), whereas reach 3 underwent
net erosion (− 4.8 Mm3). The sediment balance in reaches 1 to 3 in
the AD period has been close to zero, whereas that of reach 4 has been
identical before and after the construction of Vallabrègues Dam
(+ 3.4 Mm3), the AD balance maintained by stronger erosion after
1950 (− 9.4 Mm3). Finally, the sediment balance of the Mas Thibert
subreach has been negative for the BD and the AD periods (−3.93 Mm3
and −2.17 Mm3, respectively).
Overall, the sediment balance (accumulation–erosion) indicates that the lower Rhône was characterised by net storage
(+ 13.08 to + 17.1 Mm 3 ) during the BD period and net loss
(− 4.64 to − 11.2 Mm3) during the AD period.
6. Interpretation and discussion
The foregoing results show significant changes in river channel morphology and sediment balance that can be broadly defined in terms
of the pre-hydropower dam (BD) and post-hydropower dam (AD)
periods. This enables an evaluation of the role of these structures visa-vis other control factors in affecting the sediment balance of the
lower Rhône down to the mouth and shoreface.
M. Provansal et al. / Geomorphology 219 (2014) 27–41
37
Fig. 8. Three-dimensional evolution of the channel during the BD and AD periods. (A) Reach 2 (Avignon); (B) Reach 4 (Beaucaire–Arles). 1. CNR diversion canal in reach 2.
The discussion will successively examine the following points:
(i) pre-dam geomorphic change/sediment balance variations and
drivers of fluvial change; (ii) the impacts of dams and gravel mining
in the course of the last 50 to 60 years; (iii) differences in geomorphic
change/sediment balance variations amongst the reaches; (iv) the
delta mouth sediment balance, and (v) a comparison of the Rhône
River with other human-modified river systems.
6.1. Pre-dam geomorphic change/sediment balance variations and drivers
of fluvial change
The most significant feature of the BD period was the channel metamorphosis that occurred in the lower Rhône between the end of the
nineteenth century and the early twentieth century. This process was
characterised by a shift in channel pattern from braiding to singlethread, accompanied by reduction in the width of the active channel,
vertical and lateral accretion of the channel margins, and bed erosion.
Within the five studied reaches, much of the geomorphic change occurred prior to 1950. Channel narrowing was two-thirds complete by
the 1920–1940s. This evolution occurred several decades prior to the
construction of hydropower dams throughout the Rhône main stem
and tributaries and prior to gravel mining in the Rhône and its tributaries. The rate of channel narrowing and the three-dimensional
changes diminished and virtually ceased after 1947–1950 as the fluvial
system approached a new equilibrium state (except in reach 4). This
early trend agrees with the results of previous studies (Arnaud-Fassetta,
2003; Antonelli et al., 2004). Geomorphic evolution mainly resulted
from two control factors that acted simultaneously: (i) an overall reduction in sediment input from the catchment, and (ii) the joint influence
of engineering infrastructure designed to improve river navigation and
the spontaneous afforestation of the alluvial margins.
The reduction in suspended sediment flux since the end of the nineteenth century highlighted by measurements in Arles and further
confirmed by the reduction in sedimentation at the mouth of the
Rhône (cf. 6.4 infra) may be attributed to several factors. By extrapolation to the 300 km reach of the Rhône valley from Lyon to the Mediterranean Sea, the storage of fine sediments in the alluvial margins of the
lower Rhône during the BD period amounted to about 1.2 Mm3/y. This
is a negligible quantity, being b10–12% of the total solid load in Arles
over the period (18.8, then 9.1 Mm3/y in the middle of the twentieth
century) and, taken alone, does not suffice to explain the progressive
load diminution. We therefore propose the hypothesis that reforestation and torrent control, which have very efficiently reduced potential
erosion and favoured sediment retention in the catchment since the
end of the nineteenth century, have played a decisive role in the decline
in suspended load. This mechanism, characteristic of European Mediterranean rivers (Liquette et al., 2009; Gracia-Ruiz, 2010), is commonly
neglected because of the difficulty of compiling relevant data on the
effects of reforestation, unlike the much better documented effects of
dam retention.
