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Recent morphological channel changes in a deltaı̈c environment. The case of the Rhône River, France
Geomorphology 57 (2004) 385 – 402 www.elsevier.com/locate/geomorph
Recent morphological channel changes in a deltaı¨c environment. The case of the Rhoˆne River, France C. Antonelli *, M. Provansal, C. Vella CEREGE, Europoˆle Me´diterrane´en de l’Arbois, BP 80, Aix-en-Provence 13545, France Received 27 November 2001; received in revised form 10 March 2003; accepted 28 March 2003
Abstract Channel morphological changes were measured at two sites within the Rhoˆne delta along the main channel of the Rhoˆne River over one century. Results show an average erosion rate of 2.8 mm year 1 during the 20th century and decreasing to < 1.5 mm year 1 since the 1960s. Erosion affected site 2 more than site 1 but cross-sections can alternately be in erosion or accretion depending on bed topography and/or floor lithology. Riverbed adjustments are partly explained by the boundary shear stress and specific stream power but local factors also influence the Rhoˆne River evolution. Results compared with those obtained upstream or in other European rivers suggest that the lower Rhoˆne River has developed an original response to external stresses: the first part of the 20th century is characterised by an important degradation followed by a decrease in the incision rate since the 1960s. The initiation of the erosion phase is assumed to be linked to the end of the Little Ice Age, to the reforestation of the alpine hillsides, to the earlier management and to the reduction of the sedimentary yield (according to the literature). The second phase possibly corresponds to the relaxation period. In this perspective, the effects of dams are probably less significant than we would suppose up to now. It implies that such a large river records morphological responses which may vary all along the fluvial system in relation to the time and space scales involved. D 2003 Elsevier B.V. All rights reserved. Keywords: Channel changes; Boundary shear stress; Specific stream power; Climato-anthropic impacts; Lower Rhoˆne River
1. Introduction In a natural shifting river, channel morphology is the result of active bed-load transport. In such river systems, channel geometry corresponds to river adjustments resulting from modifications of the sedimentary load which is directly linked to land-use changes in the catchment, hydroelectric structures, and climatic changes (Parde´, 1951; Starkel, 1983; * Corresponding author. E-mail addresses: [email protected] (C. Antonelli), [email protected] (M. Provansal), [email protected] (C. Vella). 0169-555X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-555X(03)00167-3
Blum and Tornqvist, 2000). Large rivers in developed countries have been experiencing a trend of bed incision since the beginning of the 20th century (Guillen and Palanques, 1992; Warner, 1993; Gasowski, 1994; Landon and Pie´gay, 1994; Marston, 1994; Vo¨ro¨smaty et al., 1997; Rinaldi and Simon, 1998; Winterbottom, 2000). In some cases, an increase in the incision rate has been observed since the 1950s (Bravard, 1989; Bravard and Peiry, 1993; Gautier, 1994; Gasowski, 1994; Miramont et al., 1998; Leteinturier et al., 2000). The upstream Rhoˆne River and its main tributaries were also affected by this phenomenon mainly
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linked to three external stresses. First, the end of the Little Ice Age and land-use changes in the catchment area have modified the hydrologic regime and have reduced the solid discharge (Pichard, 1995; Landon, 1999; Warner, 2000). Secondly, the dykes, constructed since the end of the 19th century in order to protect people and land against floods, have isolated the Rhoˆne River from its floodplain (Poinsard, 1992; Arnaud-Fassetta, 1998). Finally, since 1945 hydroelectric plants (18 dams from Lake Geneva to the sea, Serre-Poncßon dam and its annex on the Durance River, etc.), sand and gravel mining in the river channel and its tributaries have been responsible for reducing and modifying solid transport downstream (Poinsard et al., 1989; Klingeman et al., 1994). The longitudinal profiles were used by ArnaudFassetta (1998) to demonstrate a global bed incision. The opportunity to analyse precise detailed cross-sections led us to conduct a new study of the morphological changes of the deltaı¨c Rhoˆne River.
In the deltaı¨c part of the Rhoˆne River, crosssections have been recorded since the beginning of the 20th century by the National Company of the Rhoˆne River (CNR). These data were surveyed downstream of a large catchment (98 000 km2) that integrates time and space scales which cannot be compared with the recent modifications observed on the Rhoˆne River tributaries. The main objective of this study is to examine morphological modifications of the deltaı¨c part of the Rhoˆne River over the 20th century and to compare them with the results observed in the upstream catchment and on other European hydrosystems. This study also attempts to integrate spatio-temporal parameters involved in the evolution of this deltaı¨c system.
2. Study area The Rhoˆne delta corresponds to a base-level plain where accumulation has been dominant for several
Fig. 1. Location map of Rhoˆne river delta and both studied sites. Note that Kilometric Point 0 is located at Lyon (corner map).
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millenia (Kruit, 1955; Oomkens, 1970). Hydrological and sedimentary variations (of climatic and/or anthropological origins) are expressed by deposits of varying granulometry (silt to medium-coarse sands), changes of river style (meander, crevassing), and the rapid progradation of the river mouth (Arnaud-Fassetta, 1998; Vella, 1999). At Arles, the Rhoˆne River divides into two arms (Fig. 1). The first one, the Grand Rhoˆne (Eastern branch, 50 km long), discharges 85 –90% of the total flow into the Mediterranean Sea via the Roustan grau. The second one, the Petit Rhoˆne (Western branch, 70 km long) discharges its flow into the sea via the Orgon grau. This study focuses on the Grand Rhoˆne River for which three cross-sectional fields were surveyed during the 20th century. Its mean annual water flow is 1500 m3/s at the gauging station at Arles and varies between 320 m3/s at low water level and 3800 m3/s during a 1-year flood and to 10 000 m3/s during a 100-year flood. The river width varies from 150 m at Arles to 1000 m at the river mouth. The longitudinal profile shown in Fig. 2 is uneven and characterised by riffles ( 4.75 m below the mean sea level, i.e., msl) and pools ( 18 to 21 m msl). Embankment and channelisation of the river have led to a straight to moderately sinuous channel (1,0 to
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1,3; Arnaud-Fassetta et al., submitted for publication). Two sites were selected on this branch because of their representativeness. Their overall length is 12 km. Site 1 (Fig. 3a) is located between kilometric point (KP) 292 and KP 296. From KP 292 to KP 292.6, the longitudinal profile indicates an abrupt rise of the river bed (f 1%). Between KP 292.6 and KP 294.7, the reach is characterised by the presence of the highest riffle of the channel (Terrin riffle), the depth of which has been maintained at 4.75 m msl for navigation purposes by excavation of the bedrock channel (in 1969, 1979 and 1991) and by regular dredging. On this segment, the river bed is plane (f 0.004%). Finally, from KP 294.7 to 296, the longitudinal profile decreases towards the following pool (f 0.24%). Channel width at site 1 varies between 386 and 666 m but the central part of this reach is wider (mean width = 523 m) than the other two (406 m). The river floor consists of Pleistocene conglomerates (i.e., Crau gravel) with pebbles embedded in a very cohesive silty – clayey matrix. The banks are paved (boulder armouring) nearly everywhere at site 1. Site 2 (KP 306– KP 314, Fig. 3b) corresponds to a low sinuosity section (1.14). Its longitudinal
Fig. 2. Longitudinal profile of Grand Rhoˆne (after CNR).
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Fig. 3. Map and longitudinal profile of site 1 (a) and site 2 (b).
profile is uneven whereas its width varies between 312 and 462 m. Site 2 is narrower but deeper than site 1. The central part of this reach has an ancient braided stream deposit as confirmed by a secondary channel (always submerged) and the alluvial deposits of ‘‘Pilotes’ island’’. The dominant bed material is sand with a median grain size of 370 – 410 Am.
3. Methodology 3.1. Cross-section data Numerous cross-sections have been recorded since the beginning of the 20th century and stored in the archives of the CNR. However, the majority of the cross-sections display only the navigable chan-
C. Antonelli et al. / Geomorphology 57 (2004) 385–402 Table 1 Characteristics of cross-sections recorded by CNR Date
Area (from KP to KP)
Distance between two cross-sections (m)
1908 1964 1965 1986 1999 1999
306 – 314 292 – 296 306 – 314 292 – 300 269.1 – 300 300 – 317
500 100 500 100 100 500
nel, which is generally narrower than the natural channel. Hence, only two sections possess enough data to allow a precise chronological study of the channel changes (Table 1). At site 1, 127 crosssections were surveyed each 100 m and were recorded in 1964, 1986 and 1999. At site two, 54 cross-sections were surveyed each 500 m and were recorded in 1908, 1965 and 1999. Therefore, the chronology is different on each site: data cover all the 20th century at site 2 (the longer and less
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artificial) whereas at site 1 data cover only the second part of the 20th century. These data have accorded since the 1960s, after the major humaninterventions occurred on the catchment area. All the profiles are referenced by CNR with regard to the mean sea level (msl), thus allowing comparison. The surveying methodology and vertical precision of the measurements in 1908 are unknown. However, all the other cross-sections were recorded using an echo sounder with a vertical precision of about F 20 cm. In the cross-sections recorded in 1908 and 1965, channel depths were measured manually (60 – 90 survey points by cross-section). The reading error on paper is estimated to be F 0.5 mm and corresponds to a vertical error of F 5 cm on a crosssection. The most recent profiles (1999) display between 33 and 172 survey points. All cross-sections were converted into a format compatible with BMAPR (software designed to measure beach profile and adapted to our needs). We measured the
Fig. 4. Overlaying of cross-section data: an example of channel bed evolution at sites 1 and 2.
