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\DOI{10.5802/crgeos.314}
\datereceived{2025-01-13}
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\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\TopicFR{D\'eveloppement durable, am\'enagement}
\TopicEN{Sustainable development, landscaping}
%
%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Human Environment Observatory}
%\def\cdr@specialtitle@french{Observatoire Homme Milieu (OHM)}
%\makeatother

\title{Observing social and environmental change in a large regulated
river: the Rh\^{o}ne Valley Human-Environment Observatory}

\alttitle{\`{A} l'\'{e}coute des mutations socio-environnementales d'un
grand fleuve am\'{e}nag\'{e} : l'Observatoire Hommes-Milieux Vall\'{e}e
du Rh\^{o}ne}

\author{\firstname{Carole} \lastname{Barth\'{e}l\'{e}my}\CDRorcid{0000-0003-1536-0108}\IsCorresp}
\address{Aix Marseille Univ, IRD, LPED, Marseille, France}
\address{Laboratoire Population Environnement D\'{e}veloppement, Campus
Saint-Charles, 3 place Victor Hugo, 13331 Marseille Cedex 3, France}
\email[C. Barth\'{e}l\'{e}my]{carole.barthelemy@univ-amu.fr}

\author{\firstname{Fanny} \lastname{Arnaud}\CDRorcid{0000-0002-8784-1384}}
\address{UMR 5600 EVS, CNRS, ENS de Lyon, Universit\'{e} de Lyon, France}
\email[F. Arnaud]{fanny.arnaud@ens-lyon.fr}

\author{\firstname{Brice} \lastname{Mourier}\CDRorcid{0000-0001-8881-7346}}
\address{Universit\'{e} Claude Bernard Lyon 1, LEHNA UMR 5023, CNRS, ENTPE,
F-69518, Vaulx-en-Velin, France}
\email[B. Mourier]{Brice.MOURIER@entpe.fr}

\author{\firstname{Bertrand} \lastname{Morandi}\CDRorcid{0009-0005-7730-4824}}
\address{GRAIE Campus LyonTech la Doua, 66 bd Niels Bohr -- CS 52132
69603 Villeurbanne Cedex, France}
\email[B. Morandi]{bertrand.morandi@graie.org}

\author{\firstname{Herv\'{e}} \lastname{Pi\'{e}gay}\CDRorcid{0000-0002-3864-2119}}
\addressSameAs{3}{UMR 5600 EVS, CNRS, ENS de Lyon, Universit\'{e} de Lyon, France}
\email[H. Pi\'{e}gay]{Herve.Piegay@ens-lyon.fr}

\keywords{\kwd{Rh\^{o}ne river}\kwd{Interdisciplinarity}\kwd{Relation
with local stakeholders}\kwd{Human-Environment
Observatory}\kwd{Integrated management}\kwd{Sustainable development}}

\altkeywords{\kwd{Rh\^{o}ne}\kwd{Interdisciplinarit\'{e}}\kwd{Relations
avec les acteurs locaux}\kwd{Observatoire Hommes-Milieux}\kwd{Gestion 
int\'{e}gr\'{e}e}\kwd{D\'{e}veloppement durable}}

\shortrunauthors

\begin{abstract} 
Following the major floods of the early 2000s, the Rh\^{o}ne river has
been the focus of an integrated management plan, the \textit{Plan
Rh\^{o}ne}. This plan advocates a new way of managing the river from a
sustainable development perspective. The Rh\^{o}ne Valley
Human-Environment Observatory (OHM VR) was set up in 2010 with
sustainable development as a foundation applied to the scale of the
river. The scientific objectives of the OHM VR have been to monitor the
effects of this new management approach in a perspective of a highly
anthropised socio-ecosystem, by studying the socio-environmental
responses and adaptations in the Rh\^{o}ne corridor following a change
in the mode of river management. Priority scientific themes were
developed with the aim of covering most of the questions raised by this
new mode of management: to achieve a better understanding of the
geo-historical trajectory of the river to situate temporally the crisis
events and their consequences; to provide insight into the new modes of
management from a socio-political point of view and their reception by
the local populations; to study the socio-economic processes in a
context of ecological restoration actions; to analyse the environmental
risks, in particular pollution; and to develop new tools to support the
research works and the diffusion of scientific results.
Interdisciplinarity and relations with local stakeholders provide the
framework for these investigations. This article provides an overview
of the activities of the OHM VR and their evolution since its creation,
with a particular focus on two interdisciplinary case studies that have
shaped collective scientific thinking in recent years.
\end{abstract}

\begin{altabstract} 
Suite aux inondations tr\`{e}s importantes du d\'{e}but des ann\'{e}es
2000, le fleuve Rh\^{o}ne a \'{e}t\'{e} dot\'{e} d'un vaste plan de
gestion appel\'{e} Plan Rh\^{o}ne. Celui-ci pr\^{o}ne une nouvelle
mani\`{e}re de g\'{e}rer le cours d'eau dans la perspective du
d\'{e}veloppement durable. L'Observatoire Hommes-Milieux Vall\'{e}e du
Rh\^{o}ne (OHM VR) a \'{e}t\'{e} cr\'{e}\'{e} en 2010 en prenant comme
\'{e}v\'{e}nement fondateur la mise en place de cette gestion dite
durable \`{a} l'\'{e}chelle du fleuve. Les objectifs scientifiques de
l'OHM VR consistent \`{a} \'{e}tudier les effets de cette gestion dans
une perspective de socio-\'{e}cosyst\`{e}me fortement contraint, en
\'{e}tudiant les r\'{e}ponses et les adaptations
socio-environnementales suite au changement de mode de gestion. Des
th\'{e}matiques prioritaires ont \'{e}t\'{e} \'{e}labor\'{e}es afin de
couvrir une grande partie des questionnements suscit\'{e}s par cette
nouvelle gestion : mieux comprendre la trajectoire g\'{e}o-historique
du fleuve pour situer les \'{e}v\'{e}nements de crise et leurs
cons\'{e}quences ; appr\'{e}hender les nouvelles modalit\'{e}s de
gestion d'un point de vue socio-politique et leur r\'{e}ception
aupr\`{e}s des populations ; \'{e}tudier les processus
socio-\'{e}cosyst\'{e}miques dans un contexte de restauration
\'{e}cologique ; analyser les risques environnementaux, notamment les
pollutions ; et d\'{e}velopper de nouveaux outils d'aide aux travaux de
recherche et de diffusion des r\'{e}sultats scientifiques.
L'interdisciplinarit\'{e} et les \'{e}changes avec les acteurs locaux
structurent l'ensemble de ces questionnements. Cet article dresse un
panorama des activit\'{e}s de l'OHM VR et de leur \'{e}volution depuis
sa cr\'{e}ation, et il d\'{e}veloppe en particulier deux cas
d'\'{e}tudes interdisciplinaires qui ont structur\'{e} la r\'{e}flexion
scientifique collective ces derni\`{e}res ann\'{e}es.
\end{altabstract} 

\thanks{LabEx DRIIHM (ANR-11-LABX-0010), LTSER \textit{Zone Atelier
Bassin du Rh\^{o}ne}, EUR H2O'Lyon (ANR-17-EURE-0018)}

\editornote{Article submitted by invitation}
\alteditornote{Article soumis sur invitation}

\maketitle

\vspace*{1pc}

\twocolumngrid

\end{noXML}

\defcitealias{Poirieretal2023}{ibid.}
\defcitealias{GramagliaBabut2014}{ibid.}
\defcitealias{Thoreletal2018}{ibid.}

