Contenu du sommaire : Dynamique et gestion des cours d'eau

Basing his study on original documents preserved in local archives (Ain, Rhône, Isère "départements"), the author reconstitutes the mediaeval lay-out of the Rhône river in the plain of Miribel-Jonage (upstream Lyon). He proves that, from the XlVth to the XVIIIth century, the river accentuated its braiding and moved 2-3 kilometers northward, causing many conflicts between riverine populations.

The "Mappe Sarde" of 1730 is the oldest reliable map concerning the "department" of Savoy. This paper assesses its usefulness for fluvial geomorphic studies. The map, drawn at the scale of 1/2400° , provides an accurate picture of rivers which can be used for diachronic as well as for synchronie analysis ; nevertheless, it must be used with caution, as is shown in relation to the questions scale change, the interrelationship of sheets and the verification of their reliability. The Sarde land survey allows the calculation of semi-quantitative parameters such as the index of sinuosity and braiding. The interest of such reconstruction of the fluvial landscape is exemplified by the alluvial plain of the Middle Arve river.

During the last two centuries, the Atlantic salmon has di- sappeared from most of major French rivers. Conversely, the situation has remained relatively stable in Brittany. The presence of salmon in the small Brittany rivers is the consequence of favourable ecological conditions linked to geological substrate and to climatic conditions. The salmon can pass easily over the weirs of water mills, explaining its continuing presence within these rivers. A brief historical insight into water mills in Europe and in France is presented. The situation of water mills is analysed for salmon rivers in Brittany at the end of the XVHIth century and at the beginning of the XlXth century. The example of selected rivers in the west part of France, (Normandy and Vendée) is presented from the end of the XVHIth century to the end of XlXth century. The modification of river ecosystems brought by water mills and the effect on Atlantic salmon are discussed.

As the alluvial vegetation may be considered as a indicator of river dynamics, a dendrochronological study of "black" poplars (Populus nigra) was performed in the floodplain of the lower Ain river. It confirms a metamorphosis which has affected the river over the last 30 years. The phenomenon may be attributed to the combined effects of anthropic impacts upon the river (dams, dykes...) and to the recent reduction of peak discharges.

The flash flood of Nîmes on 3rd October 1988, was due to the classical conjonction of two antagonistic air masses, to their unusual duration and dinally to the impact of a poorly adapted street network.

A series of experiments was conducted in a pebble bottomed flume with marked pebbles, in order to test the influence of pebble shape on the resistance to erosion. Critical shear stresses are about 1 .5 times greater for flat pebbles than those required for well, rounded pebbles of the same diameter. This is confirmed by observations of bed structure and composition made in a natural stream in Middle Belgium where flat and well rounded pebbles are present at the same sites.

The Miribel Canal, embanked from 1848 to 1857, is the oldest development scheme of the Rhône upstream Lyon. Since 1937, the Jons dam controls the discharge and provides a 30 cumecs minimum flow. The study deals with the relations between the long profile adjustment over a period of 130 years, the surficial bed granulometry and the flood hydrology. The archives of the Service de la Navigation du Rhône and the surveys of the Compagnie Nationale du Rhône document precisely the long profile changes. The granulometry has been surveyed along a 17 km section, using a size measurement method ; a large amount of the gravelly bed material is presently reworked by floods ; over the bars the sediment size decreases downstream due to hydraulic sorting, but the vertical incision of the river bed has provokedthe exhumation of coarse deposits of fluvioglacial and morainic origin. Currently, this material is stable and controls the long profile of the Miribel Canal.

The functioning of running water ecosystems is controlled by the interaction of several physical parameters, the knowledge of which is beiing improved. Currently, the CEMAGREF, LYON, is developing a methodology allowing the description and the modelling of three of these parameters which have been selected on the basis of their importance for the biotic communities.namely, i stream velocity, ii water depth, Hi bed material. A division of the study reach into discrete cells provides a differenciated picture of the spatial variability of the hydrodynamic parameters measured at a given discharge, the mathematical modelling of water level integrates time variability. A reach on the Filliere river (Northern French) examplifies this method.