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mean width at three levels at the first site (0, + 1 and + 2 m msl for 1964, 1986 and 1999, respectively) and at four levels at the second site (0 and + 0.5 m msl for 1908 and 1965 and 0, + 0.5, + 1, + 2 m msl for 1999). For each cross-section, the riverbed level differences between two dates were automatically measured with a 10-m interval. Then, the mean incision or aggradation profile was calculated from the comparison of these differences (using ExcelR). Finally, the eroded (or deposited) volumes were calculated in two steps. First, BmapR calculated the incised or eroded area by the overlaying of two crosssections (Fig. 4). During the next step, volumes were calculated: the incised or eroded area was multiplied by the distance between two cross-sections (i.e., 100 m at site 1; 500 m at site 2). These two steps were
repeated for each segment of both sites. In summing up each volume we obtained each site total volume. Results were sorted using the statistical method of quartiles: D20, D40, D60, D80 with DN is the incision (or deposition) value such that N percent of data results are inferior. 3.2. Hydraulic data Hydraulic data were selected in order to understand if internal factors were involved in the Rhoˆne River morphological changes. Boundary shear stress (s0) and specific stream power (x) were calculated for a 1-year flood event and for a 100-year flood event (discharge of about 3800 m3/s and 10 000 m3/ s respectively at Arles). Indeed in a recent period (1993 –1994) three major floods with a 30 –80-year
Fig. 5. Mean channel width at sites 1 and 2.
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Fig. 6. Vertical variations of channel bed at sites 1 and 2.
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recurrence occurred, therefore we took into account their potential morphogenic implications. Geometrical data (wetted area, width) were calculated using MapInfoR software. The cross-sections recorded in 1908 were not treated due to the lack of survey points at the top of the banks. Specific stream power was calculated using the Bagnold’s (1966, 1977) equation: x ¼ qgQs=w
ð1Þ
where q = specific weight of water, g = gravitational acceleration, Q = discharge, s = water-surface slope and w = bankfull width. Boundary shear stress is given by (Carson, 1987; Bravard and Petit, 1997) s0 ¼ gqRs
ð2Þ
where R = hydraulic radius. The slope used in both equations is equal to the mean water-surface slope in the Rhoˆne delta (4 10 5 m m 1; IRS, 2000).
4. Results 4.1. Morphological evolution at both sites during the 20th century The channel mean width on both sites has displayed a significant stability for 40 years (Fig. 5). At site 1, we noted a weak narrowing of the channel between 1964 and 1986 ( 1%) followed by a widening between 1986 and 1999 ( + 2%). Changes were irregular all along the reach and alternatively affected shallow or deep cross-sections. At site 2, we observed an increase of the channel width ( + 2.8%) during the first period (1908 –1965) followed by a slight narrowing ( 1.7%) between 1965 and 1999. Since the 1960s, the channel mean width has displayed a differ-
Table 2 Mean vertical changes measured at site 1 Period
1964 – 1986 1986 – 1999 1964 – 1999
Mean variation Mean (cm/year)
Standard deviation
1.58 0.50 1.21
1.14 1.57 1.05
Maximal variation (cm/year)
Minimal variation (cm/year)
+ 0.31 + 3.45 + 0.50
3.72 3.71 3.09
Table 3 Mean vertical changes measured at site 2 Period
1908 – 1965 1965 – 1999 1908 – 1999
Mean variation Mean (cm/year)
Standard deviation
3.62 1.43 2.79
0.99 2.1 0.66
Maximal variation (cm/year)
Minimal variation (cm/year)
6.43 3.1 4.25
2.38 + 0.24 1.9
ent evolution on each site; nevertheless the average of the lateral changes, which were comprised between + 4 m (site 1) and 1 m (site 2), are negligible. Calculation of the bed adjustments, mapped by quartiles, shows a deepening of the channel on both sites all over the 20th century. At site 1 (Fig. 6, Table 2), the mean incision was about 0.42 m in 35 years (i.e. 1.2 cm year 1) but the incision rate decreased from 1.56 cm year 1 during the first period (1964 – 1986) to 0.5 cm year 1 since 1986. The upstream and central parts of site 1 (from KP 292.6 to KP 294.7) at first characterised by incision have experienced deposition since 1986. The last segment (from KP 294.7 to KP 296) has remained in erosion over the whole period. As a result, the general trend has been erosion with a clear reduction in incision since 1986 (rate divided by 3). At site 2, results indicate a mean channel erosion of 2.5 m during the 20th century which was 2.8 cm year 1 (Table 3, Fig. 6). In the period 1908– 1965, the bend (KP 309– KP 311) was the most eroded area whereas between 1965 and 1999, maximum values were located upstream. On a century-scale, the bend was more sensitive to erosion. Like at site 1, the most recent period (1965 – 1999) showed a significant decrease in erosion (2.5 time less than in period 1908 – 1965). Finally, over the three periods of measurements, channel erosion has slowly decreased. The mean vertical incision has been almost identical on both sectors since the 1960s: 1.21 cm year 1 at site 1 and 1.43 cm year 1 at site 2. The first part of the 20th century corresponded to an intensive phase of erosion on site 2. Then, since the 1960s, the incision rate has been decreasing. This phenomenon was corroborated by results obtained at site 1: the common period (1960 –1990) underwent the same incision rate, and site 1 also experienced a decrease in the studied period.
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Fig. 7. Specific stream power variations at sites 1 and 2.
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Fig. 8. Boundary shear stress variations at sites 1 and 2.
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4.2. Spatial and chronological evolution of the hydraulic data The x values spatial distribution remained constant over the studied period whereas the average values rose on both sites between 1964 and 1999 and reached about 3 W m 1 in 1999 (Fig. 7). These values are weak in comparison with other rivers (Van Den Berg, 1995; Abernethy and Rutherfurd, 1998; Bravard and Petit, 1997; Knighton, 1999; Leteinturier et al., 2000; Biedenharn et al., 2000) and with upstream tributaries (Astrade and Bravard, 1999; Landon, 1999). They were strongly influenced by the gentle water-slope of the river which decreases from 0.5xupstream of the delta to 0.04xin the delta. However, for a 100-year flood, the water slope reached 0.2xin the delta, inducing a specific stream power between 5.5 and 16 W m 2 (site 1) and between 8.5 and 13 W m 1 (site 2). The boundary shear stress values increased significantly over the three periods (Fig. 8), whereas their spatial distribution remained rather constant. At site 1, s0 values were important in the deepest sections (from KP 292 to KP 292.6 and from KP 294.7 to KP 296), where their increase had been the most significant since 1964. To the contrary, on the central part of the reach (KP 292.6 –KP 294.7) values were constant. In 1999, the contrast remained clear between the deepest part (average s0 = 2.7 N m 2) and the central part (average s0 = 1.82 N m 2), although this central part had retracted downstream more than a kilometre since 1964. At site 2, the effects of the bed topography did not appear so sharply even though values were higher at the end of the bend. The chronological increase in boundary shear stress was much more important than in site 1, in particular between 1908 and 1965 (mean increase of 25%). In 1999, it overtoped 3 N m 2 everywhere. The comparison of both parameters calculated on the Grand Rhoˆne branch from Arles to the sea from recent data (1999) indicated that s0 and x varied in phase, but with different amplitudes according to sectors (Fig. 9). Upstream of site 1, variations of s0 and x were in the same range. They sharply decreased from KP 293 which corresponded to the strong rising slope of site 1 ( + 1.08%). From KP 295 up to KP 306, specific stream power increased significantly while boundary shear stress values remained low. Such a
Fig. 9. s0 and x values from the gauging-station at Arles to the sea (calculated from cross-sections recorded in 1999).
difference between the two parameters disappeared in site 2. These disparities were due to the geomorphic variables taken into account in the equations (cf. 5.1): the depth and the width of the channel could explain the range of hydraulic data calculated on both sites. Calculation of the river competence was estimated with the boundary shear stress values (s0 c grain diameter; Leopold et al., 1964; Bravard and Petit, 1997; Knighton, 1998). In the case of the lower Rhoˆne River, the competence was of about 2 mm at bankfull discharge (3400 m3 s 1) rising to 13 – 20 mm for a 100-year flood at site 1. At site 2, values were higher due to the increased water-column; between 3 and 4 mm for bankfull discharge rising to 17 –22 mm for a 100-year flood. Those results suggest that pebbles are moving only during exceptional events. However, these data must be used with caution because they underestimated some results: indeed recent observations made after weak floods (inferior to bankfull discharge) have highlighted the mobility of pebbles (D50 = 11 mm).
