Contenu du sommaire : L'interface nature-sociétés dans les hydrosystèmes fluviaux

The canalisation of the lower Durance by dykes, which began as early as the XIIIth century, has been reinforced by hydro-electric works undertaken in the 1960s, which completely changed the river hydrosystem. From a braided river in a broad bed up to 900 m in width, the Durance has been transformed into a single channel river with a narrow, stable and almost linear bed. The river's spatial organization, which was integrated into local society, has become more longitudinal and compartmentalized in character, leading to an erosion of its social and ecological cohesion.

This article looks at the changing status of a 'natural' river space - the island of Miribel-Jonage on the outskirts of Lyon. The area was created in the XIXth century as part of the process of harnessing rivers but in the 1960s its development was characterised by urban inspired models. As such it became successively a recreational area then a zone of touristic and technological development. However, in 1992, it rediscovers its 'natural' status when classified as an 'immutable zone'. At the same time, compared with the past, this area became the object of a far greater number of human, technical and social interventions designed to restore a natural equilibrium and to create a public space for the city. By seeking to incorporate the most favorable aspects of previous models, this new development concept seems to offer a greater degree of continuity rather than change.

Between Epinal and Nancy, the Moselle Valley offers 60 km of landscapes that often feature strings of artificial ponds resulting from the exploitation of alluvial deposits. However, some areas seem to have been preserved from the quarry owners' activity and present an agricultural and forestry aspect. In an attempt to explain this dichotomy in the landscape, the author will review the brief history of the quarries in the valley, as well as the problems raised by such activity and the solutions provided. This work aims at shedding light on the complex relations that exist between the river hydrosystem and some human groups whose interests, although sometimes convergent, are most of the time contradictory (quarry owners, communes, environmental protection groups. Water Agency etc.)

The Moselle river is the most morphologically active among the rivers of northeastern France. Several studies of the fluvial bed during the past twenty years have shown the strength of development processes when the river develops a strong morphological activity when it leaves the Vosges mountains and flows across the Lorraine plateau. Wherever the floodplain has been less affected by gravel pits, the present evolution of the stream bed combines both responses to hydrological events and adjustments to embanking on upstream or downstream reaches. The present diagnosis appraises the nature and the speed of channel changes, on the basis of maps and aerial photos that have been produced over the last fifty years. It accounts for the main channel features : bank-cavings and gravel bars, meanders and their cut-off, initial forms of défluviation, entrenchment or raising up of the bed bottom, etc. Such a study can be used as a suitable tool for optimal economic and environment choices in the field of water and river management.

The aim of this article is to show the effects of works on the long profile of a waterway. From Verdun-sur- le-Doubs to Lyon, the Grande Saône's morphological characteristics (gentle slope, succession of shoals) and the nature of river flow (loading floods and significant low water levels) have a particularly constraining effect on navigation. This explains the importance of human action over more than two centuries to correct the river's natural bed. With the aid of archival documents, it is possible to calculate the longitudinal development of the talweg and water levels between 1869 and 1989. Important modifications have been caused by embankments, the creation of stillwater reaches and the excavation of a navigable channel. This has resulted in a stepped waterway and the significant deepening of the channel (4,5 m on average), especially where gravels have been extracted.

This paper deals with the relationships between active channel evolution of the main tributaries of the Drôme, the Eygues and the Roubion rivers and changes that have affected watershed features since 1850. This study is based on a detailed analysis of land use changes and the history of torrent control works. It has been shown that tributaries are characterized by a generalised narrowing of the active channel which has been accelerated in the second half of the 20th century. This evolution is linked to the cumulative effects of a fluvial processes which induce a deconnec- tion of previous active alluvial surfaces and vegetation processes characterized by an increased of the potential for vegetation encroachment.