\section{The Rh\^{o}ne: an anthropised river, partially restored}
\label{sec1}

The Rh\^{o}ne River today is characteristic of a great European river
subject to anthropic pressures for several centuries, to meet the needs
of a wide range of human uses \citep{Tockneretal2022}. Connecting the
French-Swiss Alps with the Mediterranean Sea, the Rh\^{o}ne River is
812~km long and the catchment area is 98~500~km$^{2}$, with 90~500
km$^{2}$ located in France \citep{Olivieretal2022}. The mean annual
discharge is 1720 m\tsup{3}${\cdot}$s$^{-1}$ at the furthest
downstream gauging station (Beaucaire), which makes the Rh\^{o}ne River
the most important freshwater input into the northwestern Mediterranean
Sea \citep{Arnaudetal2021}. The French course of the Rh\^{o}ne River
exhibited two main phases of human intervention since the 19{th}
century which fundamentally altered the hydromorphological and
ecological functioning of the floodplain. First controlled to prevent
floods and facilitate navigation, the river was subsequently regulated
during the 20th century for the production of energy. Twenty-one
hydroelectric dams and four nuclear power plants today make the
Rh\^{o}ne River one of the main energy producers in France, supplying
respectively a quarter of nuclear electrical production and a quarter
of hydroelectricity \citep{Comby2015} (Figure~\ref{fig1}). Each year,
3.1 billion m\tsup{3} of water is extracted, including 48\% for
agricultural irrigation, 24\% for hydroelectricity production, and 16\%
for the supply of drinking water \citep{AgencedelEau2023}. The river is
the recipient of domestic and industrial effluents, and has for several
decades suffered from chronic pollution resulting in socio-political
crises related to certain pollutants, such as PCBs (polychlorobiphenyls
or Pyralene) \citep{GramagliaBabut2014}, or more recently the PFAS
(per- and polyfluoroalkyl substances) \citep{LeNaourThomas2022}.
Gravels from the Rh\^{o}ne channel and floodplain has been extensively
exploited for infrastructure construction, especially since gravel
extraction, by deepening the riverbed and locally limiting deposits,
also facilitated navigation. Today, the Rh\^{o}ne River does not
transport any bedload downstream of Lyon, and its geometry is largely
fixed.

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig01}
{\vspace*{-2pt}}
\caption{\label{fig1}Energy production on the French Rh\^{o}ne river
(past, present and planned infrastructures).} 
\end{figure*}

Overall, these anthropic pressures have radically transformed the
socio-ecosystem dynamics of the Rh\^{o}ne River by altering
hydrological, ecological and geomorphological processes, and also by
causing a deterioration in water quality and biodiversity. From the
early 1970s, and notably thanks to the French water act of 1964,
France's first law to restore water quality  and the creation of the
\textit{Agences de l'Eau} (French river basin water management
agencies) \citep{Bouleau2014}, a wide-ranging water management policy
has contributed to improving water quality and protecting rivers of
ecological interest in France. At the end of the 1990s, a major
hydraulic and ecological restoration scheme was launched by the
stakeholders involved in the management of the Rh\^{o}ne following the
publication of the first \textit{Rh\^{o}ne M\'{e}diterran\'{e}e Corse}
SDAGE (river basin masterplan), which recommended the restoration of
``a dynamic and free-flowing river''. The planned restoration scheme
consisted of increasing the minimum flows in by-passed channels
downstream of dams, rehabilitating the \textit{l\^{o}nes}, i.e.\ former
floodplain channels which host great biodiversity richness, and
restoring the main pathways of fish migration
\citep{Lamourouxetal2012}.
Moreover, along the French Rh\^{o}ne and its
main tributaries, several areas have protection status at a regional,
national or European level, including Natura 2000 sites of Community
importance ({Council Directive 92/43/EEC on the conservation of natural
habitats and of wild fauna and flora}) and special protection areas
(Council Directive 79/409/CEE on the conservation of wild birds)
\citep{Olivieretal2022}. The Rh\^{o}ne River corridor is therefore not
only characteristic of a highly anthropised river, but is also a
territory where there has been considerable investment in terms of
measures for the management and the protection of nature. Current
priorities, on both the use and the protection of water \mbox{resources,} are
and will be impacted by climate change in progress, which results in
changes in flood discharges, low-flow frequency and water temperature
\citep{AgencedelEau2023}. It is in this changing socio-environmental
context that the Rh\^{o}ne Valley Human-Environment Observatory
(\textit{Observatoire Hommes-Milieux Vall\'{e}e du Rh\^{o}ne}---OHM VR)
is conducting its research.

\section{The Rh\^{o}ne Valley Human-Environment Observatory: a
geographic and interdisciplinary connection of the Rh\^{o}ne
research}\label{sec2}
\subsection{The OHM VR conceptual framework}\label{ssec21}

The OHM VR was created in 2010. It was designed to connect communities
of researchers \mbox{working} 
\mbox{respectively} on the southern and the northern
reaches of the Rh\^{o}ne River. The observatory thus represented a way
to consider the river over its entire course and to create a dialogue
between research works that had previously been carried out in one or
other part of the river, with no existing space for exchange. The
geographical extent of the OHM VR covers the French river course from
Lake Geneva to the Mediterranean Sea, i.e.\ a 534~km long reach. The
space studied includes the channel, the banks, the floodplain and the
delta. The creation of the OHM VR offered the means to develop
exchanges between scientific disciplines.  The aim was to find common
research issues in order to develop a dialogue between all the
environmental sciences, i.e.\ the ``sciences of the Earth, life,
humankind and societies'' \citep{Chenorkian2020} working on the
Rh\^{o}ne River. To this end, the conceptual framework of the OHM VR
was built on the basis of the ternary concept ``structuring fact'',
``disrupting event'' and ``focal object'' characteristic of OHM
\citep{Chenorkian2020,Chenorkian2023}. The ``structuring fact'' is
defined by the socio-ecological framework of a highly anthropised
territory. In the case of the Rh\^{o}ne River, this means 175 years of
hydraulic engineering which fundamentally altered the fluvial
hydrosystem in response to the devastating floods of 1840 and 1856, the
river was rectified along most of its course by means of embankments,
from the Savoie department to the Mediterranean Sea. It was then
regulated from the end of the 19th century with the construction of a
complex system of longitudinal and transversal structures
(\textit{Casiers Girardon}) along the whole continuum downstream of
Lyon, to achieve a shipping channel with constant depth 
(see Figure~\ref{fig10}).
The effects of these engineering works were to
alter the river's geomorphology and sediment inputs in the delta and
along the Mediterranean coasts. A second period, extending from the
1930s to the 1980s, consisted of the construction of twenty-one
hydroelectric facilities including dams and canals. These also served
to provide water for irrigation and to promote navigation. Other
regulations along the Rh\^{o}ne River are dedicated to the water supply
for industrial processes and for the cooling systems of nuclear power
plants (Bugey, Saint-Alban, Cruas, Tricastin; see Figure~\ref{fig1}).
In the 1980s, the Rh\^{o}ne thus became a highly regulated river where
all natural processes were disturbed. The ``disrupting event'', which
is the second element of the conceptual framework of the OHMs, is
defined by an event that disrupted the spatio-temporal trajectory of
the socio-ecological framework. Floods that occurred in the Lower
Rh\^{o}ne in the early 1990s and 2000s caused extensive damages that
had a profound impact on local residents and all river stakeholders 
\citep{Comby2015}. The French State and the local {authorities} undertook
a reform of the Rh\^{o}ne management, after examining the social,
health, security and environmental consequences of the historical
engineering works. As the first inter-regional development plan for the
river, the \textit{Plan Rh\^{o}ne} was signed in 2007 with the aim of
introducing a ``global'' and ``sustainable'' management system, to
replace the sectoral management
system\footnote{\url{https://www.plan-rhone.fr/}.}. This paradigm shift
in the Rh\^{o}ne River management constituted the disrupting event of
the OHM VR. The ``focal object'', which is the last element of the
ternary conceptual framework, is defined by the highly anthropised
river corridor impacted by this new orientation in terms of public
policy (Figure~\ref{fig2}). This element is particularly well-suited
for testing interdisciplinary research approaches to meet the
socio-economic issues raised by the operational implementation of the
sustainable management of the Rh\^{o}ne River.\looseness=-1