5. Discussion 5.1. Representativeness of the incision measurements Previous studies (Arnaud-Fassetta, 1998, 2002; Arnaud-Fassetta and Provansal, 1999) described a channel incision, associated with a reduction of the river width, on both deltaı¨c branches, increasing from the end of the 19th century to the second part of the 20th century. Their conclusions were based on the
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treatment of longitudinal profiles recorded by the CNR in 1907, 1967 and 1991. These profiles were established from 85 measuring points with a 500-m interval. According to the authors, the vertical incision in the channel would be ‘‘almost uniform’’ on the whole profile, affecting the riffles rather than the pools, and would have increased by a factor 6 –7 since 1967. These conclusions are contradictory to our results, which show a deceleration in channel erosion in the second part of the 20th century and weaker erosion on the riffles than in the pools. This raises the question of the representativeness of the data used in each study (longitudinal profiles or cross-sections). In order to confront both methods, we have deducted longitudinal profile from cross-section data and calculated the incision rates on each site for each period. Results shown in Table 4 indicate that the use of longitudinal profiles led to an overestimation of the channel incision, which has two explanations: first, the navigable channel that often corresponds to the deepest and central part of the stream is regularly dredged, whereas the channel margins are not affected by such operations; secondly the deepest point focuses the highest forces applying in the riverbed thus leading to an amplification of the incision processes. In reality, different phenomena (aggradation, stability or incision) coexist on a cross-section and cannot be represented by one measurement point. Are the two studied sites of a 12-km total length, representative of the whole 50 km of the Grand Rhoˆne River? Deformations measured on site 1, located on a Pleistocene conglomerate, were probably linked to more important hydraulic stimuli than to modest events. Moreover, site 1 incision rates could be considered as minimum values for the lower Rhoˆne River. Cross-sections recorded at site 2 exhibited a
great variety of shapes, incised in sandy-silts, representative of the Grand Rhoˆne channel between Arlescity and the river mouth. Therefore the results obtained by the overlaying of cross-sections on representative sites of the Rhoˆne River are more precise. The morphological changes of both sites relied upon spatio-temporal variations of the hydraulic data, which in turn were influenced by the channel shapes. In order to understand the complex relationships existing between morphological and hydrological variables and their implications on the whole Grand Rhoˆne River, we explored statistical correlation between all these variables. Nevertheless, the morphological evolution of these two reaches is connected to a local functioning (delta scale) and furthermore to a regional functioning (catchment scale). Such an organisation requires taking into account external factors (climatic, anthropogengic) that could affect the riverbed adjustments. 5.2. Impacts of hydraulic and morphogenic data on the channel evolution The morphological modifications of the channel were not directly correlated with hydraulic data, particularly with stream power (Fig. 10). These disparities were induced by the geomorphic variables taken into account in the equations: the hydraulic radius (c channel depth) in s0 and channel width and wetted area in x. This, in turn, suggested variable channel morphology, creating different modes of liquid flow (turbulences, eddies; Fig. 11). The different values of the two parameters generated a variable morphogenesis on both sites: an increase in boundary shear stress suggests an intensification of the shearing forces at the bottom and con-
Table 4 Comparison of the incision rates from longitudinal and cross-section data Calculation from longitudinal profiles
Calculation from cross-section
Site 1 Period Mean incision (m) Mean annual incision rate (cm/year)
1964 – 1986 0.40 1.81
1986 – 1999 0.40 3.11
1964 – 1999 0.80 2.30
1964 – 1986 0.35 1.58
1986 – 1999 0.07 0.51
1964 – 1999 0.42 1.21
Site 2 Period Mean incision (m) Mean annual incision rate (cm/year)
1908 – 1965 3.07 5.39
1965 – 1999 0.26 0.76
1908 – 1999 3.33 3.66
1908 – 1965 2.06 3.62
1965 – 1999 0.49 1.43
1908 – 1999 2.54 2.79
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riverbed influenced sediment movement: the upstream strong rising slope (1.08%, KP 292 –KP 292.6; Fig. 2), and also the sharp decrease of hydraulic values on the riffle, limited the by-passing of coarse sediments. Sediment accumulation, noticed upstream after 1986, underlined the arrival of a bed-load wave, the transit of which was facilitated by high hydraulic values of the channel downstream of Arles. This sediment supply could be attributed to the major 1993 –1994 floods (40-, 80-, 30-year flood recurrence). Observations by diving and regular dredging at Arles confirmed a transit of coarse bed-load (7360 m3 in 2000, D50 = 18– 28 mm, oral com. CNR) that can be mobilised during floods, in relation to high boundary shear stress (cf. supra 4.2). Site 1 plays an essential role in the granulometric refinement, in the lower Rhoˆne River. Bank and stream samples collected downstream are finergrained (D50 < 0,4 mm; CNR dredging near the river
Fig. 10. Relations between hydraulic data and channel incision at both sites during the common period (1960 – 1999). Due to the lack of data for the 100-year flood, the diagram only shows results calculated for a 1-year flood event. As expected, channel incision seems to be more correlated with boundary shear stress but the poor statistical signification suggests that other parameters are involved in the channel degradation.
sequently an increased erosion capacity; an increase in specific stream power could explain the stream ability to adjust its channel morphology (Biedenharn et al., 2000). However, hydraulic data and incision/ aggradation values are not directly reliable because the first include mean slope and mean velocity values, whereas incision/aggradation processes are linked to floods. Therefore, other components should be involved in the geomorphologic behaviour of the lower Rhoˆne River. At site 1, the morphogenic mobility was limited by the resistant lithology of the channel bed and the bank armouring. However, the uneven topography of the
Fig. 11. Relations between s0 and x0 (first graph) and between channel width and depth at sites 1 and 2 (second graph). Both sites experience different correlations, higher significant at site 1 than at site 2. These differences could be linked to the morphology of the studied sites.
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mouth; Suanez, 1997; Arnaud-Fassetta, 1998; Antonelli et al., 2001). Pebbles (median grain size = 40 –50 mm), locally observed (Arnaud-Fassetta et al., submitted for publication), come from local sources (the Pleistocene substratum and some Holocene gravel barriers (Vella, 1999), and are reworked during floods or CNR’s dredging. At site 2, the whole bed was eroded from 1908 until 1999, with an erosion rate double than that of site 1. The stronger erosion was allowed by the lithology of the channel (Holocene sandy silts). The boundary shear stress, higher than in site 1, is able to move particles from 17 to 22 mm during 100-year floods, while bank sediments grain size is only 250– 300 Am; which could explain the bed erosion, which was more important on the concave banks, where the crosssections showed an undermining at the base. The size and the location of the lower Rhoˆne River channel deformations were partially induced by the local constraints of boundary shear stress and specific stream power, themselves bound to channel morphology: the river floor lithology (cohesive or uncohesive), granulometry (gravel, muddy sands), and topography (slope, curve) locally adjusted the hydraulic parameters. But hydraulic data are not sufficient to explain the channel morphologic evolution. The participation of local geomorphologic forces cannot mask a global erosion trend during the 20th century. 5.3. Comparison with channel bed evolution in the catchment area Previous studies explained the channel entrenchment in the delta by the effects of the reduction of solid discharge, due to hydro-climatic and anthropological changes in the catchment basin since the end of the 19th century (Arnaud-Fassetta, 2002). The lower Rhoˆne incision was thus related to the incision phase noticed upstream during the 20th century. On the upper and middle Rhoˆne River and their tributaries, channel erosion was attributed to the reduction of the solid discharges (notably the morphogenic bed-load), following land-use changes and reforestation in the catchment (Jorda, 1985; Bravard, 1989; Landon, 1999; Rinaldi and Simon, 1998; Winterbottom, 2000; Warner, 2000). The stabilization of the bed-load, as demonstrated on the torrential sections of the river, induced changes in the river style and the sinking of the
river bed (Bravard, 1989; Landon and Pie´gay, 1994; Klingeman et al., 1994). The incision trend started at different dates, but it was widespread and accelerated during the second half of the 20th century in relation to dam constructions and dredging activities. Therefore, a comparison of channel bed adjustments in the delta with those in upstream reaches and tributaries of the Rhoˆne River shows a time-lag, suggesting that the lower Rhoˆne River developed an original response to external stresses. There has been a slackening of the erosion rate since the middle of the 20th century, whereas the upstream catchment and the alpine tributaries have experienced an opposite trend, which then has to be explained. The potential effects of human and physical induced-changes are therefore described and discussed below. They probably interfere with each other to explain the channel evolution of the lower Rhoˆne. The channelisation, embankments and engineering works that started around the 1860s on the lower Rhoˆne in order to protect people against floods and to ease navigation probably explain the early beginning of the entrenchment (Arnaud-Fassetta, 2002). Straightening favours transit and reduces in-channel sediment storage. These human activities have increased the effects of the climatic changes characterised by the reduction of flood numbers at the end of the Little Ice Age until the beginning of the 20th century (Pichard, 1995). Even though the hydrological trend over the 1920 – 2000 period showed an insignificant increase in the average monthly discharge at Beaucaire, the occurrence of large floods decreased after 1910 (Fig. 12). The hydraulic energy deficit resulting from this decrease was not compensated for by the large flood events in 1993 and 1994 and could explain the decrease in the incision rate measured since the 1960s. During the first part of the 20th century both natural and human forces contributed to the change of fluvial style on the lower Rhoˆne, replacing a braiding with a meandering channel, as described on many of the rivers in Western Europe (Starkel, 1983; Probst, 1989; Bravard, 1989; Bravard and Peiry, 1993; Gautier, 1994; Miramont and Guilbert, 1997; Landon, 1999; Warner , 2000). The slowing down of the channel evolution during the second part of the 20th century could then be assumed to be the initiation of the relaxation time, leading to a state of a new ‘‘dynamic equilibrium’’ (Bravard and
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Fig. 12. Overall trend of the bed degradation in the lower Rhoˆne River. The transition from the reaction phase to the relaxation phase is unknown due to the lack of data all over the century. But the study of previous work (see text) allows us to suggest a change in the sediment balance at the end of the 19th century. (Pont et al., 2002; Antonelli, 2002)
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Petit, 1997; Brunsden, 1980 in Knighton 1998; Schumm et al., 1984). A reduction of 50% of the suspended load has been admitted since the end of the 19th century (Fig. 12): Before the Second World War, this reduction was attributed to reforestation and a decrease of major flood recurrences. After 1950, this sedimentary decrease was attributed to the hydrological deficit and dam construction. Even though studies have shown that dams could allow the transit of suspended sediments (IRS, 2000), the transit of coarse load is stopped or reduced, particularly in relationship with the weaker hydraulic energy on the old Rhoˆne’s arms (Poinsard et al., 1989; IRS, 2000). This could suppose an erosion of a ‘‘clear-water’’ type, which however is not relevant for the slowing down of the incision since the 1960s. The role played by dams in the channel incision has to be moderated because the most degraded period (1908 – 1965) corresponds to the construction of five dams whereas the next period (1965 – 1999) is synchronous with the construction of 17 dams on the whole Rhoˆne River (Fig. 12). That would indicate the limited effect of hydroelectric plants on the morphological changes on the lower Rhoˆne River. Similarly, dams generally induce downstream incision (Rinaldi and Simon, 1998; Jiongxin, 1996), but the studied sites were located more than 30 km downstream of the last dam (Vallabre`gues dam) the functioning of which began in 1971, i.e. after the decrease of the incision rates. Therefore, the slowingdown in the incision rate, since the 1960s, is paradoxically coincident with the most intensive hydroelectric management and dredging activity, i.e. with the most drastic reduction in global solid discharge. In a deltaı¨c system such as that of the Rhoˆne River, riverbed erosion cannot be attributed to the morphogenic action of a coarse bed-load (gravel, pebbles), because it is not dominant in the channel. Its presence is not disputed upstream of site 1, but pebbles observed downstream are linked to mining works or to the outcrop of a Holocene shingle bar. The pebbly load is absent downstream KP 302 and does not intervene in site 2 morphogenesis. The predominance of sands in the downstream delta has been confirmed over several millennia: the powerful channel of ‘‘Bras de Fer’’, that led to an important progradation of the river mouth in the 17– 18th century, revealed a maximal grain size inferior to 800 Am (Arnaud-Fassetta
and Provansal, 1999). Therefore, channel adjustment in the delta is driven by finer sediments and could be assumed by the erosive action of flood waters highly loaded with suspended sediments: this theory is sometimes used to explain bank erosion and channel extension (Green et al., 1999). During floods, a mobile sandy bottom layer, with a strong density (intermediate between carriage and suspension), constitutes the ‘‘active morphogenic bedload’’ (Bagnold, 1977; Leeder, 1977), which would explain the vertical incision of the channel floor. Moreover sand flux measures indicated higher concentrations of sediments at the base of the water column for moderate flows (inferior to 3000 m3/s; Antonelli et al., 2001) and diving observations showed hydraulic dunes moving on the bottom. The decrease of flood occurrence since 1960 could probably explain the recent relative stabilisation of the channel morphology in relation to a reduction in bed-load supply. Therefore, the original timing of channel entrenchment in the lower Rhoˆne River suggests a specific morphodynamic functioning, in relationship with geomorphic factors of the deltaı¨c environment. The local channelisation and embankments, associated with the decrease of major floods, influence channel entrenchment. The recent slowing of channel entrenchment could be explained both by a relaxation effect, towards a state of ‘‘dynamic equilibrium’’, and, paradoxically, by the decrease of the bed load induced by the last dam constructions. The morphogenic impacts of dam and dredging activities are thus the opposite of those in the upper and middle Rhoˆne valley. The near marine base level, which has been rising by 2 mm year 1 over the last century (Suanez et al., 1997), is certainly a complementary factor of stabilisation.
