Fanny Arnaud
email: fanny.arnaud@ens-lyon.fr.

Laurent Schmitt

Karen Johnstone

Anne-Julia Rollet

Hervé Piégay

Engineering impacts on the Upper Rhine channel and floodplain over two centuries

Keywords: Upper Rhine, Controlling factors, Channelization, Evolutionary trajectory, Braided river, Anabranching river

The reconstruction of long-term (>100 yr) channel changes is critical to understanding how natural and anthropogenic factors impact the evolution of river systems. It provides information on channel sensitivity and the limitations for channel repair in a functional river restoration context. This study presents a two-century analysis of the evolution of a large braided and anabranching river, the Upper Rhine downstream of Basel, which has experienced three phases of engineering works (nineteenth century channelization, 1930s groyne construction, and 1950s damming and by-passing). We studied historical maps, longitudinal profiles, and cross sections to investigate changes in the areas and geometries of fluvial features from 1828 to 2009. The results showed a drastic floodplain drying out following nineteenth century channelization (93% reduction of fully connected channels, 48% augmentation of partially connected channels, augmentation of disconnected water bodies by more than five times, and augmentation of terrestrialized channels of more than two times; 1828-1925).

Changes propagated downstream at 280 m yr -1 on the 50-km long river continuum. The channel exhibited bed topography of large forms in 1880, with a median pool-riffle amplitude of 4.0 m, locally reaching 6.6 m. Among the three phases of engineering works on the Upper Rhine, channelization had the most impact on planform (floodplain terrestrialization) and vertical (7 cm yr -1 bed degradation) adjustments, with short reaction times and effects lasting several decades. Damming had the greatest impact on peak flows, and can explain the lower magnitude of the channel changes in the

Introduction

The morphological and ecological consequences of human activity on river channels and floodplains over the last 200 yr have been well documented. One of the direct and most dramatic modifications is channelization, which includes all engineering procedures that either straighten, embank, enlarge, or protect an existing channel, or that involve the creation of new channels [START_REF] Brookes | Channelized Rivers: Perspectives for Environmental Management[END_REF]. Systematic river channelization works were applied in the mid-and late nineteenth century in Europe and elsewhere to satisfy demands for flood control, drainage improvement, or maintenance of navigation [START_REF] Downs | River Channel Management. Towards Sustainable Catchment Hydrosystems[END_REF][START_REF] Herget | Engineering impact on river channels in the River Rhine catchment[END_REF][START_REF] Gregory | The human role in changing river channels[END_REF]. These works have resulted in drastic landscape transformations, from multiple river channels to single-thread channel configurations, as experienced in the Danube [START_REF] Pišút | Channel evolution of the pre-channelized Danube River in Bratislava, Slovakia (1712-1886)[END_REF][START_REF] Hohensinner | Spatio-temporal habitat dynamics in a changing Danube River landscape 1812-2006[END_REF], the Rhône [START_REF] Bravard | Impact of civil engineering works on the successions of communities in a fluvial system[END_REF][START_REF] Provansal | The geomorphic evolution and sediment balance of the lower Rhône River (southern France) over the last 130 years: Hydropower dams versus other control factors[END_REF], the Rhine [START_REF] Hudson | Flood management along the Lower Mississippi and Rhine Rivers (The Netherlands) and the continuum of geomorphic adjustment[END_REF], Italian rivers [START_REF] Surian | Morphological response to river engineering and management in alluvial channels in Italy[END_REF], and braided rivers of the French Alps [START_REF] Piégay | Census and typology of braided rivers in the French Alps[END_REF].

A major consequence of straightened channels is riverbed degradation, which occurs with a short reaction time, and can reach several meters in depth in some cases [START_REF] Brookes | Channelized Rivers: Perspectives for Environmental Management[END_REF][START_REF] Surian | Morphological response to river engineering and management in alluvial channels in Italy[END_REF][START_REF] Hudson | Flood management along the Lower Mississippi and Rhine Rivers (The Netherlands) and the continuum of geomorphic adjustment[END_REF]. Bed degradation can induce lowering of the adjacent groundwater level, with a subsequent loss of river-floodplain connectivity and a tendency towards floodplain terrestrialization [START_REF] Bravard | Environmental and societal effects of channel degradation and remedial strategies[END_REF][START_REF] Girel | Biodiversity and land use history of the alpine riparian landscapes (the example of the Isère River valley, France)[END_REF]. Dikes (levees) themselves alter the lateral connectivity by closing side channels, which leads to channel abandonment [START_REF] Bravard | Impact of civil engineering works on the successions of communities in a fluvial system[END_REF][START_REF] Peiry | Dynamique fluviale historique et contemporaine du confluent Giffre-Arve (Haute-Savoie)[END_REF][START_REF] Dépret | Evolution of abandoned channels: insights on controlling factors in a multi-pressure river system[END_REF]. Some studies have revealed the magnitude of change in anabranching channel types following channelization [START_REF] Hohensinner | Reconstruction of the characteristics of a natural alluvial river-floodplain system and hydromorphological changes following human modifications: the Danube River (1812-1991)[END_REF][START_REF] Diaz-Redondo | Benchmarking fluvial dynamics for process-based river restoration: the Upper Rhine River (1816-2014)[END_REF], but the patterns of change have only rarely been detailed along a downstream continuum [START_REF] Gregory | Historical Willamette River Channel Change[END_REF]. Additionally, many channelized rivers have undergone further engineering in the twentieth century (e.g., hydropower dams and gravel mining), which have further complicated their evolutionary trajectory [START_REF] Surian | Morphological response to river engineering and management in alluvial channels in Italy[END_REF][START_REF] Provansal | The geomorphic evolution and sediment balance of the lower Rhône River (southern France) over the last 130 years: Hydropower dams versus other control factors[END_REF][START_REF] Ziliani | Reconstructing temporal changes and prediction of channel evolution in a large Alpine river: the Tagliamento River, Italy[END_REF].

Understanding the history of multi-impacted rivers over fine spatio-temporal scales is a major challenge in the context of current river restoration. Geomorphic diagnosis provides information on channel sensitivity and the limitations for channel repair. By defining realistic goals that integrate both reversible and irreversible changes for recovering a set of targeted river dynamics, rather than a static past reference state, it permits selection of the most efficient and sustainable restoration configurations [START_REF] Dufour | From the myth of a lost paradise to targeted river restoration: forget natural references and focus on human benefits[END_REF]. Exploration of the functional trajectories of change should also be useful for anticipating responses after restoration, by assessing the potential benefits and limitations for channel repair [START_REF] Mika | Inside the "black box" of river restoration: using catchment history to identify disturbance and response mechanisms to set targets for process-based restoration[END_REF]. However, the integration of long-term (>100 yr) channel change analysis into functional restoration projects remains rare [START_REF] Bravard | Historical development and integrated management of the Rhône River floodplain, from the Alps to the Camargue Delta, France[END_REF][START_REF] Eschbach | Long-term temporal trajectories to enhance restoration efficiency and sustainability on large rivers: an interdisciplinary study[END_REF][START_REF] Scorpio | Channel changes of the Adige River (Eastern Italian Alps) over the last 1000 years and identification of the historical fluvial corridor[END_REF].