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig02}
{\vspace*{-3pt}}
\caption{\label{fig2}Conceptual framework of the OHM VR: radical change
in the river management following floods of the 1990s and 2000s which
gave rise to the implementation of a sustainable development policy and
commitment by the actors to the \textit{Plan Rh\^{o}ne}
\citep{PiegayBarthelemy2014}.}
{\vspace*{-2pt}}
\end{figure*}

The community of researchers brought together in the OHM VR device has
set the objective of studying ``the responses and socio-environmental
adaptations in the Rh\^{o}ne River corridor following a change in the
mode of river management'' \citep{PiegayBarthelemy2014}. Research  was
programmed with a view to exchange with operational stakeholders: the
regional environmental management department (DREAL), the river basin
water management agency (\textit{Agence de l'Eau
Rh\^{o}ne-M\'{e}diterran\'{e}e-Corse}), the {publicly-owned} {company
to develop the Rh\^{o}ne River} (\textit{Compagnie Nationale du
Rh\^{o}ne}---CNR), the national electricity company (EDF), river
syndicates, nature reserves, etc. The research also aimed to include
local residents, all of these providing scientific support to analyse
the new \textit{Plan Rh\^{o}ne} management system. More generally, the
Rh\^{o}ne context and the conceptual framework of the OHMs have enabled
to produce knowledge of the contemporary challenges regarding the
integrated management of fluvial hydrosystems in a context of global
change. 

\subsection{The OHM VR research within the Rh\^{o}ne scientific
ecosystem}\label{ssec22}

The research carried out by the OHM VR device is part of a particular
institutional context, specific to the history of scientific production
on the Rh\^{o}ne River. Its creation responded to the need to better
interface major partnership research programs and scientific facilities
working in the southern and the northern Rh\^{o}ne sectors, while
broadening the thematic fields studied to better meet the challenge of
integrated river management. This led us to mobilize the humanities and
social sciences (SHS) in particular, given the complexity of the actors
involved and the numerous environmental issues raised by researchers at
the various sites and observatories in the Rh\^{o}ne Basin. This also
enabled us to relaunch the site \textit{Axe Rh\^{o}ne} of the existing
\textit{Zone Atelier Bassin du Rh\^{o}ne} (ZABR) for the Rh\^{o}ne
River corridor, which had stalled due to a lack of collective projects.
The OHM VR is now one of the nine sites and observatories in the
\textit{ZABR} (scientific platform from the Ecology \& Environment
Institute of the National Research Centre---CNRS; \textit{Centre
National de la Recherche Scientifique}). This structure makes it
possible to collaborate with other researchers on the Rh\^{o}ne basin
scale. The OHM VR incorporates three long-term research programs: 
(1)~the Rh\^{o}nEco program\footnote{\url{https://www.rhoneco.fr/}.}, a
unique ecological observatory that has been focused on the scientific
monitoring of the hydraulic and ecological restoration of the Rh\^{o}ne
since 1998, (2) the Rh\^{o}ne Sediment Observatory
(\textit{Observatoire des S\'{e}diments du
Rh}\^{o}ne---OSR)\footnote{\url{https://observatoire-sediments-rhone.fr/}.},
whose objective since 2009 has been to produce knowledge of the
hydro-sedimentary functioning of the river and associated contaminant
fluxes, and (3)~the research platform of the Cr\'{e}pieux-Charmy water
pumping field, labelled \textit{Site d'Etude en Ecologie 
Globale---Milieux Anthropis\'{e}s} (SEEG-MA; Research site in global
ecology---anthropised environments) by the CNRS in 2019.

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig03}
\vspace*{1pt}
\caption{\label{fig3}Scheme of the Research Data Infrastructure
supporting the OHM VR \citep[adapted from][]{Arnaudetal2020}.}
\end{figure*}

When it was set up, the OHM VR benefitted from the presence of two
dynamic university sites, Lyon and Aix-Marseille, and an already
structured and interdisciplinary community working on the Rh\^{o}ne
River. While geography was one of the first disciplines to take an
interest in the functioning of the river from the end of the 19th
century, the 1960s saw the emergence of research in chemistry on water
quality (following pollution accidents) \citep{Bouleau2014}. Ecology,
particularly hydrobiology, and fluvial geomorphology have been
mobilised since the 1980s within the framework of the \textit{Programme
Interdisciplinaire de Recherche sur l'ENvironnement} (PIREN) \textit{du
Haut-Rh\^{o}ne} (Interdisciplinary Research Program on the Environment
of the Upper Rh\^{o}ne), launched by the CNRS and the French Ministry
of the Environment to study the river's ecology in the {context} of
hydroelectric development \citep{Roux2014}. His program notably saw the
emergence of the \mbox{four-dimensional} fluvial hydrosystem concept 
\citep{AmorosPetts1993}, which was adopted by water actors on the
national scale. The social sciences, in particular sociology and
anthropology, became involved later in the Rh\^{o}ne scientific
ecosystem \citep{Chabenat1996,BarthelemyOlivier2024}. As the Lower
Rh\^{o}ne, the Camargue delta and the Upper Rh\^{o}ne are the most
studied parts of the river, the OHM VR community has been made up of
these pools of researchers in order to share their knowledge at
scientific seminars and explore new scientific questions developed in
dialogue with the river stakeholders and funded mainly via the OHM VR
call for projects.\looseness=-1

\subsection{Numeric tools for sharing and promoting the
Rh\^{o}ne research outputs}\label{ssec23}

A research observatory serves as an ideal setting for the long-term
generation, exchange, and sharing of information and knowledge
\citep{Liboureletal2009}. However, the multidisciplinary and complex
nature of socio-ecosystems makes it challenging to develop tools to
help research teams to work together effectively and to transfer
scientific knowledge to decision makers and the general public. A
Research Data Infrastructure (RDI) \citep{Latifetal2019} has been
\mbox{developed} over the years within the LabEx DRIIHM to provide tools and
services for the storage, visualization, and dissemination of
heterogeneous datasets produced by the OHMs \citep{Arnaudetal2020}. The
OHM VR early developed several tools to meet specific needs following
the pioneer ZABR investment to inform, store and share data
(Figure~\ref{fig3}):
\href{https://elvis.ens-lyon.fr/geonetwork/ohm-vr/fre/catalog.search\#/search}{metadata catalogue}, 
webGIS 
\href{https://websig.ens-lyon.fr}{G\'{e}oRh\^{o}ne}, 
online and desktop databases 
(\href{https://bdoh.irstea.fr/OBSERVATOIRE-DES-SEDIMENTS-DU-RHONE/}{BDOH},
ENS-Observatoires), 
\href{https://photo.driihm.fr/index.php?/category/vallee-du-rhone}{photo gallery}, 
and recently, the spatio-temporal interactive application (timeline)
\href{https://chrono-rhone.ens-lyon.fr/}{Chrono-Rh\^{o}ne} 
that describes significant events in the socio-ecological trajectory of
the Rh\^{o}ne River.\looseness=-1

\begin{figure*}
\vspace*{-.5pt}
\includegraphics{fig04}
\caption{\label{fig4}Geographical extent of the data produced and
collected in the OHM VR between 2011 and 2024 ($n=95$ metadata
sheets).}
\end{figure*}

The challenge for an RDI such as this lies in maintaining technical
skills (IT and geomatics engineers) over the long term, as well as
offering permanent human assistance to help researchers in achieving a
gradual cultural shift towards open science practices. However, if
these numerical tools receive regular input, they can become powerful
communication tools that can be used as quantitative indicators of the
scientific production of the observatory. The
\href{https://hal.science/OHM-VALLEE\_DU\_RHONE/browse/last}{documentary portal} 
of the OHM VR currently contains 793 documents. A textometric analysis
carried out on the metadata sheets published by the researchers running
OHM VR projects between 2011 and 2024 ($n=95$ metadata sheets;
Figure~\ref{fig4}) shows that the OHM VR produces data throughout the
\mbox{entire} French Rh\^{o}ne and its tributaries. Some sectors are more
investigated, e.g.\ the river downstream of Lyon and the Upper Rh\^{o}ne
between Lake Geneva and Br\'{e}gnier-Cordon.