6. Conclusion The study of detailed cross-sections registered over a century allows a better understanding of the recent modifications of the lower Rhoˆne River, principally characterised by channel incision. This bed sinking, highly significant at the beginning of the century has been decreasing for 40 years. Hydraulic constraints and channel morphology locally regulate the incision rates. Boundary shear stress and specific stream power partially explain the
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morphological evolution of the Rhoˆne River, but they have to be completed by lithologic and morphologic local factors. Nevertheless the initiation and duration of the incision and its recent decrease are linked to various causes that intervene at local and global scales. The incision in both sites has decreased since the 1960s, whereas the upper and middle Rhoˆne, and its tributaries, have undergone an erosive increase. Local bank management, increasing the effects of the climatic change (since the end of the Little Ice Age), has initiated the channel geomorphic evolution, which is driven by a sandy bed load. Thus the slowing-down in the incision rate is explained by the interference of several factors: the reduction of the hydraulic energy, due to the lack of large floods, the drastic reduction (about 50%) of the global solid discharge, the rise of the near marine base-level and, likely, a relaxation towards a state of ‘‘dynamic equilibrium’’ after the rapid incision in the first half of the 20th century. These observations emphasise the fact that the lower Rhoˆne River corresponds to a specific environment in which the functioning of the global hydrosystem is partly controlled by local constraints (hydraulic data, bedforms, base level, embankments). The original responses recorded in the lower Rhoˆne River, in contrast to those in the upstream catchment, raise the question of the time and space scale in a large river and suggest that different reaches in a fluvial system may exhibit various morphological responses to a common climato-anthropological history. As mentioned by Knighton (1998, p.262) ‘‘events are unlikely to be synchronous over large areas because the threshold conditions that partly determine the magnitude and direction of change are influenced by factors which are themselves spatially variable.’’ In fact, the lower Rhoˆne River appears sensitive to flood events, but remains dependent on its intrinsic structure and the nearness of the base level. References Abernethy, B., Rutherfurd, I.D., 1998. Where along a river’s length will vegetation most effectively stabilise stream banks? Geomorphology 23, 55 – 75. Antonelli, C., 2002. Flux se´dimentaires et morphogene`se re´cente dansle chenal du Rhoˆne aval. PhD thesis, University Aix-Marseille I. 279 pp. Antonelli, C., Provansal, M., Genty, P., 2001. Evaluation du trans-
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it sableux du Rhoˆne d’Arles a` la mer. Scientifique et de´cideurs: agir ensemble pour une gestion durable des syste`mes fluviaux 6-7-8 juin, Lyon. Arnaud-Fassetta, G., 1998. Dynamiques fluviales holoce`nes dans le delta du Rhoˆne. PhD thesis, Universite´ Aix-Marseille I. 329 pp. Arnaud-Fassetta, G., 2002. River channel changes in the Rhoˆne Delta (France) since the end of the Little Ice Age: geomorphological adjustment to hydroclimatic change and natural resource management. Catena 51, 141 – 172. Arnaud-Fassetta, G., Provansal, M., 1999. High frequency variations of water flux and sediment discharge during the Little Ice Age (1586 – 1725 AD) in the Rhoˆne delta (Mediterranean France). Relationship to the catchment basin. Hydrobiologia 410, 241 – 250. Arnaud-Fassetta, G., Quisserne, D., Antonelli, C., submitted for publication. Bed material discontinuum along a large and managed river: the case of the Rhoˆne River in its delta (France) after 150 years of climatic-societal disturbances. Geomorphologie: Relief, Processus, Environnement. Astrade, L., Bravard, J.P., 1999. Energy gradient and geomorphological processes along a river influenced by neotectonics (the Saoˆne river, France). Geodinamica Acta 12, 1 – 10. Bagnold, R.A., 1966. An approach to the sediment transport problem from general physics. U.S. Geological Survey Professional Paper 422-I, 1 – 37. Bagnold, R.A., 1977. Bed load transport by natural rivers. Water Resources Research 13-2, 303 – 312. Biedenharn, D.S., Thorne, C.R., Watson, C.C., 2000. Recent morphological evolution of the Lower Mississippi River. Geomorphology 34, 227 – 249. Blum, M., Tornqvist, T.E., 2000. Fluvial response to climate and sea-level changes: a review and lookforward. Sedimentology 47, 2 – 48. Bravard, J.P., 1989. La me´tamorphose des rivie`res des Alpes francßaises a` la fin du Moyen-Age et a` l’e´poque moderne. Bulletin de la Socie´te´ Ge´ographique de Lie`ge 25, 145 – 157. Bravard, J.P., Peiry, J.L., 1993. La disparition du tressage fluvial dans les Alpes francß aises sous l’effet de l’ame´nagement des cours d’eau (19 – 20 e`me sie`cle). Zeitschrift fu¨r Geomorphologie 88, 67 – 79. Bravard J.P., Petit, F., 1997. Les cours d’eau, Dynamique du syste`me fluvial. Ed. Armand Colin/Masson, Paris, France. Carson, M.A., 1987. Measures of flow intensity as predictors of bed load. Journal of Hydraulic Engineering 113-11, 1402 – 1421. Gasowski, Z., 1994. L’enfoncement du lit de la Loire. Revue de Ge´ographie de Lyon 69-1, 41 – 45. Gautier, E., 1994. Interfe´rence des facteurs anthropiques et naturels dans le processus d’incision sur une rivie`re alpine - l’exemple du Bue¨ch (Alpes du Sud). Revue de Ge´ographie de Lyon 69-1, 57 – 61. Green, T.R., Beavis, S.G., Dietrich, C.R., Jakeman, A.J., 1999. Relating stream-bank erosion to in-stream transport of suspended sediment. Hydrological Processes 13, 777 – 787. Guillen, J., Palanques, A., 1992. Sediment dynamics and hydrodynamics in the lower course of a river highly regulated by dams: the Ebro River. Sedimentology 39, 567 – 579. IRS, 2000. Etude globale pour une strate´gie de re´duction des
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risques dus aux crues du Rhoˆne. Etude du transport solide, 1e`re e´tape, rapport de synthe`se. 5 volumes, 469 p. Institution interde´partementale des bassins Rhoˆne-Saoˆne, Valence, France. Jiongxin, X., 1996. Underlying gravel layers in a large sand bed river and their influence on downstream-dam channel adjustment. Geomorphology 17, 351 – 359. Jorda, M., 1985. La torrentialite´ holoce`ne des Alpes francßaises du sud, facteurs anthropiques et parame`tres naturels de son e´volution. Cahiers ligures de Pre´histoire et Protohistoire 2, 49 – 70. Klingeman, C., Bravard, J.P., Giuliani, Y., 1994. Les impacts morphodynamiques sur un cours d’eau soumis a` un ame´nagement hydroe´lectrique a` de´rivation: le Rhoˆne en Chautagne (France). Revue de Ge´ographie de Lyon 1, 73 – 87. Knighton, A.D., 1998. Fluvial Forms and Processes. Edward Arnold, London. 383 pp. Knighton, A.D., 1999. Downstream variation in stream power. Geomorphology 29, 293 – 306. Kruit, C., 1955. Sediments of the Rhone delta; grain size and microfauna. Verhandelingen van het Koninklijk Mijnbouwkundig Genootschap. Geologische serie deel 15, 359 – 501. Landon, N., 1999. L’e´volution contemporaine du profil en long des affluents du Rhoˆne moyen. Constat re´gional et analyse d’un hydrosyste`me complexe, la Droˆme. PhD thesis, Universite´ Paris IV-Sorbonne. 560 pp. Landon, N., Pie´gay, H., 1994. L’incision de deux affluents subme´diterrane´ens du Rhoˆne: la Droˆme et l’Ade`che. Revue de Ge´ographie de Lyon 1, 63 – 72. Leeder, M.R., 1977. Bedload stresses and Bagnold’s bedform theory for water flows. Earth Surface Processes 2, 3 – 12. Leopold, L.B., Wolman, M.G., Miller, J.D., 1964. Fluvial Processes in Geomorphology Freeman, San Francisco. 522 pp. Leteinturier, B., Engels, P., Petit, F., Chiffaut, A., Malaisse, F., 2000. Morphodynamisme d’un troncßon de Loire boubonnaise depuis le XVIIIe sie`cle. Geomorphologie: Relief, Processus, Environnement 4, 239 – 252. Marston, R.A., 1994. River entrenchment in small moutain valleys of the western USA: influence of beaver, grazing and clearcut logging. Revue de Ge´ographie de Lyon 69, 11 – 15. Miramont, C., Guilbert, X., 1997. Variations historiques de la fre´quence des crues et e´volution de la morphogene`se fluviale en moyenne Durance (France, S – E). Ge´omorphologie: Relief, Processus, Environnement 4, 325 – 338. Miramont, C., Jorda, M., Pichard, G., 1998. Evolution historique de la morphogene`se et de la dynamique fluviale d’une rivie`re me´diterrane´enne: l’exemple de la moyenne Durance (France du SE). Ge´ographie Physique et Quaternaire 52, 381 – 392. Oomkens, E., 1970. Depositional sequences and sand distribution in the postglacial Rhoˆne delta complex. In: Morgan, J.P. (Ed.), Deltaic Sedimentation Modern and Ancient, pp. 198 – 212. Parde´, M., 1951. Sur le me´canisme des transports solides effectue´s par les rivie`res et les alte´rations corre´latives des lits fluviaux. Revue de Ge´ographie Alpine 39, 5 – 40.