Changes to the river are the result of both natural factors (flow, slope) and human intervention (hydraulic, agricultural and urban development schemes). Over time, the links with local societies have evolved, at times being very close and at others looser diffuse. This variation has given rise to specific fluvial landscapes. It is therefore important to retrace the development of the river over several centuries, evaluating the respective importance of natural and human factors in this process. The spatial framework of the study covers the central part of the Garonne valley, between Toulouse and Langon. Two homogeneous sections can be distinguished along this alluvial corridor : from Toulouse to the Tarn confluence and from this point to Castets-en-Dorthe. In this zone, the Garonne and its valley are of considerable importance, representing a vast area for flood expansion. The length of these sections prohibits an exhaustive study of their characteristics. Consequently, selected geographical scales are analysed. Within the two sections, two sites with similar features are studied in more details (Ondes and Caudrot).

This paper presents the first results of a study which aims at devising a hydro- geomorphic river typology of the Alsatian watershed of the Rhine rift valley. A Multiple correspondance analysis and a specific stream power map of 187 sites allow an initial morpho- dynamic régionalisation of the rivers under consideration. Use of a Hierarchical ascendant classification for the sites of the III and Rhine rivers and the division of the Eberbach river into different sections based on the longitudinal evolution of the unit stream power (an adaption of the method of Bernot et ai, 1996) shows how this initial régionalisation can be refined to produce a detailed typology. Finally, the muttivariate analyses allow a hierarchy to be established of different variables, which could be used in other geographical contexts.

In the Massif Central, the upper Loire river is influenced by hydroelectric works. Measurement of the effects shows a major impact on hydrological conditions on an annual, monthly and daily basis, as well as the elimination of medium and small-scale floods. As resources of sediments have dried up, due to reafforestation, gravel extraction and embanking works, for more than a century the Loire's river bed in the Forez area has become further entrenched. The dams associated with the hydro-electric schemes have accentuated this process by reducing the transport of bed load, drying out the substrate and considerably reducing the necessary upply of coarse material. A restoration programme designed to pave the river is envisaged although not yet started despite its urgency.

Geohistorical analysis shows recent changes in the links between flooding, territory and planning in the downstream valley of the Canche river (Pas-de-Calais). With the additional help of surveys and interviews, this approach illustrates how the present management of floods reflects society's greater understanding of flood risks. A new river management system has been set up, greater solidarity exists between the upstream and dowstream parts of the watershed, and the notion of risk management is beginning to be integrated into individual and collective strategies.

Until now the regulation of rivers and torrents streams in Umbria has been carried out mainly by works designed to reinforce river- banks and reduce the impact of flooding. The aim was to protect nearby urban settlements and industrial areas. For some time this form of artificial control has been criticised, underlining the need to find new methods of regulation which would not only respect the natural vocation of watercourses but also the fluvial ecosystems and riverine landscapes. This paper describes two case studies of the reintroduction of flood expansion areas, a traditional flood control method developed in Umbria in the XVIIIth century. Such a technique has never been really applied due to fears of conflict between rural populations and local authorities.

The Highlands of the French Central Massive are regularly affected by localized violent storms, resulting in unexpected floods. Limited knowledge of concerned streams and the dispersion of measure instruments affect the credibility of quantitative results, swifty published and recapitulated by the regional and ever sensationalist press. This article provides thought on the methods, concepts and discourse validity with regards to flooding and fluvial processes.

In the North-East of Algeria , the 'exoreic' watersheds (coastal regions of Constantine, Rhumel and Seybouse) are characterised by major contrasts in the outflow of water into the mediterranean sea : negligible flows during periods of low water compared with substantial and sudden flows associated with the period of floods. The pronounced reduction in flow during the summer season is analysed using data from approximately 20 hydrological stations. During this period, the wadis are exposed to increased pollution from urban and industrial waste. Water resources are thus shown to be particularly vulnerable to pollution due to the incapacity of the rivers to transport the pollutants. This problem is a feature both within the drainage basins (Constantine) and along the coastal fringe (Annaba). There is, there-fore, a major problem of inadequate treatment of waste water : the capacity to treat urban effluents is limited and plant treating industrial wastes operate in an impredictable manner. As throughout northern Algeria, the protection of developed hydrosystems must necessarily associate two types of action : the cleaning of waste water and the maintenance of a minimum ecological flow on the downstream side of dams.