The free keywords indicated by the researchers highlight the
predominance of \textit{casiers Girardon} and cartographical archives,
and to a lesser extent, the datasets related to sedimentation,
vegetation, floodplain, pollution, discourse, interviews and press
\mbox{(Figure~\ref{fig5}).} The
OHM VR as a platform that federates research on the Rh\^{o}ne is also
visible, with datasets that have been produced in shared works with the
OSR or Rh\^{o}nEco programs.

\begin{figure}
\includegraphics{fig05}
\vspace*{-2pt}
\caption{\label{fig5}Word cloud based on free keywords in metadata
sheets (occurrence ${\geq}$ 2). The terms ``Rh\^{o}ne'' and ``OHM VR''
have been removed to avoid distorting the figure.}
\vspace*{-2pt}
\end{figure}

\vspace*{-1pt}

\section{An interdisciplinarity approach organized to question the
implementation of sustainable development of the Rh\^{o}ne River}
\label{sec3}

\vspace*{-1pt}

The political, social and economic crisis triggered by the floods of
the 1990s and 2000s led to the creation, in 2004, of the ``\textit{Plan
Rh\^{o}ne}'' Interregional Project Contract, designed to provide local
territories with the financial and other resources needed to implement
what is known as global management or sustainable development. In the
first \textit{Plan Rh\^{o}ne}
\mbox{(2007--2013)}\footnote{\url{https://www.planrhone.fr/fileadmin/medias/Publications/General/Contrat\_Projets\_Interregional\_Plan\_rhone\_2007-1013.pdf}.}, 
sustainable development is defined (p.~7) as ``a project capable of
making the most of the \mbox{territory's} potential, managing the constraints
which it is subject and preserving its environmental assets and the
quality of life of its inhabitants''\footnote{``Un projet \`{a}
m\^{e}me de permettre tout \`{a} la fois d'exploiter au mieux les
potentialit\'{e}s de ce territoire, de g\'{e}rer les contraintes
auxquelles il est soumis et de pr\'{e}server ses richesses
environnementales et le cadre de vie de ses habitants''.}. While river
development in the post-war years was mainly focused on using the
resource for productive purposes, the 1970s--1980s were marked by
environmentalist movements at national and local levels, which led to
the initiation of research and policies related to nature protection
\citep{BarthelemySouchon2009}. The ``global sustainable development
project for the river and its valley'' advocated by the \textit{Plan
Rh\^{o}ne} (p.~3)\footnote{``Projet global de d\'{e}veloppement durable
pour le fleuve et sa vall\'{e}e''.} would then introduce a new phase,
seeking to reconcile {human uses and ecological issues}. To~respond to
this disrupting event and to study the possible modalities of this
sustainable development, the OHM VR has been structured with
transversal research themes, potentially relevant to several scientific
disciplines and enabling the interdisciplinary ``cultural melting pot''
specific to the OHMs device to function \citep{Chenorkian2020}.

\vspace*{-2pt}

\subsection{Structuring and evolutive research themes}
\label{ssec31}

\vspace*{-1pt}

The lifetime of the OHM VR allows us to look back over almost 15 years
of operation. This offers very clear evidence of the necessity of this
long timeframe to establish both a community of researchers who are
geographically and thematically distant, and common knowledge
challenges. Between 2011 and 2014, the research themes rapidly
increased from three to six (Figure~\ref{fig6}). 

\begin{figure*}
{\vspace*{1pt}}
\includegraphics{fig06}
\caption{\label{fig6}Evolution of OHM VR research themes between 2011
and 2020.}
{\vspace*{1pt}}
\end{figure*}

The first three themes were focused on: (1) study and management of
``constructed'' alluvial margins, (2) stakeholders' perceptions and
representations of the risks and challenges of the Rh\^{o}ne
\mbox{development,} and (3) approaches to modeling and predicting change. A
particular investment was made on the issue of dismantling
``constructed'' alluvial margins, i.e.\ the river banks inherited from
the Girardon engineering works \citep{Thoreletal2018}. This issue is
exemplary of the exchanges between researchers and river management
stakeholders, as it is directly linked to concerns regarding to
Rh\^{o}ne flood mitigation and river restoration issues (see 
Section~\ref{sec4}). Three years after the creation of the OHM VR, it
became clear that we needed to diversify our scientific perspectives in
order to broaden the spectrum of the management paradigm shift,
embracing a wider understanding of the socio-ecosystem and the main
environmental issues associated with the river. Six priority themes
were then identified (Figure~\ref{fig6}): 

\smallskip\noindent
(1) \textit{Geo-historical trajectory and temporal disruption of 2003.}
The aim of this theme is to resituate the change in the management
paradigm within a longer time perspective, and to integrate the
evolutions of the socio-ecosystem into its physical, biological,
chemical and social dimensions. The OHM VR conceptual scheme
(Figure~\ref{fig2}) provides a simplified temporal image of the
historical domestication of the river over a period of around 175
years. The aim is to specify this trajectory, and even to reveal
whether earlier trajectories may have influenced current
socio-environmental dynamics, such as the detailed establishment of
infrastructures and their \mbox{effects} on sediment dynamics. An additional
aim is to identify other temporal disruptions which might have
contributed to the advent of the \textit{Plan Rh\^{o}ne} (heatwave of
2003, smaller-scale floods, historical or accidental pollution, etc.). 

\smallskip\noindent
(2) \textit{Implementing a sustainable development policy.} 
Presented as a sustainable development policy, the \textit{Plan
Rh\^{o}ne} was intended to renew actions related to the river, with a
focus on transversality and shared governance. Has public action
brought about changes in the way the river is managed, particularly in
terms of environmental policies, economic development and social
relations between stakeholders? What points of convergence or
divergence can be detected in these interactions? 

\smallskip\noindent
(3) \textit{Socio-ecosystemic functioning.}
The aim of this theme is to structure a collective, systemic and
integrated approach around a shared, evolving and interdisciplinary
conceptual scheme describing or \mbox{representing} the way in which the
socio-ecosystem functions. In a sense, this is the core of the
reflexive approach, which integrates ecological dimensions by
considering the various compartments: fauna and flora, aquatic and
riparian, hydromorphological, chemical and ecotoxicological, and
socio-economic. The aim is also to produce methodological frameworks
and, potentially, indicators (physical, biological and socio-economic)
to diagnose the state of the river corridor, its natural or constructed
alluvial margins and the services provided by the river. 

\smallskip\noindent
(4) \textit{Environmental risks.}
Risk management is a major issue for the OHM VR. These include
flooding, the development of invasive species, climate change and
pollution. The aim is to quantify these risks by measuring their
potential impact on the socio-ecosystem, and also to examine how they
are perceived and represented by local residents, and the solutions
envisaged by the various stakeholders.

\smallskip\noindent
(5) \textit{Restoration and renaturation.}
At the time this theme was defined, restoration objectives for the
Rh\^{o}ne were evolving with the implementation of the river bed
re-widening scheme (see Section~\ref{sec4}), which led to a
reconsideration of the restoration actions launched previously. These
restoration actions no longer focused solely on increasing minimum
flows or rehabilitating \textit{l\^{o}nes}, but also on removing bank
protection structures, re-widening the riverbed and revitalizing
sediment transport as a result of lateral erosion or artificial gravel
supply. 