Pichard, G., 1995. Les crues sur le bas Rhoˆne de 1500 a` nos jours. Pour une histoire hydro-climatique. Me´diterrane´e 3 – 4, 105 – 116. Poinsard, D., 1992. Effets des ame´nagements fluviaux sur les de´bits liquides et solides. L’exemple du Rhoˆne dans les plaines de Miribel-Jonage et de Donze`re-Mondragon. Thesis, Universite´ Lyon III, France. 501 pp. Poinsard, D., Bravard, J.-P., Caclin, M.-C., 1989. Profil en long et granulome´trie du lit des cours d’eau ame´nage´s: l’exemple du canal de Miribel (Haut-Rhoˆne). Revue de Ge´ographie de Lyon 64-4, 240 – 251. Pont, D., Simonnet, J.P., Walter, A.V., 2002. Medium-term changes in suspended delivery to the ocean: consequences of catchment heterogeneity and river management (Rhoˆne River, France). Estuarine Coastal Shelf Science 54, 1 – 18. Probst, J.L., 1989. Hydroclimatic fluctuations of some Europeans rivers since 1800. In: Petts, G.E., Moller, H., Roux, A.L. (Eds.), Historical Change of Large Alluvial Rivers, Western Europe. Wiley, Chichester, pp. 41 – 55. Rinaldi, M., Simon, A., 1998. Bed-level adjustments in the Arno river, central Italy. Geomorphology 22, 57 – 71. Schumm, S.A., Harvey, M.D., Watson, C.C., 1984. Incised Channels. Morphology, Dynamics and Control. Water Resources Publication. 200 pp. Starkel, L., 1983. The reflection of hydrologic changes in the fluvial environment of the temperate zone during the last 15000 years. In: Gregory, K.J. (Ed.), Background to Palaeohydrology. Wiley, Chichester, pp. 213 – 235. Suanez, S., 1997. Dynamiques actuelles et re´centes de la frange littorale orientale du delta du Rhoˆne. PhD thesis Universite´ Aix-Marseille I. 282 pp. Suanez, S., Prosper-Laget, V., Provansal, M., 1997. Variations relatives du niveau marin dans le delta du Rhoˆne et a` Marseille— implications climatiques et/ou tectoniques. Comptes Rendus de l’Acade´mie des Sciences de Paris 324 (IIa), 639 – 646. Van Den Berg, J.H., 1995. Prediction of alluvial channel pattern of perennial rivers. Geomorphology 12, 259 – 279. Vella, C., 1999. Perception et e´valuation de la mobilite´ du littoral holoce`ne sur la marge orientale du delta du Rhoˆne. PhD thesis, Universite´ d’Aix-Marseille I. 217 pp. Vo¨ro¨smaty, C.J., Meybeck, M., Fekete, B., Sharma, K., 1997. The potential impact of the neo-castorization on sediment transport by the global network of rivers. Human Impact on Erosion and Sedimentation, IAHS Publ. 245, 261 – 273. Warner, R.F., 1993. Downstream changes in channel morphology and capacity in the Bellinger Valleys, NSW, Australia. Zeitschrift fu¨r Geomorphologie 88, 29 – 47. Warner, R.F., 2000. Gross channel change along the Durance River, southern France, over the last 100 years using cartographic data. Regulated Rivers: Research & Management 16, 141 – 157. Winterbottom, S.J., 2000. Medium and sort-term channel planform changes on the Rivers Tay and Tummel, Scotland. Geomorphology 34, 195 – 208.
Recent morphological channel changes in a deltaı¨c environment. The case of the Rhoˆne River, France C. Antonelli *, M. Provansal, C. Vella CEREGE, Europoˆle Me´diterrane´en de l’Arbois, BP 80, Aix-en-Provence 13545, France Received 27 November 2001; received in revised form 10 March 2003; accepted 28 March 2003
Abstract Channel morphological changes were measured at two sites within the Rhoˆne delta along the main channel of the Rhoˆne River over one century. Results show an average erosion rate of 2.8 mm year 1 during the 20th century and decreasing to < 1.5 mm year 1 since the 1960s. Erosion affected site 2 more than site 1 but cross-sections can alternately be in erosion or accretion depending on bed topography and/or floor lithology. Riverbed adjustments are partly explained by the boundary shear stress and specific stream power but local factors also influence the Rhoˆne River evolution. Results compared with those obtained upstream or in other European rivers suggest that the lower Rhoˆne River has developed an original response to external stresses: the first part of the 20th century is characterised by an important degradation followed by a decrease in the incision rate since the 1960s. The initiation of the erosion phase is assumed to be linked to the end of the Little Ice Age, to the reforestation of the alpine hillsides, to the earlier management and to the reduction of the sedimentary yield (according to the literature). The second phase possibly corresponds to the relaxation period. In this perspective, the effects of dams are probably less significant than we would suppose up to now. It implies that such a large river records morphological responses which may vary all along the fluvial system in relation to the time and space scales involved. D 2003 Elsevier B.V. All rights reserved. Keywords: Channel changes; Boundary shear stress; Specific stream power; Climato-anthropic impacts; Lower Rhoˆne River
1. Introduction In a natural shifting river, channel morphology is the result of active bed-load transport. In such river systems, channel geometry corresponds to river adjustments resulting from modifications of the sedimentary load which is directly linked to land-use changes in the catchment, hydroelectric structures, and climatic changes (Parde´, 1951; Starkel, 1983; * Corresponding author. E-mail addresses: [email protected] (C. Antonelli), [email protected] (M. Provansal), [email protected] (C. Vella). 0169-555X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-555X(03)00167-3
Blum and Tornqvist, 2000). Large rivers in developed countries have been experiencing a trend of bed incision since the beginning of the 20th century (Guillen and Palanques, 1992; Warner, 1993; Gasowski, 1994; Landon and Pie´gay, 1994; Marston, 1994; Vo¨ro¨smaty et al., 1997; Rinaldi and Simon, 1998; Winterbottom, 2000). In some cases, an increase in the incision rate has been observed since the 1950s (Bravard, 1989; Bravard and Peiry, 1993; Gautier, 1994; Gasowski, 1994; Miramont et al., 1998; Leteinturier et al., 2000). The upstream Rhoˆne River and its main tributaries were also affected by this phenomenon mainly
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linked to three external stresses. First, the end of the Little Ice Age and land-use changes in the catchment area have modified the hydrologic regime and have reduced the solid discharge (Pichard, 1995; Landon, 1999; Warner, 2000). Secondly, the dykes, constructed since the end of the 19th century in order to protect people and land against floods, have isolated the Rhoˆne River from its floodplain (Poinsard, 1992; Arnaud-Fassetta, 1998). Finally, since 1945 hydroelectric plants (18 dams from Lake Geneva to the sea, Serre-Poncßon dam and its annex on the Durance River, etc.), sand and gravel mining in the river channel and its tributaries have been responsible for reducing and modifying solid transport downstream (Poinsard et al., 1989; Klingeman et al., 1994). The longitudinal profiles were used by ArnaudFassetta (1998) to demonstrate a global bed incision. The opportunity to analyse precise detailed cross-sections led us to conduct a new study of the morphological changes of the deltaı¨c Rhoˆne River.
In the deltaı¨c part of the Rhoˆne River, crosssections have been recorded since the beginning of the 20th century by the National Company of the Rhoˆne River (CNR). These data were surveyed downstream of a large catchment (98 000 km2) that integrates time and space scales which cannot be compared with the recent modifications observed on the Rhoˆne River tributaries. The main objective of this study is to examine morphological modifications of the deltaı¨c part of the Rhoˆne River over the 20th century and to compare them with the results observed in the upstream catchment and on other European hydrosystems. This study also attempts to integrate spatio-temporal parameters involved in the evolution of this deltaı¨c system.
2. Study area The Rhoˆne delta corresponds to a base-level plain where accumulation has been dominant for several
Fig. 1. Location map of Rhoˆne river delta and both studied sites. Note that Kilometric Point 0 is located at Lyon (corner map).