This study aims to quantify the morphological effects of channelization and further engineering works on a braided and anabranching river system over substantial temporal (181 yr) and spatial (50-km long reach) scales. The research was conducted on the Upper Rhine River, which is a major environmental, cultural, and socio-economic corridor in Europe. Channelization of the river started in 1817, which makes it one of the first intensive large-scale engineering schemes undertaken on a large river anywhere in the world [START_REF] Tricart | Le cours périalpin du Rhin, du Rhône et du Danube: aménagement fluvial et dérives de l'environnement[END_REF][START_REF] Uehlinger | The Rhine River basin[END_REF]. The contemporary hydromorphological evolution (1930s-2010s) was quantified for a downstream continuum, the 50-kmlong Old Rhine downstream of Basel, by [START_REF] Arnaud | Historical geomorphic analysis (1932-2011) of a by-passed river reach in process-based restoration perspectives: the Old Rhine downstream of the Kembs diversion dam (France, Germany)[END_REF]. This study provided insights into the process-based restoration of the river reach. The present study integrates this medium-term geomorphic analysis and future restoration schemes with the long-term temporal trajectory (1828 to 2009). We aim to specify the spatio-temporal patterns of planform and vertical channel changes by responding to the following questions: What was the channel pattern and what were the dynamics prior to channelization? How did channelization affect processes and forms in terms of response times and magnitudes? What is the relative importance of channelization effects compared to the 1930s and 1950s engineering effects? What are the implications for functional river restoration?

This study combines a quantitative and qualitative analysis to outline the evolutionary trajectories of some of the main geomorphic variables of the fluvial system (active-channel width, bed level, channel types) over a continuous time period of almost 200 yr. The multi-century material provides new and complementary information to the multi-decade study by [START_REF] Arnaud | Historical geomorphic analysis (1932-2011) of a by-passed river reach in process-based restoration perspectives: the Old Rhine downstream of the Kembs diversion dam (France, Germany)[END_REF]. In particular, historical maps, longitudinal profiles, and cross sections provide an accurate description of the channel pattern before and after engineering works. Some aspects of channel evolution and restoration issues in the upstream part of the Upper Rhine could be in common with other large braided/anabranching multiimpacted rivers worldwide.

Study area

The 50-km long reach discussed in this study is located between Kembs (kilometric post [KP] 174.0) and Breisach (KP 224.8). This reach lies at the southern and upstream part of the Upper Rhine Graben, which is a 310-km long and 35-km wide spectacular subsidence zone within the European Rift system (Fig. 1A; [START_REF] Uehlinger | The Rhine River basin[END_REF]. The Upper Rhine Graben consists of Mesozoic strata with an overlying cover of marine sandy-clayey Tertiary material of more than 2000 m in depth [START_REF] Chr | Erosion, Transport and Deposition of Sediment[END_REF]. The top cover between Basel (KP 170.0) and Breisach is a large alluvial fan, which is in places up to 250-m thick, and was formed during the Last Glaciation of the Alps (≈20 ka BP) and the Younger Dryas (13-11 ka BP). In this reach, the sediments originate from the northern Swiss Alps, the Swiss Molasse Basin, and the Jura Mountains [START_REF] Kock | Formation and evolution of the Lower Terrace of the Rhine River in the area of Basel[END_REF]. After the Younger Dryas, the longitudinal profile of the Upper Rhine adjusted to changes in the controlling variables of the fluvial system: (1) a decrease in the water and sediment flux following the Holocene warming (Schmitt et al., 2016), (2) sediment trapping in lakes formed after the retreat of the Rhine Glacier area and which control 60% of the river network upstream of Basel [START_REF] Walser | Le bassin du Rhin à l'amont de Bâle et l'influence des lacs sur le régime du fleuve[END_REF], and (3) neotectonics, notably an uplift of the Upper Rhine Graben upstream from Mulhouse and a subsidence downstream [START_REF] Nivière | Kinematic evolution of a tectonic wedge above a flat-lying décollement: The Alpine foreland at the interface between the Jura Mountains (Northern Alps) and the Upper Rhine graben[END_REF][START_REF] Schmitt | Paléodynamique fluviale holocène dans le compartiment sud-occidental du fossé rhénan (France)[END_REF]. These resulted in erosion of the alluvial fan down to a depth of 30 m [START_REF] Kock | Formation and evolution of the Lower Terrace of the Rhine River in the area of Basel[END_REF][START_REF] Schmitt | Paléodynamique fluviale holocène dans le compartiment sud-occidental du fossé rhénan (France)[END_REF].

The early nineteenth century Upper Rhine downstream of Basel can be classified as a graveldominated, braided [START_REF] Carbiener | Evolution of major pattern models and processes of alluvial forest of the Rhine in the rift valley (France/Germany)[END_REF], and anabranching [START_REF] Nanson | Anabranching rivers: their cause, character and classification[END_REF][START_REF] Herget | Engineering impact on river channels in the River Rhine catchment[END_REF][START_REF] Schmitt | Paléodynamique fluviale holocène dans le compartiment sud-occidental du fossé rhénan (France)[END_REF] river (Fig. 1C). Farther downstream, the Upper Rhine exhibited a progressively anabranching (mainly anastomoses; downstream of Breisach) and meandering (downstream of Strasbourg) river pattern [START_REF] Carbiener | Evolution of major pattern models and processes of alluvial forest of the Rhine in the rift valley (France/Germany)[END_REF][START_REF] Diaz-Redondo | Benchmarking fluvial dynamics for process-based river restoration: the Upper Rhine River (1816-2014)[END_REF]. [START_REF] Schmitt | Paléodynamique fluviale holocène dans le compartiment sud-occidental du fossé rhénan (France)[END_REF] showed that the longitudinal bed slope variation and the Holocene bed-level evolution were the two discriminating variables explaining the longitudinal geomorphic continuum of the pre-channelized Upper Rhine. (C) The 1828 cartography for a section near Blodelsheim showing the complex braided and anabranching system (see feature legend in Fig. 3). The thalweg is the blue line and the planned position of the 200-m wide channelized river is mapped in red.

Discharge measurements started in 1808 in Basel, which makes for one of the longest uninterrupted hydrological series in Europe [START_REF] Wetter | The largest floods in the High Rhine basin since 1268 assessed from documentary and instrumental evidence[END_REF]). The First Jura Waters Correction Project (1868-1890) in the upstream Swiss basin diverted the Aare River to Lake Biel to dry up the floodplain for agricultural development and delay flood peaks at the confluence with the Emme [START_REF] Wetter | The largest floods in the High Rhine basin since 1268 assessed from documentary and instrumental evidence[END_REF].