\smallskip\noindent
(6) \textit{New tools for assessing human-environment interactions.}
The aim of this sixth theme is to promote research on innovative tools
for characterizing, assessing and predicting socio-ecosystem
trajectories, as well as for compiling and sharing information with
various stakeholders, thus enriching the observatory's research data
infrastructure (Figure~\ref{fig3}).

\smallskip

The six themes defined here cover the full range of issues involved in
the new management of the river. This structure of six themes has
persisted until today, however, their content has changed over time, in
particular following the re-editions of the \textit{Plan Rh\^{o}ne}:
whereas the first edition of the plan (2007--2013) emphasized the issues
of ``global project'' and ``sustainable development'', the second edition
(2015--2020)\footnote{\url{https://www.plan-rhone.fr/fileadmin/medias/Publications/General/CPIER\_2015\_2020\_SIGNE\_VDEF\_BD.pdf}.} 
rather highlighted the notion of ``integrated management'', notably
through a new axis entitled ``Multifunctional management of the waters
and environments of the Rh\^{o}ne and Sa\^{o}ne rivers''. The notion of
``transition'' (ecological, energy) also made its appearance in the
second \textit{Plan Rh\^{o}ne}. The third current plan, now called the
\textit{Plan Rh\^{o}ne-Sa\^{o}ne}
(2021--2027)\footnote{\url{https://www.plan-rhone.fr/fileadmin/medias/Publications/General/CPIER\_Sept23\_enPages.pdf}.}, 
confirms this evolution by displaying a ``new ambition'': ``the
ecological and climatic transition of activities and territories''.
These semantic shifts thus led the OHM VR community to revise its
research themes in 2020 (Figure~\ref{fig6}) in order to better follow
and question the changing concerns of river\break managers.

From a scientific point of view, the creation of the OHM VR has enabled
the emergence and refinement of certain research questions specific to
the observatory: how to go beyond the segmented legacies of previous
management and envisage a ``human-environment'' co-evolution based on
the knowledge and indicators defined in the OHM VR? How does the
socio-ecosystem function and react to new management practices and
certain environmental conditions? How do public and private action
support these changes? What about the relationship with the river and
the expectations of local\break residents? 

\subsection{Thematic analysis of the OHM VR research projects}
\label{ssec32}

Figure~\ref{fig7} shows the research themes addressed in the projects
selected during the first period of \mbox{operation} of the OHM VR (between
2010 and 2016, the year of its first scientific evaluation), and during
the {second} period, from 2017 to 2024. This corresponds respectively to
36 and 44 projects funded during each period, including 13 inter-OHM
projects. The number of projects increased overall for all research
themes, with the exception of the most widely covered theme on the
socio-ecosystemic functioning, which decreased slightly during the
second period. Projects on environmental risks and geo-historical
trajectory occupy second place in terms of funded projects. Depending
on the project, the risk theme is addressed from the angle of flooding,
climate change, water resources, water or sediment pollution, and
ecotoxicological risks, with some projects also considering risks to
human health. Projects on integrated river management (theme~2), with a
focus more specifically on the shift in management paradigm at the core
of the OHM VR, and concerning in priority the field of SHS, were
strengthened during the second period.

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig07}
\vspace*{-2.5pt}
\caption{\label{fig7}Number of research projects funded by the OHM VR,
by theme and by period (a project can be part of several themes).}
\end{figure*}

An analysis of specificities conducted on the project titles confirms
this thematic division and its evolution. Figure~\ref{fig8} highlights
the importance of projects funded in the first period in the
\textit{casiers of \mbox{Girardon}}, and also the socio-ecosystemic
functioning and the risks through climatic processes. The second period
exhibited more research on the geo-historical trajectory (``19{th}
century'', ``Little Ice Age''), as well as on restoration and
management issues.

\begin{figure*}
\includegraphics{fig08}
\vspace*{-2.5pt}
\caption{\label{fig8}Analysis of specificities from the titles of 81
projects funded by the OHM VR: (A) between 2010 and 2016; (B) between
2017 and 2024.}
\vspace*{-3pt}
\end{figure*}

\subsection{The success of the interdisciplinary dialogue}
\label{ssec33}

Over the 15 years of operation of the OHM VR, the consolidation of
finalised research questions, more related to the river stakeholders,
was also highlighted by an analysis of the publications on the
Rh\^{o}ne River carried out by \citet{Poirieretal2023}. This involved a
content analysis of 97 peer-reviewed scientific articles (71 were
written in English and 26 in French) written by ZABR and OHM VR
researchers working on the river and published between 2006 and 2016. 
The  authors show that the appearance of articles related to certain
societal concerns is concomitant to the advent of the OHM VR in 2010,
suggesting that the OHM VR acted as a catalyser to consolidate and
develop this type of research. Fairly symmetrically with the research
themes most investigated by the OHM VR (Figure~\ref{fig7}), the
articles studied by \citetalias{Poirieretal2023} are mostly divided between
physico-chemical aspects (53\% of articles), biological aspects (32\%)
and societal aspects (11\%). On the other hand, \citetalias{Poirieretal2023}
show that the development of research works focused on societal issues
was also supported by the ZABR and the co-construction of projects with
the \textit{Agence de l'Eau Rh\^{o}ne-M\'{e}diterran\'{e}e-Corse}
(e.g., the RhonAVel'Eau project). 

In order to encourage interdisciplinarity, the OHM VR has from its
creation requested that projects submitted to the APR be
interdisciplinary, while {remaining} open to disciplinary proposals with
the aim of encouraging the emergence of unexplored research fields.
Thus, all projects funded by the OHM VR between 2010 and 2024 involve
at least two \mbox{disciplines,} within the same domain (e.g., geography and
sociology within the SHS) or between distinct domains (e.g., chemistry
and history) (Figure~\ref{fig9}). In this context, we can reasonably
hypothesize that interdisciplinary projects combining SHS, Engineering
Sciences and Natural Sciences emerge after a few years of operation of
the OHM device, the latter playing the role of catalyser as
conceptualised by \citet{Chenorkian2020}.
                                
\begin{figure}
{\vspace*{4pt}}
\includegraphics{fig09}
{\vspace*{3pt}}
\caption{\label{fig9}Disciplines involved in projects funded by the OHM
VR.}
\end{figure}

Another fundamental aspect of the OHM device is exchanges with local
stakeholders. This is particularly well-developed on the Rh\^{o}ne
River, due to the history of scientific work and its close links with
operational actors. According to \citet{Tressetal2005}, the research
carried out in the OHM VR is part of partnership between researchers
and non-academic actors in the co-production of knowledge. This is
illustrated by the Rh\^{o}nEco and OSR programs, which for over fifteen
years have been co-constructed by researchers from different
disciplines, CNR engineers, the \textit{Agence de l'Eau
Rh\^{o}ne-M\'{e}diterran\'{e}e-Corse} and other stakeholders to produce
knowledge to meet the challenges of integrated river management, thus
constituting unique programs of their kind on an international scale.
These collaborations are also visible in the projects funded by the OHM
VR, which in recent years has accounted for several projects involving
both scientists and river managers. 

In the following section, we have chosen to develop two case studies
that are representative of the implementation of interdisciplinarity on
the Rh\^{o}ne River, the first involving dialogues between researchers
from different disciplines, and the second involving exchanges between
researchers and river managers. 