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millenia (Kruit, 1955; Oomkens, 1970). Hydrological and sedimentary variations (of climatic and/or anthropological origins) are expressed by deposits of varying granulometry (silt to medium-coarse sands), changes of river style (meander, crevassing), and the rapid progradation of the river mouth (Arnaud-Fassetta, 1998; Vella, 1999). At Arles, the Rhoˆne River divides into two arms (Fig. 1). The first one, the Grand Rhoˆne (Eastern branch, 50 km long), discharges 85 –90% of the total flow into the Mediterranean Sea via the Roustan grau. The second one, the Petit Rhoˆne (Western branch, 70 km long) discharges its flow into the sea via the Orgon grau. This study focuses on the Grand Rhoˆne River for which three cross-sectional fields were surveyed during the 20th century. Its mean annual water flow is 1500 m3/s at the gauging station at Arles and varies between 320 m3/s at low water level and 3800 m3/s during a 1-year flood and to 10 000 m3/s during a 100-year flood. The river width varies from 150 m at Arles to 1000 m at the river mouth. The longitudinal profile shown in Fig. 2 is uneven and characterised by riffles ( 4.75 m below the mean sea level, i.e., msl) and pools ( 18 to 21 m msl). Embankment and channelisation of the river have led to a straight to moderately sinuous channel (1,0 to
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1,3; Arnaud-Fassetta et al., submitted for publication). Two sites were selected on this branch because of their representativeness. Their overall length is 12 km. Site 1 (Fig. 3a) is located between kilometric point (KP) 292 and KP 296. From KP 292 to KP 292.6, the longitudinal profile indicates an abrupt rise of the river bed (f 1%). Between KP 292.6 and KP 294.7, the reach is characterised by the presence of the highest riffle of the channel (Terrin riffle), the depth of which has been maintained at 4.75 m msl for navigation purposes by excavation of the bedrock channel (in 1969, 1979 and 1991) and by regular dredging. On this segment, the river bed is plane (f 0.004%). Finally, from KP 294.7 to 296, the longitudinal profile decreases towards the following pool (f 0.24%). Channel width at site 1 varies between 386 and 666 m but the central part of this reach is wider (mean width = 523 m) than the other two (406 m). The river floor consists of Pleistocene conglomerates (i.e., Crau gravel) with pebbles embedded in a very cohesive silty – clayey matrix. The banks are paved (boulder armouring) nearly everywhere at site 1. Site 2 (KP 306– KP 314, Fig. 3b) corresponds to a low sinuosity section (1.14). Its longitudinal
Fig. 2. Longitudinal profile of Grand Rhoˆne (after CNR).
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Fig. 3. Map and longitudinal profile of site 1 (a) and site 2 (b).
profile is uneven whereas its width varies between 312 and 462 m. Site 2 is narrower but deeper than site 1. The central part of this reach has an ancient braided stream deposit as confirmed by a secondary channel (always submerged) and the alluvial deposits of ‘‘Pilotes’ island’’. The dominant bed material is sand with a median grain size of 370 – 410 Am.
3. Methodology 3.1. Cross-section data Numerous cross-sections have been recorded since the beginning of the 20th century and stored in the archives of the CNR. However, the majority of the cross-sections display only the navigable chan-
C. Antonelli et al. / Geomorphology 57 (2004) 385–402 Table 1 Characteristics of cross-sections recorded by CNR Date
Area (from KP to KP)
Distance between two cross-sections (m)
1908 1964 1965 1986 1999 1999
306 – 314 292 – 296 306 – 314 292 – 300 269.1 – 300 300 – 317
500 100 500 100 100 500
nel, which is generally narrower than the natural channel. Hence, only two sections possess enough data to allow a precise chronological study of the channel changes (Table 1). At site 1, 127 crosssections were surveyed each 100 m and were recorded in 1964, 1986 and 1999. At site two, 54 cross-sections were surveyed each 500 m and were recorded in 1908, 1965 and 1999. Therefore, the chronology is different on each site: data cover all the 20th century at site 2 (the longer and less
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artificial) whereas at site 1 data cover only the second part of the 20th century. These data have accorded since the 1960s, after the major humaninterventions occurred on the catchment area. All the profiles are referenced by CNR with regard to the mean sea level (msl), thus allowing comparison. The surveying methodology and vertical precision of the measurements in 1908 are unknown. However, all the other cross-sections were recorded using an echo sounder with a vertical precision of about F 20 cm. In the cross-sections recorded in 1908 and 1965, channel depths were measured manually (60 – 90 survey points by cross-section). The reading error on paper is estimated to be F 0.5 mm and corresponds to a vertical error of F 5 cm on a crosssection. The most recent profiles (1999) display between 33 and 172 survey points. All cross-sections were converted into a format compatible with BMAPR (software designed to measure beach profile and adapted to our needs). We measured the
Fig. 4. Overlaying of cross-section data: an example of channel bed evolution at sites 1 and 2.
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mean width at three levels at the first site (0, + 1 and + 2 m msl for 1964, 1986 and 1999, respectively) and at four levels at the second site (0 and + 0.5 m msl for 1908 and 1965 and 0, + 0.5, + 1, + 2 m msl for 1999). For each cross-section, the riverbed level differences between two dates were automatically measured with a 10-m interval. Then, the mean incision or aggradation profile was calculated from the comparison of these differences (using ExcelR). Finally, the eroded (or deposited) volumes were calculated in two steps. First, BmapR calculated the incised or eroded area by the overlaying of two crosssections (Fig. 4). During the next step, volumes were calculated: the incised or eroded area was multiplied by the distance between two cross-sections (i.e., 100 m at site 1; 500 m at site 2). These two steps were
repeated for each segment of both sites. In summing up each volume we obtained each site total volume. Results were sorted using the statistical method of quartiles: D20, D40, D60, D80 with DN is the incision (or deposition) value such that N percent of data results are inferior. 3.2. Hydraulic data Hydraulic data were selected in order to understand if internal factors were involved in the Rhoˆne River morphological changes. Boundary shear stress (s0) and specific stream power (x) were calculated for a 1-year flood event and for a 100-year flood event (discharge of about 3800 m3/s and 10 000 m3/ s respectively at Arles). Indeed in a recent period (1993 –1994) three major floods with a 30 –80-year
Fig. 5. Mean channel width at sites 1 and 2.
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Fig. 6. Vertical variations of channel bed at sites 1 and 2.
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recurrence occurred, therefore we took into account their potential morphogenic implications. Geometrical data (wetted area, width) were calculated using MapInfoR software. The cross-sections recorded in 1908 were not treated due to the lack of survey points at the top of the banks. Specific stream power was calculated using the Bagnold’s (1966, 1977) equation: x ¼ qgQs=w
ð1Þ
where q = specific weight of water, g = gravitational acceleration, Q = discharge, s = water-surface slope and w = bankfull width. Boundary shear stress is given by (Carson, 1987; Bravard and Petit, 1997) s0 ¼ gqRs
ð2Þ
where R = hydraulic radius. The slope used in both equations is equal to the mean water-surface slope in the Rhoˆne delta (4 10 5 m m 1; IRS, 2000).
4. Results 4.1. Morphological evolution at both sites during the 20th century The channel mean width on both sites has displayed a significant stability for 40 years (Fig. 5). At site 1, we noted a weak narrowing of the channel between 1964 and 1986 ( 1%) followed by a widening between 1986 and 1999 ( + 2%). Changes were irregular all along the reach and alternatively affected shallow or deep cross-sections. At site 2, we observed an increase of the channel width ( + 2.8%) during the first period (1908 –1965) followed by a slight narrowing ( 1.7%) between 1965 and 1999. Since the 1960s, the channel mean width has displayed a differ-
Table 2 Mean vertical changes measured at site 1 Period
1964 – 1986 1986 – 1999 1964 – 1999
Mean variation Mean (cm/year)
Standard deviation
1.58 0.50 1.21
1.14 1.57 1.05
Maximal variation (cm/year)
Minimal variation (cm/year)
+ 0.31 + 3.45 + 0.50
3.72 3.71 3.09
Table 3 Mean vertical changes measured at site 2 Period
1908 – 1965 1965 – 1999 1908 – 1999
Mean variation Mean (cm/year)
Standard deviation
3.62 1.43 2.79
0.99 2.1 0.66
Maximal variation (cm/year)
Minimal variation (cm/year)
6.43 3.1 4.25
2.38 + 0.24 1.9
ent evolution on each site; nevertheless the average of the lateral changes, which were comprised between + 4 m (site 1) and 1 m (site 2), are negligible. Calculation of the bed adjustments, mapped by quartiles, shows a deepening of the channel on both sites all over the 20th century. At site 1 (Fig. 6, Table 2), the mean incision was about 0.42 m in 35 years (i.e. 1.2 cm year 1) but the incision rate decreased from 1.56 cm year 1 during the first period (1964 – 1986) to 0.5 cm year 1 since 1986. The upstream and central parts of site 1 (from KP 292.6 to KP 294.7) at first characterised by incision have experienced deposition since 1986. The last segment (from KP 294.7 to KP 296) has remained in erosion over the whole period. As a result, the general trend has been erosion with a clear reduction in incision since 1986 (rate divided by 3). At site 2, results indicate a mean channel erosion of 2.5 m during the 20th century which was 2.8 cm year 1 (Table 3, Fig. 6). In the period 1908– 1965, the bend (KP 309– KP 311) was the most eroded area whereas between 1965 and 1999, maximum values were located upstream. On a century-scale, the bend was more sensitive to erosion. Like at site 1, the most recent period (1965 – 1999) showed a significant decrease in erosion (2.5 time less than in period 1908 – 1965). Finally, over the three periods of measurements, channel erosion has slowly decreased. The mean vertical incision has been almost identical on both sectors since the 1960s: 1.21 cm year 1 at site 1 and 1.43 cm year 1 at site 2. The first part of the 20th century corresponded to an intensive phase of erosion on site 2. Then, since the 1960s, the incision rate has been decreasing. This phenomenon was corroborated by results obtained at site 1: the common period (1960 –1990) underwent the same incision rate, and site 1 also experienced a decrease in the studied period.