These engineering works substantially modified the downstream flow regime, so that the contemporary hydrology of the Upper Rhine is usually analyzed as starting in 1891 (CECR, 1978). The mean annual discharge at the Basel gauging station over the period 1891-2011 is 1050 m 3 s -1

, and the 2-, 10-and 100-yr flood discharges are ~2800, 3700, and 4600 m 3 s -1 , respectively. The top three major floods recorded in Basel since 1808 occurred in the nineteenth century in September 1852

(5640 m 3 s -1 ), June 1876 (5700 m 3 s -1 ), and September 1881 (5280 m 3 s -1 ; Fig. 2). Three recent major floods have occurred since the 1990s, with discharges of 4640, 5090, and 4810 m 3 s -1 recorded in May 1994, May 1999, and August 2007, respectively (top seven major floods recorded in Basel since 1808). The number of days that the mean daily discharge exceeded the 2-yr flood discharge varied between 0 and 27 days per year (1869-2011; Fig. 2; discharge information from [START_REF] Ghezzi | Die Abflussverhältnisse des Rheins in Basel[END_REF] and the Swiss Federal Office for the Environment). Over centuries, the growing population in the Upper Rhine floodplain took protective measures against the shifting river. The first artificial meander cuts date from the fourteenth century, but the effect of such actions was localized and did not affect the natural river dynamics [START_REF] Uehlinger | The Rhine River basin[END_REF]. The nineteenth century channelization belongs to the period of systematic stabilization of large rivers in Europe, performed on a regional instead of a local scale [START_REF] Herget | Engineering impact on river channels in the River Rhine catchment[END_REF]. Channelization was conducted on the study reach between 1842 and 1876 following the plans of the Badenese hydraulic engineer J.G. [START_REF] Tulla | Mémoire sur la rectification du cours du Rhin[END_REF]. The primary objectives were to narrow and shorten the river course to cause bed degradation and subsequently reduce overflowing, and to provide flood control, border fixation, navigation improvement, and assist agriculture. The 3.5-km wide complex channel system was changed into a 200-m wide channelized river (Fig. 1C) by the construction of dikes (levees) over 600 to 900 m longitudinal sections. The 50-m long intervals between these sections allowed major floods to exit to high-flow dikes built several hundred meters behind. In the 1920s, bed degradation at Istein (KP 178.0) reached the Jurassic limestone substratum, thus forming rapids on the bedrock outcrop, and impeding navigation between Germany and Switzerland [START_REF] Humbert | Rhin. Encyclopédie de l'Alsace[END_REF][START_REF] Tricart | Le cours périalpin du Rhin, du Rhône et du Danube: aménagement fluvial et dérives de l'environnement[END_REF]. Regularization works were then conducted from 1931 to 1939 following the plans of the Badenese engineer M. Honsell, inspired by works by H. Girardon on the Rhône and M. Fargue on the Garonne [START_REF] Humbert | Rhin. Encyclopédie de l'Alsace[END_REF]. Lateral groyne fields were constructed downstream from the Istein bedrock outcrop to create a 75-m wide shipping channel under a sinusoidal pattern assumed to stabilize the longitudinal profile [START_REF] Schneider | Zusammenfassende darstellung der Rheinregulierung Strassburg/Kehl-Istein. Baukommission des Unternehmens "Regulierung des Rheins zwischen Strassburg/Kehl und Istein[END_REF]. Finally, damming (1932) and construction of the Grand Canal d'Alsace (1932 to 1959) were aimed at hydropower generation and navigation improvement. The Kembs Dam conveys most of the flow (maximum discharge: 1400 m 3 s -1 ) into a lateral canal that supplies the power plants of Kembs, Ottmarsheim, Fessenheim, and Vogelgrün (Fig. 1B). A minimum flow (50-150 m 3 s -1

) is maintained in the by-passed Old Rhine for 81% of the time [START_REF] Arnaud | Historical geomorphic analysis (1932-2011) of a by-passed river reach in process-based restoration perspectives: the Old Rhine downstream of the Kembs diversion dam (France, Germany)[END_REF].

Material and methods

Planform changes over the period 1828-1925 were investigated for the 3.5-km wide hydrosystem area. This area (12,629 ha) was delimited by the farthest side channels connected upstream and downstream prior to channelization (1828), with a 100-m buffer applied (Fig. 1C). Vertical changes over the period 1880-2009 were investigated in the 200-m wide single-thread channel.

Planform data

Two maps constructed before channelization (1828, 1838), one constructed towards the completion of channelization (1872), and one constructed fifty years later (1925) were identified (Table 1). The 1828 and 1838 editions (1:20,000) were produced for the planning of river channelization and to map the changing French-German border. Thus, they are considered as more detailed descriptions of the Upper Rhine fluvial and riverine landscape in comparison with former editions (e.g., Cassini maps; [START_REF] Bravard | Cartography of rivers in France[END_REF]. Fewer features and less detailed delineation are presented on the 1872 (1:20,000) and 1925 (1:25,000) maps (Fig. 3). The maps were not accurately dated, but as their purpose was to define international borders and river engineering, the mapped aquatic channel was assumed to represent the mean discharge, as is usual in cartography [START_REF] Diaz-Redondo | Benchmarking fluvial dynamics for process-based river restoration: the Upper Rhine River (1816-2014)[END_REF].

The 1828 and 1925 maps were rectified to a 2008 orthophotograph using ArcMap v.10.1. The 1838 and 1872 maps were rectified to the 1828 map sheets using triangulation posts mapped on each sheet. The resulting root mean square error (RMSE) ranged from 6.8 to 19.0 m per map sheet. a Source: before 1869, [START_REF] Ghezzi | Die Abflussverhältnisse des Rheins in Basel[END_REF]; after 1869, Swiss Federal Office for the Environment.

Natural and artificial water bodies, unvegetated gravel/sand bars, vegetation boundaries, terrestrialized channels, and anthropogenic features (croplands/villages/roads, dikes/embankments) on the 1828, 1872 and 1925 maps were digitized at a 1:5000 scale. Differences in form, position, color, texture, and legend information were used when available (Fig. 3). Natural water bodies were subclassified into three channel types according to their hydrological connectivity [START_REF] Gregory | Historical Willamette River Channel Change[END_REF][START_REF] Hohensinner | Reconstruction of the characteristics of a natural alluvial river-floodplain system and hydromorphological changes following human modifications: the Danube River (1812-1991)[END_REF][START_REF] Diaz-Redondo | Benchmarking fluvial dynamics for process-based river restoration: the Upper Rhine River (1816-2014)[END_REF]: fully connected channels (including the mainstem, side channels, and tributaries), partially connected channels (channels connected only at the upstream or downstream end, including backwaters) and fully disconnected water bodies (oxbow lakes). Terrestrialized channels are former channels that are now full of sediments. Terrestrialization, i.e., the passage from the aquatic to the terrestrial stage, could have occurred according two main processes: filling by coarse and fine sediment deposits, and/or lowering of the water level resulting from main channel incision [START_REF] Bornette | River incision and decennial vegetation dynamics in cutoff channels[END_REF][START_REF] Dépret | Evolution of abandoned channels: insights on controlling factors in a multi-pressure river system[END_REF]. The delineation of features on the 1925 map was not as accurate as previous editions, and the legend colors (e.g., blue for water bodies) and symbols (e.g., vegetated areas) were not systematically applied to the different sheets. This could induce errors in the identification of features, e.g., connected

and disconnected channels could be confused, or terrestrialized channels could be mistaken for roads, dikes, or vegetation. Retro-interpretation from the 1925 to 1828 maps helped distinguish some features with respect to the river-floodplain succession stages.

Assessment of the early nineteenth century lateral channel migration was based on the thalweg comparison in 1828 and 1838 for 5-km long channel segments, i.e., half a map sheet (n = 11).

Channel sinuosity was calculated using the same 5-km long channel segments of the thalwegs of 1828 and 1838, and the channelized river centerline of 1872, with the three dates being compared with the 1828 hydrosystem centerline. The lateral distributions of the channel types based on three cross sections located on the upstream (KP 179.5), middle (KP 197), and downstream (KP 216.5) sections of the river reach were also analyzed. On these cross sections, the number of channels, channel width, and distance to the channelized river centerline were calculated for the three dates.