As far as the Rhône channel itself is concerned, the engineering installations between the 1880s and 1920s, well before dams were constructed, were aimed at producing a single-thread channel that would
self-dredge its bed. This artificial deepening of the channel further
strongly enhanced the natural erosion occurring in response to a sediment deficit. Despite the decrease in specific stream power (Bravard,
2010), the amplitude and speed of channel degradation can be explained by the high critical shear stresses coupled with a bedload fining
trend (decrease in grain size from Caderousse to the delta; Arnaud-
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M. Provansal et al. / Geomorphology 219 (2014) 27–41
Fassetta et al., 2003). The reduction in large floods since the end of the
nineteenth century (Pichard, 1995) enabled vegetation to colonise the
fluvial margins. As vegetation grew dense, it increasingly favoured the
trapping of fines, therefore favouring growth of the size and elevation
of banks while also stabilising bars, as demonstrated by GIS analysis of
reach 4 (Fig. 8B). Afforestation of the alluvial margins occurred quite
early, as in reach 4 and subreach 5a, between 1906 and 1950 in the former, and as early as in the latter (Fig. 6). The strong chronological coincidence of these changes with engineering works and the speed of the
ensuing morphological changes seem to indicate that these works
have been the main driver of these changes.
6.2. The impacts of dams and gravel mining in the course of the last
50 years
As far as the AD period is concerned, the overall storage in Alpine
reservoir dams (fine and coarse sediment) at the catchment scale is estimated at about 200 Mm3, i.e., 4 Mm3/y (EDF unpublished data). This
storage concerns the entire grain size spectrum. The proportion of
coarse sediments is likely to be high in the middle and upper reaches
of the Rhône and their feeder catchments. No studies have been
conducted on this aspect of the Rhône basin, but over the last 10–
20 years much of the temporarily stored fine sediments are evacuated
by artificial flash flows (Eyrolle et al., 2012). This is an important aspect
as far as the Rhône sediment balance is concerned, even if pre-dam land
use changes and reforestation in the hitherto strongly eroded Mediterranean mountains had already significantly reduced the sediment that
could potentially be stored by dams. The removal (mainly of coarse sediments) caused by mining activities between 1970 and 1990 has further
exacerbated these effects. The estimated total extracted volume is about
13.5 Mm3 (IRS, 2000), i.e., 0.83 Mm3/y from the Rhône main stem (from
Lyon to the sea) and 70.54 Mm3, i.e., 3.5 Mm3/y from tributaries, the
Mediterranean tributaries (mainly the Durance) having been the most
affected (63 Mm3, i.e., 2.06 Mm3/y). The combined total amount of sediment trapped by reservoir dams and extracted by mining represents
only a small fraction of the overall reduction in suspended load
(−12 Mm3/y) and in bedload (about 10–15 Mm3/y).
Variations in river liquid discharge, especially the frequency of floods
exceeding 8000 m3/s, play a decisive role in solid transport (Pont et al.,
2002). Floods of this magnitude (Fig. 2) were important in the course of
the first half of the twentieth century, but the AD period coincided with
one of several decades of weak floods (1960–1990). Since 1993, the
Rhône liquid discharge has risen once again to high levels comparable
to those of the end of the nineteenth century. The mean maximum discharge was 6217 m3/s before the transitional upstream dam-building
period, and 6220 m3/s after. The secular decreasing trend in suspended
load cannot therefore be attributed to a change in liquid discharge.
In addition to the aforementioned factors, a lag effect also results
from bedload time travel from the upstream catchment to the lower
Rhône reaches. Considering a transfer speed of 0.1 to 1 km/y in the
upper Rhône or tributaries (Petit et al., 1996; Rollet, 2007), the impact
on the lower Rhône River has probably been weak. This is not the case
farther upstream in the middle and upper Rhône or in the tributaries
(i.e., 50 to 300 km upstream of the lower Rhône) where the sediment
deficit caused by reservoir trapping favoured a variety of processes
that included bed remobilisation, bed incision and/or armouring, and
reactivation of bank mobility (Rollet, 2007; Liébault et al., 2008;
Chapuis, 2012). On the Rhône main stem, bedload transfer has also
been irregular, disrupted by local discontinuities (e.g., gorges) and the
overall decrease in specific stream power, as clearly demonstrated by
Bravard (2010). The geomorphic impacts expected from Alpine reservoir dams and gravel mining have, thus, not really affected the lower
Rhône owing to the inertia that characterises the downstream translation of these impacts.