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Fig. 7. Specific stream power variations at sites 1 and 2.
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Fig. 8. Boundary shear stress variations at sites 1 and 2.
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4.2. Spatial and chronological evolution of the hydraulic data The x values spatial distribution remained constant over the studied period whereas the average values rose on both sites between 1964 and 1999 and reached about 3 W m 1 in 1999 (Fig. 7). These values are weak in comparison with other rivers (Van Den Berg, 1995; Abernethy and Rutherfurd, 1998; Bravard and Petit, 1997; Knighton, 1999; Leteinturier et al., 2000; Biedenharn et al., 2000) and with upstream tributaries (Astrade and Bravard, 1999; Landon, 1999). They were strongly influenced by the gentle water-slope of the river which decreases from 0.5xupstream of the delta to 0.04xin the delta. However, for a 100-year flood, the water slope reached 0.2xin the delta, inducing a specific stream power between 5.5 and 16 W m 2 (site 1) and between 8.5 and 13 W m 1 (site 2). The boundary shear stress values increased significantly over the three periods (Fig. 8), whereas their spatial distribution remained rather constant. At site 1, s0 values were important in the deepest sections (from KP 292 to KP 292.6 and from KP 294.7 to KP 296), where their increase had been the most significant since 1964. To the contrary, on the central part of the reach (KP 292.6 –KP 294.7) values were constant. In 1999, the contrast remained clear between the deepest part (average s0 = 2.7 N m 2) and the central part (average s0 = 1.82 N m 2), although this central part had retracted downstream more than a kilometre since 1964. At site 2, the effects of the bed topography did not appear so sharply even though values were higher at the end of the bend. The chronological increase in boundary shear stress was much more important than in site 1, in particular between 1908 and 1965 (mean increase of 25%). In 1999, it overtoped 3 N m 2 everywhere. The comparison of both parameters calculated on the Grand Rhoˆne branch from Arles to the sea from recent data (1999) indicated that s0 and x varied in phase, but with different amplitudes according to sectors (Fig. 9). Upstream of site 1, variations of s0 and x were in the same range. They sharply decreased from KP 293 which corresponded to the strong rising slope of site 1 ( + 1.08%). From KP 295 up to KP 306, specific stream power increased significantly while boundary shear stress values remained low. Such a
Fig. 9. s0 and x values from the gauging-station at Arles to the sea (calculated from cross-sections recorded in 1999).
difference between the two parameters disappeared in site 2. These disparities were due to the geomorphic variables taken into account in the equations (cf. 5.1): the depth and the width of the channel could explain the range of hydraulic data calculated on both sites. Calculation of the river competence was estimated with the boundary shear stress values (s0 c grain diameter; Leopold et al., 1964; Bravard and Petit, 1997; Knighton, 1998). In the case of the lower Rhoˆne River, the competence was of about 2 mm at bankfull discharge (3400 m3 s 1) rising to 13 – 20 mm for a 100-year flood at site 1. At site 2, values were higher due to the increased water-column; between 3 and 4 mm for bankfull discharge rising to 17 –22 mm for a 100-year flood. Those results suggest that pebbles are moving only during exceptional events. However, these data must be used with caution because they underestimated some results: indeed recent observations made after weak floods (inferior to bankfull discharge) have highlighted the mobility of pebbles (D50 = 11 mm).
5. Discussion 5.1. Representativeness of the incision measurements Previous studies (Arnaud-Fassetta, 1998, 2002; Arnaud-Fassetta and Provansal, 1999) described a channel incision, associated with a reduction of the river width, on both deltaı¨c branches, increasing from the end of the 19th century to the second part of the 20th century. Their conclusions were based on the
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treatment of longitudinal profiles recorded by the CNR in 1907, 1967 and 1991. These profiles were established from 85 measuring points with a 500-m interval. According to the authors, the vertical incision in the channel would be ‘‘almost uniform’’ on the whole profile, affecting the riffles rather than the pools, and would have increased by a factor 6 –7 since 1967. These conclusions are contradictory to our results, which show a deceleration in channel erosion in the second part of the 20th century and weaker erosion on the riffles than in the pools. This raises the question of the representativeness of the data used in each study (longitudinal profiles or cross-sections). In order to confront both methods, we have deducted longitudinal profile from cross-section data and calculated the incision rates on each site for each period. Results shown in Table 4 indicate that the use of longitudinal profiles led to an overestimation of the channel incision, which has two explanations: first, the navigable channel that often corresponds to the deepest and central part of the stream is regularly dredged, whereas the channel margins are not affected by such operations; secondly the deepest point focuses the highest forces applying in the riverbed thus leading to an amplification of the incision processes. In reality, different phenomena (aggradation, stability or incision) coexist on a cross-section and cannot be represented by one measurement point. Are the two studied sites of a 12-km total length, representative of the whole 50 km of the Grand Rhoˆne River? Deformations measured on site 1, located on a Pleistocene conglomerate, were probably linked to more important hydraulic stimuli than to modest events. Moreover, site 1 incision rates could be considered as minimum values for the lower Rhoˆne River. Cross-sections recorded at site 2 exhibited a
great variety of shapes, incised in sandy-silts, representative of the Grand Rhoˆne channel between Arlescity and the river mouth. Therefore the results obtained by the overlaying of cross-sections on representative sites of the Rhoˆne River are more precise. The morphological changes of both sites relied upon spatio-temporal variations of the hydraulic data, which in turn were influenced by the channel shapes. In order to understand the complex relationships existing between morphological and hydrological variables and their implications on the whole Grand Rhoˆne River, we explored statistical correlation between all these variables. Nevertheless, the morphological evolution of these two reaches is connected to a local functioning (delta scale) and furthermore to a regional functioning (catchment scale). Such an organisation requires taking into account external factors (climatic, anthropogengic) that could affect the riverbed adjustments. 5.2. Impacts of hydraulic and morphogenic data on the channel evolution The morphological modifications of the channel were not directly correlated with hydraulic data, particularly with stream power (Fig. 10). These disparities were induced by the geomorphic variables taken into account in the equations: the hydraulic radius (c channel depth) in s0 and channel width and wetted area in x. This, in turn, suggested variable channel morphology, creating different modes of liquid flow (turbulences, eddies; Fig. 11). The different values of the two parameters generated a variable morphogenesis on both sites: an increase in boundary shear stress suggests an intensification of the shearing forces at the bottom and con-
Table 4 Comparison of the incision rates from longitudinal and cross-section data Calculation from longitudinal profiles
Calculation from cross-section
Site 1 Period Mean incision (m) Mean annual incision rate (cm/year)
1964 – 1986 0.40 1.81
1986 – 1999 0.40 3.11
1964 – 1999 0.80 2.30
1964 – 1986 0.35 1.58
1986 – 1999 0.07 0.51
1964 – 1999 0.42 1.21
Site 2 Period Mean incision (m) Mean annual incision rate (cm/year)
1908 – 1965 3.07 5.39
1965 – 1999 0.26 0.76
1908 – 1999 3.33 3.66
1908 – 1965 2.06 3.62
1965 – 1999 0.49 1.43
1908 – 1999 2.54 2.79
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riverbed influenced sediment movement: the upstream strong rising slope (1.08%, KP 292 –KP 292.6; Fig. 2), and also the sharp decrease of hydraulic values on the riffle, limited the by-passing of coarse sediments. Sediment accumulation, noticed upstream after 1986, underlined the arrival of a bed-load wave, the transit of which was facilitated by high hydraulic values of the channel downstream of Arles. This sediment supply could be attributed to the major 1993 –1994 floods (40-, 80-, 30-year flood recurrence). Observations by diving and regular dredging at Arles confirmed a transit of coarse bed-load (7360 m3 in 2000, D50 = 18– 28 mm, oral com. CNR) that can be mobilised during floods, in relation to high boundary shear stress (cf. supra 4.2). Site 1 plays an essential role in the granulometric refinement, in the lower Rhoˆne River. Bank and stream samples collected downstream are finergrained (D50 < 0,4 mm; CNR dredging near the river
Fig. 10. Relations between hydraulic data and channel incision at both sites during the common period (1960 – 1999). Due to the lack of data for the 100-year flood, the diagram only shows results calculated for a 1-year flood event. As expected, channel incision seems to be more correlated with boundary shear stress but the poor statistical signification suggests that other parameters are involved in the channel degradation.
sequently an increased erosion capacity; an increase in specific stream power could explain the stream ability to adjust its channel morphology (Biedenharn et al., 2000). However, hydraulic data and incision/ aggradation values are not directly reliable because the first include mean slope and mean velocity values, whereas incision/aggradation processes are linked to floods. Therefore, other components should be involved in the geomorphologic behaviour of the lower Rhoˆne River. At site 1, the morphogenic mobility was limited by the resistant lithology of the channel bed and the bank armouring. However, the uneven topography of the
Fig. 11. Relations between s0 and x0 (first graph) and between channel width and depth at sites 1 and 2 (second graph). Both sites experience different correlations, higher significant at site 1 than at site 2. These differences could be linked to the morphology of the studied sites.