\vspace*{-2pt}

\section{Two interdisciplinary case studies: the analysis of the PCB
crisis and the dismantling of the Rh\^{o}ne's constructed
margins}\label{sec4}

\vspace*{-2pt}

\subsection{Examine pollutant crises over the long term: from PCBs to
PFAS}\label{ssec41}

\vspace*{-1pt}

Most of the research carried out on the theme of pollution in the OHM
VR has been done on an interdisciplinary basis, involving the
environmental sciences in the broadest sense of the term. This research
associates disciplines from the life and earth sciences (sedimentology
and geochemistry, for example), or results from a dialogue with the SHS
(sociology, social geography). Publications, in particular that of
\citet{GramagliaBabut2014} on PCB pollution of the Rh\^{o}ne, have
contributed to highlight the close and complex links that exist between
the often toxic chemical substances resulting from our modes of
industrial production and consumption patterns, and the social,
political and economic frameworks that consider them differently
according to the periods and social groups concerned.

Annual scientific seminars are an important part of the OHM device,
providing an opportunity for researchers to exchange ideas. It was at
the OHM VR annual scientific seminar in 2020 that the issue of PCB
pollution in the Rh\^{o}ne was debated again, some ten years after the
crisis of 2008. While surveys of users and local residents show the
permanence, since the first PCB alert in the 1980s, of concerns about
the presence of these pollutants in the sediments and fish of the
Rh\^{o}ne \citepalias{GramagliaBabut2014}, measurements carried out as part
of the OSR program converge, on the contrary, towards a decrease in
their concentration since the 2000s \citep{Mourieretal2025}. This
exchange between experts in sociology, sedimentology and geochemistry
highlighted the discrepancies between certain apprehensions of
non-scientists and scientific measurements of pollution in the
environment. Among riverside residents of the Rh\^{o}ne, the issue of
pollution, and in {particular} PCBs, occupies a special place in social
\mbox{representations.} It is a \mbox{stabilized} representation, forming part of
what Jean-Claude Abric calls ``the hard core'' of representations
(\citeyear{Abric1987}), stemming from a long history of pollution on the Rh\^{o}ne,
exacerbated by the period of rediscovery and management of the PCB
crisis in the 2000s. Indeed, the accidental rediscovery of the presence
of this pollutant in two fish upstream of the Lyon conurbation
generated, from 2005 onwards, a large-scale process of publicization,
controversy and regulation which went beyond the Rh\^{o}ne basin, since
it notably led the French State to set up a national PCB action plan in
2008 \citep{Combyetal2014}. Thus, as is often the case with pollution
issues, knowledge needs far exceed monitoring capacities, and the
managers invested in this type of crisis are often different from those
managing the \textit{Plan Rh\^{o}ne}.

Ongoing concerns about PCBs can still be observed today, for example
during the consultations organized between December 2023 and February
2024 by the \textit{Commission Nationale du D\'{e}bat Public},
concerning a new hydroelectric dam project upstream of
Lyon\footnote{Field notes, Carole Barth\'{e}l\'{e}my.}, which
has since been abandoned. Local residents and users expressed their
concerns about the presence of PCBs in old sediments in the area
affected by the dam, which is geographically close to the plant
responsible for PCB production, and their possible release into the
environment. However, since 1997, concentrations have been in decline
throughout the river continuum, including upstream of Lyon, and are
even below regulatory thresholds (norms established for benthic
invertebrates) \citep{Mourieretal2014,Dendieveletal2020}.

The discussions that took place during the OHM VR seminar in 2020
continued and led an interdisciplinary group of researchers to write a
chapter in the collective work ``\textit{Contaminations, contaminants,
contaminer}'' (Contamination, contaminants, contaminate), coordinated
by the LabEx DRIIHM \citep{Mourieretal2025}. The cross-disciplinary
approach between SHS, sedimentology and geochemistry firstly
highlighted the need to work on a long time scale in order to compare
environmental and socio-political trajectories. Indeed, the detection
of PCBs in sediment cores makes it possible to track their presence in
the environment over \mbox{several} decades. However, this temporality does
not correspond to their socio-political acceptance: the toxicity of
PCBs on the environment and human health was debated for ``only''
fifteen years or so (out of a period of 80 years since they were first
marketed in 1930), in the 1980s, 1990s and, above all, between 2005 and
2011. This result underlines the time lag between the
``materialization'' of pollution (measurements and monitoring) and
society's assumption of responsibility for this materiality when it
becomes problematic in terms of public health and environmental
protection, and breaks away from the simple commercial use of the
substance\break in question. 

For \citet{GramagliaBabut2014}, even if the PCB crisis of the 2000s had
moved some lines in the public handling of pollution, there were still
many administrative, scientific and political obstacles to overcome
before we could imagine other ways of managing current and future
industrial pollution, other than in a short period of
socio-environmental crisis. History may well be proving them right,
with the advent of a new crisis around the Rh\^{o}ne, regarding PFAS.
These ``eternal pollutants'' include over 4000 chemical compounds with
anti-adhesive, waterproofing and heat-resistant properties, used in
many industrial fields and in everyday consumer products
\citep{Pauletal2009}. In May 2022, a \textit{France
T\'{e}l\'{e}visions} documentary revealed the abnormal presence of
these pollutants in the vicinity of a PFAS-producing industrial complex
in Pierre-B\'{e}nite, with measurements carried out in the air, soil,
drinking water and breast milk of local residents. Since then, this
media scandal has generated a movement of mobilization on the part of
local residents and nature protection associations, as well as State
services and local authorities \citep{LeNaourThomas2022}.  It has also
brought to light scientific work in ecotoxicology and geochemistry
carried out by Rh\^{o}ne researchers in the 2010s, which already showed
major contamination of all compartments of the fluvial hydrosystem over
several tens of kilometres downstream of the industrial complex
\citep{Babutetal2016,Mourieretal2019}, but which had not been followed
by any action on the part of the public authorities.

In this context, the interdisciplinary research carried out at the OHM
VR and its and its openness to new research fronts may enable the
observatory to take on a ``whistle-blower'' role, or at least to reveal
the mechanisms of socio-environmental crises that risk being repeated
in time and space \citep{LeNaourThomas2023,MourierArnaud2023}.

Last, the topic of pollution has led to joint projects with other OHM:
two in the south of France: Bassin Minier de Provence and Littoral
M\'{e}diterran\'{e}en, and an International OHM, Estarreja (Portugal).
Six inter-OHM projects focused on two perspectives. The first was to
compare measures of pollution in different industrial pollution
contexts between the Rh\^{o}ne and Estarreja sectors\footnote{Project
\og{}~\'{E}valuation des risques \'{e}cotoxicologiques et pour la
sant\'{e} humaine de contaminants organiques dans la zone d'Estarreja
et dans des friches industrielles de la vall\'{e}e du Rh\^{o}ne~\fg{},
JP Bedell, 2016.}. The second analytical perspective involved
questioning lay people's relationships to pollution, by bringing
together scientific experts and activists from associations committed
to gaining recognition for pollution and the associated
risks\footnote{Project  \og{}~Pollutions, mobilisations
environnementales et territoires industrialis\'{e}s : les cas de Fos
sur Mer, de Gardanne et du littoral sud marseillais~\fg{}, J. Rouchier,
I. Laffont-Schwob, 2019.}. These projects between OHMs enable
comparisons to be extended, for a better understanding of
pollution-related ``crises''.