The story is a bit different for the run-of-the-river dams constructed
by CNR, and which do not constitute obstacles during floods. About
0.5 Mm3/y would have been stored along the banks in the bypassed
reaches in the Rhône valley, from Lyon to the sea, over the last 50–
60 years, also a negligible quantity (5 to 8%) relative to the total flux
measured in Arles (9.1 to 6.1 Mm3/y) over the same period. However,
bedload cannot pass over these dams, nor attain the bypassed channel
(IRS, 2000; Bravard and Clemens, 2008). The channel morphology of
the bypassed reaches 1, 2, 3, associated with the Caderousse, Avignon
(Fig. 8A) and Vallabrègues Dams, is practically fixed due to the reduced
managed discharge. The overall AD stability of these three reaches has
been further reinforced by the reduction in sediment input and in
large floods over the period 1960–1990.
In contrast, reach 4 downstream of Vallabrègues Dam (Fig. 8B) has
been characterised by significant bed incision (−9.4 Mm3) and strong
recent accumulation in the margins (+ 12.8 Mm3). This trend disappears 10 km downstream, in the Grand Rhône reach (5) (Antonelli
et al., 2004). Significant accumulation of sand and silt on the top of the
bank in reach 4 may be due to flushing of trapped sediments through
the dams during floods. Isotope dating (Fig. 4) has confirmed that
these deposits are associated with large floods (50-to-100 yr RI flood
events) that occurred between 1993 and 2003 (Provansal et al., 2010).
In reach 5 (Grand Rhône), bedload mobility is fastest downstream of
the Terrin riffle (Fig. 1) from where sand has been readily transported
downstream to generate the positive sediment balance at the mouth
(5.1 Mm3, Fig. 7B).
6.3. Differences in geomorphic change/sediment balance variations
amongst the reaches
In addition to the impacts of changes at the catchment scale, of river
engineering, and of dams and mining, differences in sediment budgets
amongst the five reaches (Fig. 7C and D) can also be explained in
terms of lithological and structural controls that impacted the local fluvial dynamics. Erosion of the alluvial bed has been lower in reaches 1
and 2, where outcrops of limestone in the gorge limit such erosion,
than in reaches 3 to 5 farther downstream where the channel cuts
through soft Holocene deposits (Fig. 1). The GIS analysis shows the effects of local discontinuities in reach 2 (Fig. 8A, 1902–1976 before the
dam building), where bedload eroded upstream is blocked along the
hard limestone outcrops of Avignon. Moreover, the large alluvial fan
formed at the confluence with the Durance has created a local base
level that reduces the bed slope of the Rhône River thereby weakening
its erosive capacity in reaches 1 and 2. Sediment accumulation has
also been uneven. Accumulation in reaches 1 and 2 has been related respectively to channel narrowing at the downstream gorge and to the
obstacle created by the alluvial fan at the confluence with the Durance.
This retention of sediment explains the reduction in accumulation
further downstream in reach 3.
6.4. The delta-lobe sediment balance
Sediment accumulation at the mouth occurred to the tune
of + 3.4 Mm 3 /y prior to 1970, and + 1.2 Mm 3 /y since then, i.e.,
a more than 50% drop. The sandy fraction, which represents the
upper 8 m of sedimentation, diminishes in the same proportions, from
1.7 to 0.5 Mm3/y. These variations clearly highlight the strong link between changes in the lower Rhône sediment budget and river-mouth
sedimentation, with important implications in terms of secular deltaic
geomorphic change. The volume of coarse sediments eroded in
the lower Rhône during the pre-dam period was between 51.7 and
64.1 Mm3, depending on estimates for reach 5 (Fig. 7B). Budget calculations (IRS, 2000) and dredging conducted by the CNR upstream of Arles
have shown that the rate of bedload displacement is b 0.2 Mm3/y, at a
speed of 0.2 km/y. This value is consistent with estimates proposed by
Petit et al. (1996) and Bravard and Petit (2000). In the delta (reach 5),
the transport of coarse sediments is limited due to the sharp reduction
in slope. Pebbles and gravels were never transported beyond the Terrin
M. Provansal et al. / Geomorphology 219 (2014) 27–41
riffle at PK 292–296 (Antonelli, 2002; Provansal et al., 2003) either in
the course of the last 130 years or during geological times (Vella et al.,
2005). Because bedload is dominantly composed of gravels and pebbles
upstream of the delta, we hypothesise that the coarse sediment eroded
in reaches 1 to 4, more than 50–70 km upstream of the mouth, has not
been transported down to the river mouth, where only sands and muds
are present. We may therefore pursue this reasoning by considering
that bedload extractions, all of which have been carried out upstream
of the Terrin riffle in the Rhône main stem and its tributaries, have not
directly contributed to the reduction in sediment supply to the coast.