The longitudinal patterns of change in channel type areas were detailed by extracting areas for 500-m long channel segments (n = 99) and calculating the differences per period. Homogeneous subreaches were statistically delineated using the "Hubert test" tool of the FluvialCorridor toolbox developed by [START_REF] Roux | FluvialCorridor": A new ArcGIS toolbox package for multiscale riverscape exploration[END_REF]. The Hubert test for stationarity [START_REF] Hubert | The segmentation procedure as a tool for discrete modeling of hydrometeorological regimes[END_REF] is especially successful for identifying longitudinal discontinuities on univariate series. It provides subreach segmentation when the difference of the mean between two consecutive segments is significant. Then, the overall propagation rate of channel changes (R, in m y -1 ) was calculated as:

= ∑ × × ( 1
)
where 2 is the mean kilometric point of subreaches where the highest changes in channel type area occurred during the period 2 (1925-1872; 53 yr), the period 1 is 1872-1828, and is the number of channel types ( = 4: fully connected, partially connected, fully disconnected, terrestrialized channels).

Finally, rates of change in active-channel width (sum of connected channels and unvegetated gravel bars) pre-and post-channelization were compared with the post-damming period using the dataset reported in [START_REF] Arnaud | Historical geomorphic analysis (1932-2011) of a by-passed river reach in process-based restoration perspectives: the Old Rhine downstream of the Kembs diversion dam (France, Germany)[END_REF].

12 Fig. 3. Examples of feature delineation on three old maps.

Topographic data

We compared from 197 to 240 thalweg points to determine trends of bed-level changes on the 50-km long study reach. Thalweg profiles were compiled for five dates from 1880 to 2009 (Table 2); for 1880 Riffles and pools were identified from a linear regression fitted to the thalweg profiles; the maximumsize residuals above the regression line identifying riffles, and the minimum-size residuals identifying pools [START_REF] Vetter | Riffle-pool morphometry and stage-dependant morphodynamics of a large floodplain river (Vereinigte Mulde, Sachsen-Anhalt, Germany)[END_REF]. The median amplitude and spacing of pool-riffle sequences were calculated, as well as the ratio D/W, with D as the pool-to-pool spacing, and W as the median active-channel width between dikes (W = 209 m in 1880 and 1935; W = 153, 110, and 111 m in the 1950s, 1990s, and 2010s, respectively; see Fig. 4C).

Six cross-section series were also compared (Table 2). The spacing of the surveys in 1884 was approximately 2000 m (n = 21), while in 1923 it was 100 m (n = 60). Bed-level information was given relative to the low-flow water level (1884), or no bed-level information was given (1923). These cross sections were analyzed qualitatively to determine channel geometry, especially the presence of lateral bars and islands. Changes in channel geometry after regularization and damming were quantified by comparing 14 cross sections performed between 1930 and 2009 (Table 2). These were selected according to a relatively constant spacing on the study reach (~3000 m). Atypical geometries in comparison with adjacent cross sections were excluded.

Results

Planform changes (1828-2008)

Prior to channelization, natural areas (water bodies, bars, vegetation, terrestrialized channels) occupied 76% of the total hydrosystem area (Fig. 4A). The remaining 24% of anthropogenic areas consisted mainly of croplands. Some high-flow dikes and local embankments (1%) were constructed at bends (see the 1828 cartography in Fig. 3), with fully connected channels being the prevalent channel type prior to channelization (91% of the area of natural water bodies; Fig. 4B).

Lateral analysis showed a very complex pattern with numerous channels, up to 18 on the downstream cross section (Fig. 5C-1). The largest channels (up to 578 m wide; Fig. 5B-1) were present in the center of the hydrosystem, while narrow channels (less than 20 m wide) were present further from the mainstem, up to 2.4 km away (Fig. 5C-1). The thalweg lines from 1828 and 1838 were a median of 142 m distant to each other, thus highlighting the lateral channel migration prior to channelization. The channel sinuosity was fairly constant, with median values of 1.1 and 1.2 for 1828 and 1838, respectively; it had decreased to 1.0 in 1872. In 1828, 50% of the 5-km long segments had a sinuosity lower than 1.0 because the hydrosystem centerline used as a reference was locally more sinuous than the 1872 channelized river centerline. The channelization induced a drastic floodplain drying out (Fig. 4A): between 1828 and 1925, natural water bodies decreased from 29% to 7% of the total hydrosystem area, vegetated areas increased from 39% to 47%, terrestrialized channels increased from 1% to 3%, and bars virtually disappeared (6.6% to 0.1%). Changes in natural water bodies and vegetation were expressed mainly between 1828 and 1872, the same time period during which the augmentation of artificial water bodies occurred. Anthropogenic features (mainly croplands) increased at a constant rate between 1828 and 1925, from 24% to 36% of the hydrosystem area. Partially connected channels became the prevalent channel type in 1925 (53% of the area of natural water bodies; Fig. 4B). Fewer channels were also present per cross section, not exceeding 13 channels in 1872 (Fig. 5C-2) and 10 channels in 1925 (Fig. 5C-3) (including disconnected water bodies). The channels also became narrower, not exceeding a width of 263 m in 1872 (Fig. 5A-2) and 197 m in 1925 (Fig. 5B-3). Channels located away in the floodplain experienced changed hydrological connectivity, from fully connected to partially connected, but were still present in 1925, contrary to most of the channels located near the mainstem (Fig. 5A-3 to 5C-3; see also maps in Fig. 3). The median active-channel width decreased from 902 to 302 m over the period 1828-1925, which is a 67% channel narrowing (Fig. 4C). Channel width then decreased further to 111 m in 2008, which gives an 88% total channel narrowing over the 181-yr period. The greatest narrowing occurred between 1828 and 1872, with a median rate of 7.4 m yr -1 . If we assume that narrowing was driven by channelization, then the median rate was 10.9 m yr -1 over the period 1842-1872. After 1956, the median rate was below 1.3 m yr -1 , and it became asymptotic after 1991.

The 1828 hydrosystem extent was longitudinally heterogeneous: it was narrower upstream of KP 191 and wider downstream (Fig. 6A). It controlled the longitudinal pattern of channels in 1828, with a higher fully connected channel area downstream of this point, as well as dikes and embankments aimed at stabilizing the braided and anabranching channels. Changes over 1828-1925 propagated longitudinally (see dotted regression lines and arrows in Fig. 6B): over the period 1828-1872, the reduction in fully connected channel area was the highest between KP 190 and 199 (rate of -1.08 ha yr -1 m -1

). Then, over the 1872-1925 period, it was highest between KP 202.5 and 215, but at a lower rate than in the previous period (-0.33 ha yr -1 m -1 ). The augmentation of the terrestrialized channel area over the 1828-1872 period was the highest between KP 180 and 186.5 (rate of 0.16 ha yr -1 m -1

), while over the 1872-1925 period it was the highest between KP 192.5 and 215, although at a slightly lower rate than over the previous period (0.09 ha yr -1 m -1 ). The partially connected channel area initially increased upstream of KP 181.5 (0.16 ha yr -1 m -1 ), then decreased between KP 185.5 and 203 (-0.06 ha yr -1 m -1

), before increasing downstream of KP 208 (0.09 ha yr -1 m -1

). Finally, the area of disconnected water bodies initially increased at a low rate below KP 178.5 (0.01 ha yr -1 m -1 ), then increased at a higher rate than in the previous period below KP 198.5 (0.03 ha yr -1 m -1

). The overall propagation rate of channel changes based on the four channel types in Fig. 6 was 280 m yr -1 . Points indicate the discontinuities detected with the Hubert test.