\vspace*{-2pt}
\subsection{When the OHM VR responds to public action on flood
management}\label{ssec42}
\vspace*{-2pt}

Built between 1884 and 1938, the so-called \textit{casiers Girardon}
(named after the engineer who supervised the works) aimed to improve
the navigability of the Rh\^{o}ne River between Lyon and the
Mediterranean Sea. Their main function was to stabilize a navigation
channel both laterally and vertically, by promoting the movement of
sediments that ``clogged up'' the channel and their deposition within
\textit{casiers} constructed along the riverbanks (Figure~\ref{fig10}).

\begin{figure*}
\includegraphics{fig10}
\caption{\label{fig10}(A) Map of the \textit{Passage des Pillettes} at
Tain-l'Hermitage before and after the Girardon works, presented at the
\textit{Exposition Universelle} of 1889. Source: Rh\^{o}ne Department
Archives \citep{Souquet2022}. (B)~Aerial view from July 1920 showing
the Girardon aquatic \textit{casiers} on the left bank of the Rh\^{o}ne
at Vienne. Source: IGN, \textit{Remonter le temps}.} 
{\vspace*{-4pt}}
\end{figure*}

The Girardon works fulfilled their function, as over time they led to
channel incision, a reduction in channel width, and the infilling of
aquatic \textit{casiers} with coarse and fine sediments 
\citep{Tenaetal2020,Vauclinetal2020,Seignemartinetal2023}. These
sediments, particularly after the reduction of flows downstream of
diversion dams, provided fertile surfaces that were rapidly colonised
by the alluvial forest \citep{Janssenetal2021}. These impacts, which
resulted in an increased \mbox{disconnection} between the river and its
floodplain, had been demonstrated by researchers, especially
geomorphologists, since the 1990s. The Pierre-B\'{e}nite reach, located
south of Lyon, is a site particularly affected by the
hydromorphological impacts of the \mbox{Girardon} and damming works. It serves
as a pioneering site for ecological restoration, with actions initiated
by the CNR as early as 1999--2000, including a tenfold increase in the
minimum flow downstream of the dam and the rehabilitation of
disconnected \textit{l\^{o}nes} \citep{CollectifRhonEco2016}.

This body of knowledge was to be solicited following the catastrophic
floods that affected the Lower Rh\^{o}ne from December 1 to 5, 2003.
The tributaries, impacted by heavy rainfall at that time of year,
contributed to a flow of 11~500 m$^{3}$/s measured at Beaucaire (a
100-year return period). The damage in the affected areas was estimated
at one billion euros. The implementation of a global flood prevention
strategy was then partly based on consultations jointly organized by
the State authorities and the three Regions bordering the Rh\^{o}ne
between 2004 and 2006, to respond to the outcry and the anger of local
residents \citep{BarthelemySouchon2009,Comby2015,Guerrin2014}. These
consultations gathered several hundred people. Representatives of
associations formed in reaction to the floods, as well as individuals,
initially expressed themselves with strong emotion. A common theme in
their remarks was criticism of the CNR for not having sufficiently
``dredged'' the river and for allowing it to ``fill up''
\citep{BarthelemySouchon2009}.

These two terms refer to an argument that was once widely shared,
linking the sedimentation process in the riverbed to the acceleration
and amplification of flooding. This argument was based on empirical
knowledge held by local elected representatives and residents, stemming
from their observations, such as a damaged bridge pile, a
\textit{l\^{o}ne} that had filled up, or a transformation in the course
of the river observed during previous floods. Local memory was thus
mobilized, generating criticism of the French State, via the CNR, for
allegedly neglecting the management of the Rh\^{o}ne and its
tributaries, a feeling intensified by the occurrence of the flood.
Observations made during the consultations showed that the anger was
gradually \mbox{attenuated} when representatives from the State services and
the CNR \mbox{provided} explanations. The regulatory framework was clarified,
including the prohibition of gravel extraction from the riverbed since
the 1992 Water Act, as well as the administrative {responsibilities} of
the CNR, which is tasked with ensuring that its facilities do not
``worsen the waterline''. Scientists, particularly geomorphologists, who
have been working for several years in the field of river restoration,
were also solicited and they spoke publicly. They explained why
dredging coarse sediments was unnecessary, as small or medium-scale
floods could take could do it \citep{ProvansalBravard2009}. However,
they debated the issue of fine sediments, which are present on the
constructed margins inherited from the Girardon works and which could
indeed be removed \citep{Bravardetal2005}. This argument, expressed by
a scientist, gained consensus as it partly addressed the
representations and expectations of local residents, who wished to
``dredge'' the river \citep{Thoreletal2018}. The CNR supported the
proposal to ``reactivate'' the alluvial margins, which fell within its
mission of global river management. It can also be assumed that
proposing a concrete action, which seemed achievable within a
reasonable timeframe, acted as a common value in the post-flood
Rh\^{o}ne context \citep{Guerrin2014}. For the OHM VR, these discussions
helped identify the need for fundamental knowledge and the necessity of
defining indicators, which led to the production of numerous doctoral
theses and research projects on this topic. These research efforts thus
allowed for questioning and refining what initially seemed self-evident
during the flooding crisis.

Proposed by the scientists, a master plan for reactivating the fluvial
dynamics of the Rh\^{o}ne's margins was developed between 2009 and
2013. The aim was to improve knowledge of the impacts of the Girardon
engineering works through the study of changes in fluvial dynamics,
land use, topography, sediment dynamics, flooding, and the landscape
over the past 150~years. The document identifies elements of
vulnerability, ecological and patrimonial issues, and initiates
reflections on the hydraulic feasibility of sediment remobilization
through the widening of the Rh\^{o}ne's margins. This would involve, in
by-passed sections, mobilize sediments stored on the margins to restore
fluvial dynamics and increase the flow conveyance of the former river
channel \citep{GaydouBravard2013}.

However, before dismantling structures dating from the 19th century,
the scientific community aimed to assess their potential effects in
terms of sediment transport, as well as the impacts on existing
biodiversity. In 2005, the (re)discovery of the Rh\^{o}ne pollution by
PCBs added a degree of complexity to the project, as the potential
remobilization of contaminated sediments stored on alluvial margins
posed environmental and health risks. The studies revealed that
pollutant stocks were generally higher in the sediments of secondary
channels and in certain floodplain compartments compared to
pre-engineering sediments. This can be attributed to a matrix effect
linked to their finer grain size and the industrial era during which
they were deposited \citep{Vauclinetal2021}. Indeed, the most
concentrated pollutant stocks were found in sedimentation zones formed
after the construction of dams (post-1970), along the river continuum
downstream of Lyon to the confluence with the Is\`{e}re. Although the
quantified PCB stocks in certain areas are relatively modest compared
to the industrial quantities rejected in the environment and
transported by the river, they nevertheless represent a complex legacy
to consider. Within the OHM VR, the funding of several doctoral theses
\citep{Rapple2018,Seignemartin2020}, a postdoctoral project
\citep{Thoreletal2018}, and around fifteen research projects have
provided interdisciplinary insights into the functioning of constructed
alluvial margins. The first major finding was the identification that
the majority of the \textit{casiers of Girardon} were terrestrialized
\citepalias{Thoreletal2018}: 90\% of the Girardon groyne fields exhibit
aging and homogeneous riparian forest. The remaining 10\% of structures
that have maintained their aquatic features constitute \textit{a
contrario} ``novel ecosystems''. These functional ecosystems can serve
as feeding sites for fish, carbon or nitrogen sinks (denitrification),
and contribute to the diversity of riverine habitats along with their
associated fauna and flora. The second major finding, published in 2018
(\citeauthor{Thoreletal2018}), does not advocate for a single
management approach for the Rh\^{o}ne's constructed margins. Instead,
it emphasizes the integration of results from several disciplines:
hydrology, geomorphology, ecology, chemistry, sociology, and geography.
This interdisciplinary synthesis led to a conceptual model designed to
support decision-making based on the full set of data acquired. As a
result, \citetalias{Thoreletal2018} proposed four alternative trajectories
for restoring the Rh\^{o}ne's river margins: (i) dismantling
terrestrialized \textit{casiers} with large volumes of potentially
mobile bedload gravel and low levels of sediment pollution; (ii)
dismantling terrestrialized \textit{casiers} in cases with fewer
benefits (mobile gravel) and higher risks (polluted sediment), but
which would enhance riparian habitat diversity and decrease flood risks
from the point of view of residents; (iii) reconnecting isolated
aquatic \textit{casiers}; and (iv) preserving aquatic \textit{casiers}
with suitable water quality and hydraulic connectivity to foster a more
heterogeneous habitat mosaic.