Consequently, bedload input to the mouth of the Rhône is largely
provided by channel erosion in the last 36 km downstream of the Terrin
riffle. Dense bathymetric surveys close to the mouth highlight the mobility of hydraulic sand dunes during flood discharges exceeding 2900
m3/s (Vassas et al., 2007), providing evidence for sandy bedload transport down to the mouth. From these observations, we infer that the
bed material eroded and transferred to the river mouth downstream
of the Terrin riffle is about 0.52 and 0.08 Mm3/y, prior to and after
dam building, respectively. These values are very low compared to the
sediment accumulation at the mouth, thus suggesting that suspension
load constitutes the main agent of mouth accretion.
Shoreface sedimentation off the mouth of the Grand Rhône (Sabatier
et al., 2006; Fanget et al., 2012) provides evidence for the progressive
decline in suspension load measured in Arles since the beginning of
the twentieth century. This decrease (mean value of about 12 Mm3/y,
i.e., 60% of the initial volume) is proportionally comparable to the diminution in the volume of deposits off the mouth (about 50%), thus
underscoring the strong relationship between the Rhône suspension
load and river-mouth sedimentation.
6.5. Comparison of the Rhône River with other human-modified river
systems
The lower Rhône is located downstream of a large catchment, partially influenced by Mediterranean climate, in a historical context of reforestation and climatic changes characteristic of the mountainous
European countries since the end of the nineteenth century. In some
Mediterranean fluvial systems, lateral geomorphic mobility continued
at least during the course of the first half of the twentieth century and
much of the geomorphic modification occurred after World War II, as
has been reported for the Ebro (Vericat and Batalla, 2005; Ollero,
2010), the Tagus (Uribelarrea et al., 2003), and the Toscan rivers in
Italy (Rinaldi, 2003), but also for non-Mediterranean rivers such as the
Sacramento in California (Michalkova et al., 2010; Micheli and Larsen,
2010). This post-World War II period has been characterised by the
impacts of hydropower dams, especially given the fact that water in
dam reservoirs is also used for irrigation in the relatively dry Mediterranean climatic context compared to more humid climates. Further
impacts have been generated by intense riverbed mining, as in the Po
(Simeoni and Corbau, 2009) and in smaller coastal rivers of northeastern Spain (Rovira et al., 2005). By reducing sediment supply to the
coast, reservoir storage, estimated at more than 95% of the river load
for the Nile (Stanley, 1996) and the Ebro (Guillen and Palanques,
1992), has indirectly resulted in delta retreat and coastal erosion. We
should stress the fact, however, that most of these studies have identified neither the effects of local river management nor the timing of
the propagation of the sediment deficit (Batalla and Kondolf, 2005;
Ollero, 2010).
On other large European rivers such as the Rhine and the Danube,
important observed changes occurred as early as in the lower Rhône
(i.e., end of the nineteenth-beginning of the twentieth centuries,
Hohensinner et al., 2004). On the lower Danube and delta, the relatively
small reduction in sediment discharge (25–30%) following damming
(the Iron–Gate Dams, 1884 and 1970) has been attributed to input of
downstream tributaries (Salit et al., in press) and to increased erosion
of the river bottom, banks, and bars in the lower course of the river
39
amplified by meander cutoffs in distributaries in the delta in the
1990s (Jugaru Tiron et al., 2009).
Data of the type collated here for the Rhône are much more exhaustive than for many of the large rivers in Asia that are now iconic in terms
of the effects of dams on their sediment flux. Wang et al. (2011) determined sediment budgets for the five largest rivers in east and Southeast
Asia (Yellow, Yangtze, Pearl, Red, and Mekong), known to be important
contributors of terrigenous sediment to the western Pacific Ocean. Time
series of data on annual water discharges and sediment fluxes of these
rivers covering the period 1950–2008 indicate that anthropogenic
causes involving dams and land use have been responsible for longterm (decadal scale) decreases in sediment flux to the ocean (Wang
et al., 2011).