Cross-section analysis

The surveys in 1884 indicated a strong heterogeneity in the geometry of the corrected channel.

Among the 21 cross sections studied, all were asymmetric: 70% with one or more side channels and 30% single channel with shoals or lateral bars (Fig. 8A). Among the 60 cross sections studied in 1923, 60% had at least two channels and 40% had a single channel with shoals. Furthermore, the spacing of the surveys in 1923 (every 100 m along 1-km long reaches) emphasized the lateral thalweg migration (Fig. 8B).

Comparison of the 14 cross sections between 1930 and 2009 shows a strong simplification of the channel geometry over time. The main evolution trend was a two-channel mid-shoal bed configuration, which evolved in a single flat channel bed. Significant changes occurred in the 1930s. For example, the cross section at KP 205.82 (Fig. 8C) exhibited more changes in the three years 1931-1934 than over the twenty years 1934-1950s. The channel and the margins (groyne fields) gradually differentiated between 1950 and 2009: the channel bed was either incised or stable, and the margins aggraded on almost all the 14 cross sections. The only cross section that did not exhibit margin aggradation was located upstream of the Istein outcrop, where no groynes were built.

Discussion

Temporal trends and magnitudes of change

Prior to channelization, the Upper Rhine exhibited a highly complex braided and anabranching pattern, with numerous gravel/sand bars and extensive vegetated islands. This pattern was relatively common on large alpine rivers such as the Danube [START_REF] Hohensinner | Reconstruction of the characteristics of a natural alluvial river-floodplain system and hydromorphological changes following human modifications: the Danube River (1812-1991)[END_REF] and the Rhône [START_REF] Bravard | Impact of civil engineering works on the successions of communities in a fluvial system[END_REF]. Secondary channels located near the braided mainstem were laterally active anabranches (type 5 on the [START_REF] Nanson | Anabranching rivers: their cause, character and classification[END_REF] classification), as observed from the examination of the 1828 and 1838 maps (see also Figs. 3 and5). The lateral channel migration may be caused by the high frequency of flood events at the beginning of the nineteenth century (one 100-yr flood and three 10-yr floods from 1817 to 1831; see Fig. 2), at a period when the bedload transport was still active and the main channel was not yet incised, and the system was therefore very responsive in the lateral dimension. High channel shifting, which was exacerbated in the active bedload transport conditions, was also observed on alpine braided rivers in France by [START_REF] Lallias-Tacon | Use of airborne LiDAR and historical aerial photos for characterising the history of braided river floodplain morphology and vegetation responses[END_REF]. On the Upper Rhine, the active hydrological period in the early nineteenth century could be linked to the Little Ice Age, which may have had increased the flood frequency and the associated morphodynamics [START_REF] Eschbach | Long-term temporal trajectories to enhance restoration efficiency and sustainability on large rivers: an interdisciplinary study[END_REF]. A similar phenomenon was observed by [START_REF] Bonnefont | The morphodynamic behaviour of Moselle River before its harnessings[END_REF] on the Moselle River (tributary of the Middle Rhine) in the eighteenth century. However, the effect of the Little Ice Age on the morphodynamics augmentation of the Upper Rhine has not yet been validated

and awaits further testing [START_REF] Wetter | The largest floods in the High Rhine basin since 1268 assessed from documentary and instrumental evidence[END_REF][START_REF] Eschbach | Long-term temporal trajectories to enhance restoration efficiency and sustainability on large rivers: an interdisciplinary study[END_REF].

The study reach exhibited, on the margin of the multiple channel complex, anastomosing channels (type 1 on the Nanson and Knighton classification, 1996; see Fig. 1C). The presence of these anastomoses is referred to by Giessen, Muhlbach and Brunnenwasser in local literature on the Rhine [START_REF] Carbiener | Cas type de Rhinau-Daubensand : l'évolution du paysage rhénan dans la région de Rhinau, au coeur du secteur des Giessen, des Muhlbach et Brunnenwasser[END_REF]. Narrow and sinuous anastomoses located farther away in the floodplain were still hydrologically connected on the 1872 and 1925 maps (Figs. 3 and5).

Anastomoses were more present in the downstream sectors of the Upper Rhine near Strasbourg [START_REF] Carbiener | Evolution of major pattern models and processes of alluvial forest of the Rhine in the rift valley (France/Germany)[END_REF].

The study reach (bed slope: 0.001 m m -1 ) reacted very sensitively to channelization, which is in accord with several studies. On the Austrian Danube (bed slope: 0.0005 m m -1

), [START_REF] Hohensinner | Spatio-temporal habitat dynamics in a changing Danube River landscape 1812-2006[END_REF] showed that the initial channelization phase (1821-1829) affected only 4% of the river length, but that it led to significant modifications in aquatic and riverine habitats compared with the "natural" phase (before 1821). Over the following decades, the age structure of the floodplain complex was shown to be strongly correlated with the degree of channelization. On the Piave River (bed slope: 0.003-0.006 m m -1 ), [START_REF] Surian | Morphological response to river engineering and management in alluvial channels in Italy[END_REF] showed an intensification of the channel narrowing process since the 1960s, linked with a major increase in bank protection structures, dams, diversions, and gravel mining since the 1950s. From a review of several Italian rivers, the authors reported short reaction times for channel narrowing and bed degradation following channelization and further river engineering.

On the Rhine, we found a 93% reduction in fully connected channel area, a 513% augmentation of the area of disconnected water bodies, and a 98% reduction of bar area over a 97-yr period. These changes are higher than measured on the Austrian Danube over a 179-yr period (65% reduction in fully connected channels, double-augmentation of disconnected water bodies, 94% reduction in bars; [START_REF] Hohensinner | Reconstruction of the characteristics of a natural alluvial river-floodplain system and hydromorphological changes following human modifications: the Danube River (1812-1991)[END_REF]. The rate of channel narrowing on the Rhine (0.80% yr -1

; 1842-1925) was high compared with those measured on five reaches of the channelized Lower Rhône (0.24-0.85% yr - 1 ; 1855-1944;[START_REF] Provansal | The geomorphic evolution and sediment balance of the lower Rhône River (southern France) over the last 130 years: Hydropower dams versus other control factors[END_REF]. Additionally, the bed degradation magnitude (4.1 m in median value and 7.6 m in maximum value; 1880-1935) was in the same range or higher than a great diversity of channelized rivers in Italy (3-4 m commonly measured, with up to 10 m; [START_REF] Surian | Morphological response to river engineering and management in alluvial channels in Italy[END_REF], the Northern Alps (up to 8-14 m; [START_REF] Peiry | L'incision des rivières dans les Alpes du nord : état de la question[END_REF], Poland (up to 3.1 m; Zawiejska and Wyzga, 2010), and the Mississippi River (3.5-6 m on average; higher than 10 m in a number of reaches; [START_REF] Hudson | Flood management along the Lower Mississippi and Rhine Rivers (The Netherlands) and the continuum of geomorphic adjustment[END_REF]. Furthermore, the heterogeneous shape of the Rhine cross sections from the 1880s to the 1920s was similarly observed on the Isère and Arc rivers [START_REF] Girel | Biodiversity and land use history of the alpine riparian landscapes (the example of the Isère River valley, France)[END_REF]. On these systems, intensive morphodynamics took place after the nineteenth century river straightening, with the formation of regularly spaced (500 to 700 m) alternate bars between dikes.

