Contenu du sommaire : Le Rhône, l'axe et la vallée

The Rhône below Lyon has been modified on a major scale as the result of a century of improvement works (embankments and hydroelectric dams) ; these works have aged and are now being repaired. This article looks at such action on the by-pass section of the Rhône at Pierre-Bénite. An audit was carried out of the river's ecogeographical functional organisation using a pluridisciplinary approach based on geomorphological, batrachological and phyto- ecological indicators. From the results, various proposals have been made to assist in defining a programme of rehabilitation for the river's flood plain ftaking account of both abandoned channels and their wooded margins.

A precise estimation of the discharge of suspended sediments of the Rhône, near to its mouth (Arles), was carried out using daily measures between June 1994 and May 1995. This period is characterised by an important average discharge and a flood with a likely recurrence of once every 30 years. A preliminary analysis taking account of the activity of the main tributaries shows two major types of floods : floods originating in the south, due largely to the influence of the Cevennes and the Mediterranean alps, and these originating from the north, fed predominantly by the Saône and/or upper Rhône. The average daily load of suspended material is 123 mg/l I (6to 3612 mg/l). In a year the mass of material transported in suspension is estimated at 13.9 million tonnes for a total water discharge of 69,81 109 m3. In periods of flood (discharge > à 3000 m3/s), representing 78 days of the year (22% of the period), the discharge of suspended sediments equals 11.3 million tonnes, or 81.1% of the total mass of transported material discharge. The flood of november 1994 alone transported 5.2 million tonnes. Examination of paired-values (water discharge - daily flows of suspended material - fig. 6) shows that for a given flow suspended material is transported by floads originating from the north than those from the south. Applying linear regression models to daily discharge values of the Rhône at Beaucaire since 1961 gives an estimated annual discharge of suspended sediments of 6.2 million tonnes. This calculation shows the very considerable variation in such discharges between years, with figures ranging from 1.4 to 16.4 million tonnes. Discussion centers on the role of watershed management, modifications to land use and climatic variability with respect to the mass of suspended material transported by the Rhône.

Three main periods described the history of fluvial ecosystems of the Rhône valley during the last centuries. Direct and indirect human impacts increased during this evolution. A reduction in the morphological diversity of the regulated hydrosystem has caused a reduction in biological diversity. It is more difficult to establish a global diagnosis for the terrestrial environments than for the aquatic environments. However, several tendencies can be identified. The future of natural areas of the Lower Rhône is quite uncertain despite the increasing place of the nature in the socio- economical f unctionning of the valley.

Until 1960, navigation of the Rhône was difficult, but since then, the river has been accessible to push-barge convoys and fluvio-maritime vessels between Lyon and Fos-sur-Mer. Despite periods of economic difficulty, this has resulted in an increase and diversification of traffic. Simultaneously, the fleet of Rhône vessels has been partialy renewed and new ports created. These ports, which have been planned as multi-modal platforms, offer a wider range of services (storage and packaging) while river-based leisure and tourist activities havec developed. An overall assessment of these changes shaws numerous positive effects for the whole valley, especially with respect to urban development.

Improvements carried out along the Rhône by the Compagnie nationale du Rhône and by other organisations such as the Commissariat à l'énergie atomique or the SNCF were undertaken to meet national needs, taking little account of regional interests. The resulting damage to the region now requires rectifying. This might take the form of various regional improvements or preferential energy tariffs, financed by Electricité de France which currently benefits from its privileged position as the sole producer of electricity along the Rhône.

Despite large scale development works carried out over a long period along the river, areas of the middle Rhône remain organised around the main north-south lines of communications and display clear signs of under-development.