The nature and intensity of geomorphic and sedimentary impacts
engendered by drivers of environmental change in human-modified
river systems are thus variable. They depend on catchment size and geology, the history of land occupation and use, as well as on the climatic
context, the chronology and degree of river management, and finally, at
the reach-scale, on process and sedimentary connectivity between
neighbouring reaches, as has been shown here for the lower Rhône.
This downstream part of the river combines impacts common not
only to large European rivers that have been heavily engineered in the
nineteenth century, notably the Danube and the Rhine, but also to
many of the Mediterranean rivers on the more humid western
European margins where early rural exodus and stabilisation of
torrents and mountain slopes occurred over the same period. Discharge
regulation and sediment trapping by dams, as well as aggregate extractions since the end of the World War II, are factors in the geomorphic
transformation of the lower Rhône but not determining ones. This original combination of pre- and post-dam impacts has also generated a
time and space lag in geomorphic change between the Mediterranean
tributaries, which are important sediment purveyors, and the Rhône
main stem.
7. Conclusion
Changes in the sediment balance of the lower Rhône River and
its consequences on channel morphology have been discernible since
the end of the nineteenth century and were most prominent in the beginning of the twentieth century. This early evolution was caused by
two simultaneous control factors with combined effects. Engineering
works on the river, aimed at improving navigation, led rapidly to
anthropogenic fluvial metamorphosis, characterised by significant
channel narrowing, bed incision, and storage of fine sediment in the
alluvial margins. This transformation was further reinforced by spontaneous riparian afforestation. The impact of river engineering works was
exacerbated by an overall reduction in sediment inputs from the drainage basin associated with land use changes that likely prevailed in the
Southern Alps from the end of the nineteenth century. These included
massive rural exodus, a decline in mountain agriculture, and engineered
torrent control followed by systematic reforestation. An important consequence of the changes that occurred on the lower Rhône was a significant
reduction in sediment delivery to the river mouth, with consequences on
the stability of the Rhône delta and adjacent shorelines.
Alpine reservoir dams and mining activities mainly in the tributaries
over the last 50–60 years have had weaker impacts. The sources of sediment, torrential in origin, had already been exhausted before dams
were constructed within the Alpine basins. Downstream transmission
of the geomorphic impacts of the reservoir dams was slow within an already stabilised catchment. A reduction in the frequency of large floods
between 1960 and 1990 has further downplayed these impacts. On
the main stem river, the cascade of CNR hydropower dams assured
stabilisation of the perturbed system by engendering segmentation of
bedload transfer. Only downstream of the last hydropower dam was
equilibrium not achieved in the second half of the twentieth century,
such that the transfer of sandy bedload has persisted, assuring
40
M. Provansal et al. / Geomorphology 219 (2014) 27–41
maintenance of sand supply, albeit in diminished quantities, to the
Rhône delta. The lower Rhône River had already undergone significant
local engineering transformations prior to the downstream propagation
of the effects of these upstream dam installations.
Finally, the detailed quantitative morphological and sedimentbalance analysis in space and time conducted in this study has enabled
a clear hierarchy of control factors on the lower Rhône River within the
source-to-sink fluvial continuum. Recognition of this point is fundamental in terms of sustainable management of fluvial systems. It permits identification of different courses of action that can be taken in
order to improve management strategies relating to economic activities,
flood risks, and ecology (Downs and Thorne, 2000; Dufour et al., 2008;
Glenn et al., 2008).
Acknowledgements
The authors acknowledge the input of several Masters students on
the geomorphic evolution of the lower Rhône River. This work expands
on previously published studies and includes results from several unpublished Masters and Ph.D. studies conducted at Aix-Marseille University. Thanks to P. Pentsch and N. André-Poyaud for their help in drawing
the figures. Our thanks to Michal Tal and to four anonymous reviewers
for their constructive reviews. Editor-in-Chief Richard Marston is also
thanked for his salient constructive remarks on the revised manuscript.
This work was supported by the French National Research Agency
(ANR) through the ANR807-EXTREMA project (contract # ANR-06VULN-005, 2007–2011, Vulnérabilité, milieux et climat) and by the
research cluster of the Observatoire des Sediments du Rhone (OSR).
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