In the two-century long temporal trajectory of the Upper Rhine, bed degradation rates following channelization (~7 cm yr -1 ; 1880-1950) were more than 10 times higher than after damming and bypassing (~0.4 cm yr -1 ; 1950-2009). Furthermore, active channel narrowing affected a wider channel system in the nineteenth century, and thus it was logically much more important following channelization (10.9 m yr -1 ; 1842-1872) than following damming and by-passing (1.3 to 0.1 m yr -1 ; 1956-2008; see [START_REF] Arnaud | Historical geomorphic analysis (1932-2011) of a by-passed river reach in process-based restoration perspectives: the Old Rhine downstream of the Kembs diversion dam (France, Germany)[END_REF] for details of the post-damming temporal trajectory).

Groynes had no effect on the bed degradation rate, which has remained constant since channelization. However, groynes rapidly simplified the cross-sectional geometry and induced a significant decrease in the pool-riffle spacing. The decrease in the pool-riffle spacing continued after damming and by-passing. The values of D/W (4.7-7.5) are consistent with the literature on pool-riffle geometry (5-7; [START_REF] Leopold | Fluvial Processes in Geomorphology[END_REF][START_REF] Gregory | Stability of the pool-riffle sequence in changing river channels[END_REF]. Interestingly, the ratio D/W increased during the studied period, especially since the 1990s (4.7-5.2 between 1880 and 1950; higher than 7.0 between 1990 and 2009), when the active channel was narrower with vegetation encroachment on the dewatered groyne fields. An increase in D/W following channelization and damming was described by [START_REF] Gregory | Stability of the pool-riffle sequence in changing river channels[END_REF] in a review of several case studies.

Bed grain size surveys performed in 2011 indicated that partial bed armoring (armoring degree between 1.1 and 2.1) developed after channelization [START_REF] Arnaud | Historical geomorphic analysis (1932-2011) of a by-passed river reach in process-based restoration perspectives: the Old Rhine downstream of the Kembs diversion dam (France, Germany)[END_REF]. This could explain why the sediment loss calculated in the by-passed Old Rhine (550 m 3 km -1 yr -1 ; 1990-2010; [START_REF] Arnaud | Historical geomorphic analysis (1932-2011) of a by-passed river reach in process-based restoration perspectives: the Old Rhine downstream of the Kembs diversion dam (France, Germany)[END_REF] was much lower than measured on the by-passed Rhône at Chautagne (1250 to 1450 m 3 km -1 yr -1

; 1970-1980;[START_REF] Klingeman | Hydropower reach bypassing and dewatering impacts in gravel-bed rivers[END_REF]. This reach of the Upper Rhône did not undergo channelization prior to damming, and the riverbed is not armored. This may explain the higher sediment loss and bed incision rates.

Channelization was the most impactful engineering phase in terms of planform and vertical changes on the Upper Rhine. Similarly, on the Lower Rhône [START_REF] Provansal | The geomorphic evolution and sediment balance of the lower Rhône River (southern France) over the last 130 years: Hydropower dams versus other control factors[END_REF], much of the geomorphic change occurred several decades prior to the 1950s damming and 1970s gravel mining, as a joint consequence of channelization in the mid-nineteenth century and the overall reduction in sediment input from the Rhône basin. With regard to the long-term timescale of the Upper Rhine, [START_REF] Kock | Formation and evolution of the Lower Terrace of the Rhine River in the area of Basel[END_REF] found that the highest level of the Lower Terrace (accumulation surface) in the area of Basel is approximately 30 m higher than the present Rhine level. The Lower Terrace was thus incised by about 30 m within not more than 12 ka, which represents a mean incision rate of 2.5 mm yr -1 [START_REF] Kock | Formation and evolution of the Lower Terrace of the Rhine River in the area of Basel[END_REF]. In the present study, channelization thus increased the bed incision process by a factor of 28 (7 cm yr -1

). This can be explained by the narrower channel geometry without flow reduction, which increased the hydraulic competence without any possibility of lateral erosion. It therefore drastically increased incision of the river bottom. In contrast, damming was the engineering phase with the most impact on peak flows. The frequency with which the critical discharge for bedload entrainment was exceeded (Q c = 550 m 3 s -1 ) was reduced by a factor of 22 following damming [START_REF] Arnaud | Historical geomorphic analysis (1932-2011) of a by-passed river reach in process-based restoration perspectives: the Old Rhine downstream of the Kembs diversion dam (France, Germany)[END_REF].

This can explain the lower magnitude of channel changes in the most recent period.

Spatial patterns and linkages between planform and bed-level changes

We found the longitudinal pattern of channel adjustments over time to be somewhat complex, depending on the channel type and the spatial arrangement of the channels prior to channelization (Fig. 6). Side channels continued to evolve after channelization, as seen on the 1925 map, meaning that the relaxation period for adjustment to the new straightened river pattern covered several decades after the works were completed. The overall propagation rate of channel changes (280 m yr -1 ) is slightly lower than those found in the literature for a watershed reforestation context (French Southern Prealps streams: 300 to 500 m yr -1 ; [START_REF] Liébault | Landuse change, sediment production and channel response in upland regions[END_REF] or damming context (Ain River, tributary of the French Rhône: ~500 m yr -1; [START_REF] Rollet | Assessment of consequences of sediment deficit on a gravel river bed downstream of dams in restoration perspectives: application of a multicriteria, hierarchical and spatially explicit diagnosis[END_REF]. The rivers in these studies were affected by different anthropogenic disturbances; however, the higher rates could also be explained by higher energy river systems and closer sediment sources. On the study reach, the spatial pattern of channel adjustments could be controlled by bed degradation, which decreased the hydrological connectivity between the main channel and its floodplain both vertically and laterally. Some studies demonstrated lowering of the groundwater level based on historical piezometric records on small sectors of the Upper Rhine (records for 1828-1955 in the Harth forest, [START_REF] Gendrin | Etude de la nappe phréatique de la plaine du Haut-Rhin[END_REF]records for 1907-1938near Ottmarsheim, Simler et al., 1979). However, water-level records showed a higher rate of decrease on the upstream reach of the Upper Rhine, which then propagated downstream (CHR, 1977, p.148): between 1828 and 1880, the change in the water level reached a maximum rate of -4 cm yr -1

at KP 186 (approaching -3 cm yr -1 at KP 199.5), then between 1880 and 1920 this maximum was -9 cm yr -1 at KP 199.5 (approaching -8 cm yr -1 at KP 186). The change in water level was less pronounced downstream of KP 215, with a rate of -3 and -7 cm yr -1 over the two periods. This result is consistent with our comparison of thalweg profiles between 1880 and 1935, which showed that bed degradation was attenuated downstream of KP 212 (Fig. 7C). Therefore, information on planform and vertical changes over the nineteenth-early twentieth century seems to validate the hypothesis that the longitudinal pattern of floodplain drying out was controlled by the longitudinal pattern of bed degradation.