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mouth; Suanez, 1997; Arnaud-Fassetta, 1998; Antonelli et al., 2001). Pebbles (median grain size = 40 –50 mm), locally observed (Arnaud-Fassetta et al., submitted for publication), come from local sources (the Pleistocene substratum and some Holocene gravel barriers (Vella, 1999), and are reworked during floods or CNR’s dredging. At site 2, the whole bed was eroded from 1908 until 1999, with an erosion rate double than that of site 1. The stronger erosion was allowed by the lithology of the channel (Holocene sandy silts). The boundary shear stress, higher than in site 1, is able to move particles from 17 to 22 mm during 100-year floods, while bank sediments grain size is only 250– 300 Am; which could explain the bed erosion, which was more important on the concave banks, where the crosssections showed an undermining at the base. The size and the location of the lower Rhoˆne River channel deformations were partially induced by the local constraints of boundary shear stress and specific stream power, themselves bound to channel morphology: the river floor lithology (cohesive or uncohesive), granulometry (gravel, muddy sands), and topography (slope, curve) locally adjusted the hydraulic parameters. But hydraulic data are not sufficient to explain the channel morphologic evolution. The participation of local geomorphologic forces cannot mask a global erosion trend during the 20th century. 5.3. Comparison with channel bed evolution in the catchment area Previous studies explained the channel entrenchment in the delta by the effects of the reduction of solid discharge, due to hydro-climatic and anthropological changes in the catchment basin since the end of the 19th century (Arnaud-Fassetta, 2002). The lower Rhoˆne incision was thus related to the incision phase noticed upstream during the 20th century. On the upper and middle Rhoˆne River and their tributaries, channel erosion was attributed to the reduction of the solid discharges (notably the morphogenic bed-load), following land-use changes and reforestation in the catchment (Jorda, 1985; Bravard, 1989; Landon, 1999; Rinaldi and Simon, 1998; Winterbottom, 2000; Warner, 2000). The stabilization of the bed-load, as demonstrated on the torrential sections of the river, induced changes in the river style and the sinking of the
river bed (Bravard, 1989; Landon and Pie´gay, 1994; Klingeman et al., 1994). The incision trend started at different dates, but it was widespread and accelerated during the second half of the 20th century in relation to dam constructions and dredging activities. Therefore, a comparison of channel bed adjustments in the delta with those in upstream reaches and tributaries of the Rhoˆne River shows a time-lag, suggesting that the lower Rhoˆne River developed an original response to external stresses. There has been a slackening of the erosion rate since the middle of the 20th century, whereas the upstream catchment and the alpine tributaries have experienced an opposite trend, which then has to be explained. The potential effects of human and physical induced-changes are therefore described and discussed below. They probably interfere with each other to explain the channel evolution of the lower Rhoˆne. The channelisation, embankments and engineering works that started around the 1860s on the lower Rhoˆne in order to protect people against floods and to ease navigation probably explain the early beginning of the entrenchment (Arnaud-Fassetta, 2002). Straightening favours transit and reduces in-channel sediment storage. These human activities have increased the effects of the climatic changes characterised by the reduction of flood numbers at the end of the Little Ice Age until the beginning of the 20th century (Pichard, 1995). Even though the hydrological trend over the 1920 – 2000 period showed an insignificant increase in the average monthly discharge at Beaucaire, the occurrence of large floods decreased after 1910 (Fig. 12). The hydraulic energy deficit resulting from this decrease was not compensated for by the large flood events in 1993 and 1994 and could explain the decrease in the incision rate measured since the 1960s. During the first part of the 20th century both natural and human forces contributed to the change of fluvial style on the lower Rhoˆne, replacing a braiding with a meandering channel, as described on many of the rivers in Western Europe (Starkel, 1983; Probst, 1989; Bravard, 1989; Bravard and Peiry, 1993; Gautier, 1994; Miramont and Guilbert, 1997; Landon, 1999; Warner , 2000). The slowing down of the channel evolution during the second part of the 20th century could then be assumed to be the initiation of the relaxation time, leading to a state of a new ‘‘dynamic equilibrium’’ (Bravard and
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Fig. 12. Overall trend of the bed degradation in the lower Rhoˆne River. The transition from the reaction phase to the relaxation phase is unknown due to the lack of data all over the century. But the study of previous work (see text) allows us to suggest a change in the sediment balance at the end of the 19th century. (Pont et al., 2002; Antonelli, 2002)
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Petit, 1997; Brunsden, 1980 in Knighton 1998; Schumm et al., 1984). A reduction of 50% of the suspended load has been admitted since the end of the 19th century (Fig. 12): Before the Second World War, this reduction was attributed to reforestation and a decrease of major flood recurrences. After 1950, this sedimentary decrease was attributed to the hydrological deficit and dam construction. Even though studies have shown that dams could allow the transit of suspended sediments (IRS, 2000), the transit of coarse load is stopped or reduced, particularly in relationship with the weaker hydraulic energy on the old Rhoˆne’s arms (Poinsard et al., 1989; IRS, 2000). This could suppose an erosion of a ‘‘clear-water’’ type, which however is not relevant for the slowing down of the incision since the 1960s. The role played by dams in the channel incision has to be moderated because the most degraded period (1908 – 1965) corresponds to the construction of five dams whereas the next period (1965 – 1999) is synchronous with the construction of 17 dams on the whole Rhoˆne River (Fig. 12). That would indicate the limited effect of hydroelectric plants on the morphological changes on the lower Rhoˆne River. Similarly, dams generally induce downstream incision (Rinaldi and Simon, 1998; Jiongxin, 1996), but the studied sites were located more than 30 km downstream of the last dam (Vallabre`gues dam) the functioning of which began in 1971, i.e. after the decrease of the incision rates. Therefore, the slowingdown in the incision rate, since the 1960s, is paradoxically coincident with the most intensive hydroelectric management and dredging activity, i.e. with the most drastic reduction in global solid discharge. In a deltaı¨c system such as that of the Rhoˆne River, riverbed erosion cannot be attributed to the morphogenic action of a coarse bed-load (gravel, pebbles), because it is not dominant in the channel. Its presence is not disputed upstream of site 1, but pebbles observed downstream are linked to mining works or to the outcrop of a Holocene shingle bar. The pebbly load is absent downstream KP 302 and does not intervene in site 2 morphogenesis. The predominance of sands in the downstream delta has been confirmed over several millennia: the powerful channel of ‘‘Bras de Fer’’, that led to an important progradation of the river mouth in the 17– 18th century, revealed a maximal grain size inferior to 800 Am (Arnaud-Fassetta
and Provansal, 1999). Therefore, channel adjustment in the delta is driven by finer sediments and could be assumed by the erosive action of flood waters highly loaded with suspended sediments: this theory is sometimes used to explain bank erosion and channel extension (Green et al., 1999). During floods, a mobile sandy bottom layer, with a strong density (intermediate between carriage and suspension), constitutes the ‘‘active morphogenic bedload’’ (Bagnold, 1977; Leeder, 1977), which would explain the vertical incision of the channel floor. Moreover sand flux measures indicated higher concentrations of sediments at the base of the water column for moderate flows (inferior to 3000 m3/s; Antonelli et al., 2001) and diving observations showed hydraulic dunes moving on the bottom. The decrease of flood occurrence since 1960 could probably explain the recent relative stabilisation of the channel morphology in relation to a reduction in bed-load supply. Therefore, the original timing of channel entrenchment in the lower Rhoˆne River suggests a specific morphodynamic functioning, in relationship with geomorphic factors of the deltaı¨c environment. The local channelisation and embankments, associated with the decrease of major floods, influence channel entrenchment. The recent slowing of channel entrenchment could be explained both by a relaxation effect, towards a state of ‘‘dynamic equilibrium’’, and, paradoxically, by the decrease of the bed load induced by the last dam constructions. The morphogenic impacts of dam and dredging activities are thus the opposite of those in the upper and middle Rhoˆne valley. The near marine base level, which has been rising by 2 mm year 1 over the last century (Suanez et al., 1997), is certainly a complementary factor of stabilisation.
6. Conclusion The study of detailed cross-sections registered over a century allows a better understanding of the recent modifications of the lower Rhoˆne River, principally characterised by channel incision. This bed sinking, highly significant at the beginning of the century has been decreasing for 40 years. Hydraulic constraints and channel morphology locally regulate the incision rates. Boundary shear stress and specific stream power partially explain the
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morphological evolution of the Rhoˆne River, but they have to be completed by lithologic and morphologic local factors. Nevertheless the initiation and duration of the incision and its recent decrease are linked to various causes that intervene at local and global scales. The incision in both sites has decreased since the 1960s, whereas the upper and middle Rhoˆne, and its tributaries, have undergone an erosive increase. Local bank management, increasing the effects of the climatic change (since the end of the Little Ice Age), has initiated the channel geomorphic evolution, which is driven by a sandy bed load. Thus the slowing-down in the incision rate is explained by the interference of several factors: the reduction of the hydraulic energy, due to the lack of large floods, the drastic reduction (about 50%) of the global solid discharge, the rise of the near marine base-level and, likely, a relaxation towards a state of ‘‘dynamic equilibrium’’ after the rapid incision in the first half of the 20th century. These observations emphasise the fact that the lower Rhoˆne River corresponds to a specific environment in which the functioning of the global hydrosystem is partly controlled by local constraints (hydraulic data, bedforms, base level, embankments). The original responses recorded in the lower Rhoˆne River, in contrast to those in the upstream catchment, raise the question of the time and space scale in a large river and suggest that different reaches in a fluvial system may exhibit various morphological responses to a common climato-anthropological history. As mentioned by Knighton (1998, p.262) ‘‘events are unlikely to be synchronous over large areas because the threshold conditions that partly determine the magnitude and direction of change are influenced by factors which are themselves spatially variable.’’ In fact, the lower Rhoˆne River appears sensitive to flood events, but remains dependent on its intrinsic structure and the nearness of the base level. References Abernethy, B., Rutherfurd, I.D., 1998. Where along a river’s length will vegetation most effectively stabilise stream banks? Geomorphology 23, 55 – 75. Antonelli, C., 2002. Flux se´dimentaires et morphogene`se re´cente dansle chenal du Rhoˆne aval. PhD thesis, University Aix-Marseille I. 279 pp. Antonelli, C., Provansal, M., Genty, P., 2001. Evaluation du trans-
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