Contenu du sommaire : La rivière, un corridor naturel à gérer

The S.D.A.G.E. (Planning and Management Master Plans for Water Ressources) were instituted by the French Water Law of January 1992. This Law is designed to apply concepts resulting from interdisciplinary studies bringing together academics and managers. Examples of such an approach include the balanced management of water resources and the need to conserve the complexity of hydrosystems to ensure suxtainable development. The S.D.A.G.E. are legal documents which set-out a coherent, planning strategy for water resources and aquatic environments over the next 10- 15 years. They fix the basic policy objectives and the means by which to attain them. The Rhône-Mediteranean-Corsica region is used as a case study to show the native of an S.D.A.G.E. with respect to the management of rivers and their related environments.

During the 19th and 20th centuries, engineering works have modified the functioning of the river Rhône giving priority to navigation, hydroelectric power production, irrigation, urban and industrial wastewater collection. Today, the river's management needs to take account of society's demand for the development of environmental quality standards and the protection of natural ecosystems. As a result, an action plan has been implemented to restore aquatic communities through improvements to the river, especially in by-pass river sections, aquatic floodplain habitats and floodplain forests, and through the restoration of migration routes for long-distance migrating fish. Examples are given of operations which are underway : methodological developments, pilot operations to restore by-pass sections and the restoration of migration routes and reproduction areas for the migrating fish (Alosa fallax, twaite shad). Study of the technical aspects is completed by emphasizing the role of local partners in the realisation of projects.

The French Water Law has established tools for the planning and management of water resources with the aim of creating a long-term balance between satisfying different demands and the preservation of natural environments. These tools are innovative in seeking to take account, in the planning process, of the practices and values of different actors. A description of how ecosystems work and of different uses made of rivers, as well as the values that the actors attribute to them, needs to be integrated into a common conceptual framework. This article outlines an approach based on the construction of territories by giving a spatial dimension to ecosystems, river uses and the resources of different actors ; it also shows the value of G.I.S. in this approach.

Classification of river morphology is but one of the many components to be accounted for in establishing a global, long-term approach to river management. In the proposed method, homogeneous regions are delineated on the basis of factors that determine drainage structure : longitudinal profiles and valley side slopes, channel forms and valley characteristics, drainage density and stream size. These criteria have been hierarchically classified to enable changes of scale from the individual river segment to the drainage basin. The regionali- zation process, based primarily on the interpretation of thematic and topographic maps, is verified through field work and statistical analysis. The method is presented in the light of results obtained on the Loire basin, and is currently being applied to the Rhône basin.

The global functioning of a stream must be known if its management is to be improved. However, it's often impossible to undertake a detailed study of the whole stream and its watershed due to contraints of time and cost. The use of functional typology enables the identification of different reaches, each with its one functional characteristics, and the selection of representative sites for more detailed study. This approach involves a synthesis of standard geomorphological and hydrobiological data. Large reaches and the variables for their identification are shown by a descriptive and objective analysis. These variables are slope, bankfull width and watershed geology. This result proves that "specific strength" calculation is an appropriate tool for longitudinal classification. The functional typology allows management methods to be adapted to each reach : an example of this method is provided by the Vidourle, a mediterranean coastal river in Languedoc.

Since the 17th century, engineers have modified the dispersion patterns of flows (water, energy, matter, nutrient) in order to protect croplands and improve yields. Engineering- works controlled the flows and consequently disturbed the habitats. However, they were also developed in order to protect and improve soils and crops. Hence, the methods used a century ago are sometimes similar to the soft bioengineering techniques which are presently developed to restore and manage rivers. Various examples of old techniques aiming at controlling river flows (e.g. drainage, dredging, warping, irrigation, fertilization) are described. Through the establishment of habitats favourable to new plant communities, these anthropogenic disturbances have created a heterogenous landscape. A sustainable management of this interesting and fragmented ecocomplex must be developed. It is shown that numerous techniques used two centuries ago might be improved to restore floodplains and develop buffer-zones.

Integrated management of rivers and their margins is recommended by the 1992 French Water Law and is favoured by public opinion. Management options are no longer based only on hydraulic or ecological considerations, but also on concepts developed by the human sciences (economics, sociology, human geography). A good illustration is provided by global studies of conservation strategies for the eroded floodplains of high energy tributaries of the Rhône. Social surveys are considered as a tool to assist mediation between river users and managers. They allow identification of the different groups of actors, of their perception of rivers and of potential conflicts in the use of rivers. Surveys are also a usefull way of registering actor's opinions of future management strategies ; this may be illustrated by conservation strategies applied to several rivers in south-eastern France. Finally, social surveys can be considered a communication tool allowing managers to inform actors of their objectives.

Since the 18th century, the talwegs of the tributaries of the middle Rhône have been modified substantially due to increased population pressure. Entrenchment observed on rivers in the southeast of France is explained by watershed afforestation, landuse changes, embanking works and gravel extraction. This situation is largely the result of the combined effects of development and management policies. The 1992 Water Law instituted an integrated approach to river management ensuring that different uses were compatible with maintaining a natural balance. However, this has not always been the case and certain actors still favour the previous pattern of development. Despite this, new management methods should allow the restoration of long profiles and of physical environment in general.

Over recent years, the governmental "Loire Grandeur Nature" Plan and the European "Life Loire Nature" Program helped renew interest in the Loire River. Due to complex fluvial dynamics and to the diversity of human actions, it is difficult to determine the river's optimum streamway. Various experiments linked to the European Life-Loire-Nature program have been carried out, two examples of which are presented in this paper. Concerning the river Allier, the rapid action of the conservationists' associations had been triggered out by the lateral instability of the channel and by the increase in engineering works. Downstream, on the Loire River, the protection of the fluvial system is based on the requirement to protect the flood- plain. The equilibrium of the fluvial system is based on the preservation of the upstream channel.