The bed degradation process differed in the downstream reaches of the Upper Rhine. For example, the change in water level was only -1 cm yr -1 (1828-1880) at Strasbourg, which is located 125 km downstream of our study reach (CHR, 1977). [START_REF] Eschbach | Long-term temporal trajectories to enhance restoration efficiency and sustainability on large rivers: an interdisciplinary study[END_REF] reported a 70% reduction of bar area and a 50% reduction of connected channel area on the 4-km long Rohrschollen island near Strasbourg (1833-1876), which are less than the values of 98% and 81%, respectively, on our study reach . [START_REF] Diaz-Redondo | Benchmarking fluvial dynamics for process-based river restoration: the Upper Rhine River (1816-2014)[END_REF] studied changes in habitat structure on a 10-km long reach near Iffezheim, which is located 160 km downstream of our study reach. This reach firstly exhibited bed degradation (-3 cm yr -1 ; 1828-1880), then bed aggradation resulting from the upstream bedload supply (+1 cm yr -1 ; 1880-1920) (CHR, 1977). The authors found lower magnitudes of floodplain drying out , with fully connected channels decreasing from 33% to 12% of the floodplain area (versus 26% to 2% on our study reach; 1828-1925), partially connected channels increasing from 3% to 8% (versus 2% to 3%), and disconnected water bodies increasing from 1% to 2% (versus 0.7% to 18%). Downstream attenuation of the bed degradation process was observed on the Rhône downstream of Lyon [START_REF] Parrot | Analyse spatio-temporelle de la morphologie du chenal du Rhône du Léman à la Méditerranée[END_REF], with bed degradation being very intense in the first kilometers of the channelized reach (Pierre-Bénite reach;1897-1953). This reach changed into a more effective bedload transport configuration because of the bed incision, without upstream bedload input;

thus, it was assumed to have fed the downstream reaches and limited the downstream propagation of the sediment deficit. On the Rhine, if the magnitude of planform and vertical changes decreased downstream, the transformation of gravel bars and islands into vegetation or agricultural land following channelization was observed all along the Upper Rhine continuum (Basel to Lauterburg; 185-km long)

by [START_REF] Ricaurte | The distribution and environmental state of vegetated islands within human-impacted European rivers[END_REF], similar to the changes exhibited on the Upper Danube (Austria-Slovakia) and

Olt River (Romania).

Following this study and the findings of [START_REF] Arnaud | Historical geomorphic analysis (1932-2011) of a by-passed river reach in process-based restoration perspectives: the Old Rhine downstream of the Kembs diversion dam (France, Germany)[END_REF], we synthetized the channel adjustments in the study reach over the past 200 yr (Fig. 9). Complex cause-effect relationships between natural/anthropogenic controlling factors and hydromorphological processes took place in the 3.5-km wide multiple channel system, then in the 200-m wide channelized river. The effects of long-term (Holocene and Little Ice Age heritage) and basin-scale (Swiss engineering works) drivers on the two controlling variables of the fluvial system (the sediment supply and the flow regime) were also indicated. Interestingly, hydromorphological processes lasting several decades had further impact on engineering settlement (solid grey arrows in Fig. 9). For example, the 1930s groyne field construction took place to stabilize the long profile because the nineteenth century channelization increased flow velocity and gravel bar mobility, which complicated navigation between Germany and Switzerland. The damming and by-passing schemes were initiated at the same period, not only for hydroelectric purposes, but also because the Istein bedrock outcrop could not be crossed by boats on 332 days per year in the 1920s, and the construction of a lateral canal was needed [START_REF] Humbert | Rhin. Encyclopédie de l'Alsace[END_REF].

Implications for river restoration

This historical analysis showed that the Upper Rhine followed a non-linear evolutionary trend over the last two centuries (Fig. 10). The flood regime, sediment supply, and fluvial corridor width changed either progressively or in steps. These changes were either permanent, such as channelization, or temporary, such as floods [START_REF] Dufour | From the myth of a lost paradise to targeted river restoration: forget natural references and focus on human benefits[END_REF]. The responses of the geomorphic variables were either synchronous or were assumed to be delayed. Thresholds were reached in the adjustments of the bed-level, active-channel width, and bed grain-size. Currently, the system seems adjusted to the new conditions of the controlling variables, but its morphology remains sensitive to floods [START_REF] Arnaud | Historical geomorphic analysis (1932-2011) of a by-passed river reach in process-based restoration perspectives: the Old Rhine downstream of the Kembs diversion dam (France, Germany)[END_REF]. This model provides a knowledge framework with which river restoration scenarios can be discussed. The present-day challenges for the Old Rhine are to increase bedform mobility to create hydraulic and habitat diversity based on artificial gravel augmentation [START_REF] Arnaud | Monitoring gravel augmentation in a large regulated river and implications for process-based restoration[END_REF], and to allow bank reerosion through dike removal [START_REF] Chardon | Geomorphic effects of gravel augmentation and bank re-erosion on the Old Rhine River downstream from the Kembs dam (France, Germany)[END_REF]. Such strategies, if implemented on a large scale, would move the evolutionary trends of the response variables towards a more dynamic system (see dotted arrows in Fig. 10). Our retrospective analysis of side channels has helped to specify the nature of changes (e.g., changes in hydrological connectivity, vegetation, or cropland) and their spatial and temporal magnitudes (Fig. 6). This is critical information for identifying sites for functional channel restoration and determining their sustainability. In 2013, a 7-km long former channel was reconnected to the Old Rhine, immediately below the Kembs Dam [START_REF] Reynier | Restoration of an old bed of the river Rhine on the Kembs island: Innovative processes and tools in an ambitious site's service[END_REF]. This is a first encouraging measure, because the possibilities for former channel restoration are very limited given the intense floodplain artificialization on the study reach. The downstream reaches of the Upper Rhine are also highly urbanized, but they are less incised (see Section 4.2); thus, floodplain channel restoration is more feasible [START_REF] Diaz-Redondo | Benchmarking fluvial dynamics for process-based river restoration: the Upper Rhine River (1816-2014)[END_REF][START_REF] Eschbach | Long-term temporal trajectories to enhance restoration efficiency and sustainability on large rivers: an interdisciplinary study[END_REF]. Another encouraging large-scale program concerns flood control on the whole of the Upper Rhine (Integrated Rhine Programme; [START_REF] Kuhn | The Integrated Rhine Programme. Flood control and restoration of former floodplains along the Upper Rhine[END_REF]. On the German side of the Old Rhine, the floodplain surface is being lowered to create flood retention areas of up to 25 million m 3 capacity. By enhancing lateral connectivity in these retention areas, the aim is also to improve the mosaic of biotopes in the Rhine floodplain [START_REF] Kuhn | The Integrated Rhine Programme. Flood control and restoration of former floodplains along the Upper Rhine[END_REF].

Conclusion

This study provides new insights on the long-term (181 yr) trajectory of a large multi-impacted braided and anabranching river. Among the three phases of engineering works that affected the Upper Rhine (channelization, groyne construction, damming), channelization had the most impact on planform (floodplain terrestrialization, end of channel shifting) and vertical (intensive bed degradation) changes, with a short reaction time and effects over several decades. Damming had the most impact on peak flows, which explains the lower magnitude of channel changes in the most recent period. The complex pattern of a 50-km long continuum of the Rhine floodplain prior to major pressures has been quantified

for the first time, as well as the downstream propagation of the changes. The present Old Rhine follows an evolutionary trend inherited from the nineteenth century, but is adjusted to the controlling variables imposed by damming since the 1950s, especially peak flood reduction, which decreased the frequency of critical flows for bedload transport. Finally, the nature and magnitude of the changes in side channels, which are here quantified in space and time, are critical contributions to the understanding of large braided and anabranching multi-impacted rivers in a functional restoration context.