It is noteworthy that flood risk was primarily considered in the public
debate through the lens of perceptions held by residents and local
elected representatives. The measured hydraulic effects of the Girardon
works on water levels and peak flows, as well as the expected response
of the restored channel, were not investigated by
\citet{GaydouBravard2013}, as no explicit feedback on this aspect had
yet been identified \citep{Thoreletal2018}. This highlights that an
integrated management approach for a socio-ecosystem must emerge from a
consensus balancing biophysical processes and with societal\break
expectations.

The master plan for reactivating the fluvial dynamics of the
Rh\^{o}ne's margins was integrated into the SDAGE \textit{Rh\^{o}ne
M\'{e}diterran\'{e}e Corse}. Dismantling work was first tested in
2016--2017 at the P\'{e}age-de-Roussillon Nature Reserve. In 2021, the
first operational implementation was carried out on the Baix-Logis-Neuf
reach, consisting of the removal of 880 meters of dikes and seven
\textit{casier Girardon}, the rehabilitation of a \textit{l\^{o}ne},
and the artificial supply of 12~000~m$^{3}$ of gravel into the
Rh\^{o}ne channel. Several reaches are currently in the planning stages
(Pierre-B\'{e}nite, Saint-Vallier, Donz\`{e}re). These works are
accompanied by monitoring efforts co-designed and coordinated between
the CNR and research teams from OSR and Rh\^{o}nEco programs.

The OHM VR, by facilitating dialogue across multiple disciplines, has
contributed to producing not only knowledge but also a research
approach that takes into account the inherent complexity of the
Rh\^{o}ne River functioning. The management of the Rh\^{o}ne's
constructed margins serves as a prime example of complex science for
anthropo-ecosystems that can only be studied and managed from this
perspective. This type of integrated approach is a key element of the
interdisciplinarity practice demonstrated within the OHM. Through this
example, we can also observe how and why research themes emerge and
develop in the OHM, and how, depending on their degree of maturity,
they are addressed in other research platforms of the Rh\^{o}ne
scientific ecosystem, and/or integrated into new river management
strategies, translated and adopted by managers. These research efforts
have notably led to a revision of the Rh\^{o}ne restoration policy,
initially proposed in the 1990s, by emphasizing a restoration approach
more based on river processes.

\section{Conclusion and perspectives: from one disruption to another
{\ldots}}\label{sec5}

The Rh\^{o}ne is the paradigmatic example of an anthropized river,
whose physical, ecological, and \mbox{social} trajectories have been
conditioned since the 19th century by human use, resource exploitation,
and the perceptions held about the river. The Rh\^{o}ne has been marked
by successive ruptures and crises that question not only the modes of
river management but also the various impacts it has been subjected to,
which in turn affect the sustainability of this socio-ecosystem. The
disrupting event of the OHM VR raises questions about the
socio-environmental significance of a management approach that
prioritizes sustainability and the integration of key issues. Twenty
years have passed since the catastrophic floods of 2003, revealing
several conclusive elements. 

A first key element relates to the capacity of the OHM VR to
consistently address pollution issues and socio-ecological dynamics
across multiple disciplinary fields. The research notably revealed a
temporal gap between the production and use of industrial substances
and the concentrations measured in the environment, alongside the
socio-political crises that emerge. These findings highlight the
complexity of risk interpretation by local populations and the
necessity of taking control of these crises, beyond action plans, by
the relevant State services. For both PCBs and PFAS, the persistence of
long-chains compounds in sediments presents an additional challenge:
their accumulation poses long-term risks and could undermine certain
ecological restoration efforts that might remobilize these old stocks.
This situation also puts pressure on drinking water production systems,
confronted with the challenge of eliminating these persistent
substances from water extracted from the Rh\^{o}ne's alluvial aquifers.
Looking ahead, the work of the OHM VR opens the possibility of
re-examining the conceptual framework of OHMs in order to better
integrate the \mbox{recurrent} \mbox{pollution-related} crises and the
stakeholders involved in their management.

A second key element of the OHM VR is related to the proposal and
support for a new approach to ecological restoration that promotes
fluvial dynamics based on processes without increasing flood risk. This
evolution benefited from interdisciplinary research, intended to
demonstrate the complex interdependencies characteristic of this type
of project, to better assess induced risks, morphological adjustments
and potential ecological responses of the restored river, while leaving
space for managers to engage with these findings. The hydraulic and
ecological restoration issues addressed in the specific Rh\^{o}nEco and
OSR research programs have benefitted from the interdisciplinary
environment of the OHM VR, particularly through insights from the
social sciences. The participation of local residents and the
uncertainties that may accompany restoration projects have become a
more recent focus in the OHM VR \citep{Cottet2022}. Research and exchanges
between disciplines find support in the call for projects of the LabEx
DRIIHM, which offers smaller-scale projects that make it easier to take
risks and explore new pioneering research fronts. 

Approaches in the humanities and social sciences conducted in the OHM
VR have provided an analytical perspective on the implementation of the
\textit{Plan Rh\^{o}ne}, particularly in terms of its shared governance
objectives. The long-term period of observation reveals some
achievements in the inter-region and inter-services modes of
functioning, such as the effective technical and social management of
flood risks, particularly in the Lower Rh\^{o}ne, and restoration
efforts along the entire river. However, monitoring also highlights a
political exhaustion, leading to a technical and administrative
routinization of the \textit{Plan Rh\^{o}ne-Sa\^{o}ne}
\citep{BarthelemyComby2019}. 

Will the experience acquired from integrated management be able to
address what are now the major challenges shaping the future of the
Rh\^{o}ne River? The first challenge concerns a government decision in
favour of reindustrialization and energy independency, within the
context of meeting European greenhouse gas emission reduction targets.
Therefore, the Rh\^{o}ne upstream of Lyon has been targeted by two
major energy production projects, something that had not happened for
forty years: a hydroelectric dam upstream of the confluence with the
Ain (subsequently abandoned), and two new reactors at the Bugey nuclear
site. How might these projects, announced in the name of the energy
transition, impact the existing socio-environmental dynamics? 

The second challenge, which the OHM VR is already working on, concerns
the effects of climate change. While projections and models are
\mbox{beginning} to clearly outline its potential impacts
\citep{Explore22024}, what about the response of ecosystems, the
sharing of water resource with cross-border issues, transformations in
agricultural needs (ongoing \mbox{discussions} about extending the Aqua
Domitia \mbox{infrastructure,} water intake towards Occitania to supply the
\textit{Pyr\'{e}n\'{e}es Orientales} \citep{Barthelemyetal2024}), and
the growing desire to develop cooling spaces or even swimming areas in
the cities [ARCO ANR project, 2024--2027]? The sustainable management
of the Rh\^{o}ne River is therefore facing considerable challenges, and
it will be essential to continue monitoring and supporting it within
the OHM VR in this context of transition.

\section*{Acknowledgements}

This research was funded by the LabEx DRIIHM, the French program
\textit{Investissements d'Avenir} (ANR-11-LABX-0010) managed by the
French National Research Agency (ANR). The authors work within the
LTSER \textit{Zone Atelier Bassin du Rh\^{o}ne} and the EUR H2O'Lyon
(ANR-17-EURE-0018). We thank Lise Vaudor for creating the statistical
figures.

\CDRGrant[ANR]{ANR-11-LABX-0010}
\CDRGrant[ANR]{ANR-17-EURE-0018}

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

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