Fig. 1 .

 1 Fig. 1. (A) Geologic-tectonic setting of the Rhine basin (after Frings et al., 2014). (B) Extent of the early nineteenth century complex channel area, the present Old Rhine and the Grand Canal d'Alsace.

Fig. 2 .

 2 Fig. 2. Maximum, mean, and minimum annual discharge series at Basel and in the Old Rhine for the period 1808-2011. The discharge diverted into the Grand Canal d'Alsace was progressively increased between 1932 and 1983, from 850 to 1400 m 3 s -1 , to support growing navigation traffic and hydroelectricity requirements. Red points indicate the number of days per year that the mean daily discharge exceeded the 2-yr flood discharge.

  (a few years after the completion of channelization), 1935 (during the regularization and three years after damming and by-passing of the upstream part of the study reach), 1950-56 (during the bypassing scheme),1985-93, and 2009 (after by-passing). The three most recent thalweg profiles were extracted from the cross-section series reported in[START_REF] Arnaud | Historical geomorphic analysis (1932-2011) of a by-passed river reach in process-based restoration perspectives: the Old Rhine downstream of the Kembs diversion dam (France, Germany)[END_REF]. These cross sections were surveyed every 200 m and at the same location on each of the three dates. The historical paper documents from 1880 and 1935 describe the thalweg profile under a continuous line from upstream to downstream. As kilometric posts are located on the right (German) dike of the channelized river and their position had not changed since the nineteenth century, we could apply the same 200-m spacing to the historical profiles. Bed-level comparisons were possible after adjustment of the historical vertical datum to the current vertical datum (

Fig. 4 .

 4 Fig. 4. (A) Changes in fluvial and anthropogenic features related to the hydrosystem area over 1828-1925. (B) Changes in the types of channel connection related to the area of natural water bodies over 1828-1925. (C) Changes in active-channel width over 1828-2008. Box plots provide the 10 th , 25 th , 50 th , 75 th , and 90 th percentiles and mean values.

Fig. 5 .

 5 Fig. 5. Lateral distribution of aquatic and terrestrialized channels on three cross sections in 1828, 1872, and 1925.

Fig. 6 .

 6 Fig. 6. (A) Cumulative areas of the hydrosystem extent, the channel types and dikes/embankments in a downstream direction in 1828. (B) Cumulative rates of change in area of the channel types in a downstream direction between 1828 and 1872 (in black), and between 1872 and 1925 (in grey;

Fig. 8 .

 8 Fig. 8. Selection of cross sections showing the channel geometry (A) in 1884; (B) in 1923 (3 successive cross sections); (C) from 1931 to 2009. See Table 2 for details on data sources. Cross sections in 1884 and 1931-2009 have been reproduced. Cross sections in 1923 are from scanned original documents.

Fig. 9 .

 9 Fig. 9. Synthesis of the Upper Rhine channel adjustments to natural and anthropogenic controlling factors over the past 200 yr. This figure describes cause-effect relationships "a impacts b" (solid black arrows), "a inversely impacts b" (dotted black arrows), and "impacts of hydromorphological processes on engineering settlement" (solid grey arrows) demonstrated during this study. The potential feedback mechanism between active channel narrowing and bed degradation (dotted grey arrows) was discussed by Arnaud et al. (2015).

Fig. 10 .

 10 Fig. 10. Controlling variables (in black) and response variables (in grey) of channel morphodynamics over the past 200 yr. The bed grain-size trend information is from Arnaud et al. (2015). Dotted arrows indicate the potential river restoration trends. The ordinate axis is in relative units. *The fluvial corridor width is the artificially constrained width in which most of the flow runs (e.g.: the 200-m wide corridor following channelization; the 75-m wide corridor following regularization). It differs from the active-channel width because some channels and gravel bars were still remnant outside the embankments. The fluvial corridor width is indicated as a controlling variable because it was directly modified by anthropogenic activities, such as floods and sediment supply.

Table 1 .

 1 Maps used in the planform analysis.

BW: black and white; C: colored. Y: legend available; N: no legend. RMSE: root mean square error. BNU: Bibliothèque Nationale et Universitaire de Strasbourg; PCA: Petite Camargue Alsacienne; Arch. Dép. 68: Archives Départementales du Haut-Rhin.

Table 2

 2

	Year	Type	Source	Covered reach (kilometric post)	Space interval	Number of surveys	Vertical datum	Vertical adjustment to NN n.S. a
	1880	L	WSA	174.1-220.8	~ 1 point per 200 m	233 points	AP	+ 10 cm
	1935	L	SNS	178.0-220.8	~ 1 point per 200 m	213 points	DHHN12	+ 11 cm
				174.4-179.8 (1950-56)
	1950-56	L	WSA	180.04-194.4 (1950-52)	~ 1 point per 200 m	197 points	NN n.S.
				194.61-214.0 (1952-56)
				174.4-179.8 (1985)
	1985-93	L	WSA	180.04-209.93 (1993)	~ 1 point per 200 m	231 points	NN n.S.
				210.0-220.8 (1985)
	2009	L	WSA	174.1-220.8	~ 1 point per 200 m	240 points	NN n.S.
	1884	CS	Arch. Dép. 68	174.0-218.0	~ 1 CS per 3 km	21 CS	*
	1923	CS	Arch. Gén. Karlsruhe	178.0-182.0 196.0-198.0	~ 1 CS per 100 m	60 CS	**
	1930-37	CS	WSA	178.8-218.0	~ 1 CS per 3 km	14 CS	DHHN12	+ 11 cm
	1950-56	CS	WSA	175.6-213.0	~ 1 CS per 3 km	14 CS	NN n.S.
				175.6-178.8 (1985)
	1985-93	CS	WSA	182.91-205.82 (1993)	~ 1 CS per 3 km	14 CS	NN n.S.
				211.4-218.0 (1985)
	2009	CS	WSA	175.6-218.0	~ 1 CS per 3 km	14 CS	NN n.S.

; vertical datum information from R. Ostermann and H.

Lehmacher, Regierungspräsidium Freiburg, 2010).

Table 2 .

 2 Topographic data used in the vertical analysis.

	Amsterdamer Pegel; DHHN12: Deutsches Haupthöhennetz 1912; NN n.S.: Normal Null neues
	System; * bed level relative to the low-flow water level; ** no bed-level information; a Mean values of
	vertical datum differences between Basel and Breisach.

L: longitudinal profile (thalweg); CS: cross-sectional profile; WSA: Wasser-und Schifffahrtsamt Freiburg; SNS: Service de la Navigation de Strasbourg; Arch. Gén.: Archives Générales; AP:

Acknowledgements

This research was supported by the European Regional Development Fund within the project "Redynamization of the Old Rhine" (FEDER INTERREG IV Upper Rhine) and by Electricité de France within the research collaboration "Sediment reintroduction into the Old Rhine from controlled bank erosion". We thank Dorothée Hoenen for GIS assistance and Karl Embleton for proof-reading. Finally, we thank the two reviewers for their comments that substantially improved the quality of this manuscript.