Contenu du sommaire

The natural and half-natural fluvial landscapes of the Rhône's valley in Péage-de-Roussillon area have been modified by different human interventions. Their improvements by the riparians had consequences on their composition but never on their dynamic. On the other hand, the dams of the XlXth Century have stopped the fluvial dynamic and favoured the forest places. The planning of the river by the Compagnie Nationale du Rhône have differenciated sectors partly saved, and sectors upset where new landscapes have appeared. Whatever their genesis may be, it is necessary to preserve the most interesting environment.

Floodable lowlands of Rhône and Po valleys are particulary attractive mediums because they offer a great range of possibilities for human exploitation. Presence of soils often excellent, phreatic and drinking waters, the good quality of environment for fishing, camping and recreation in general, navigation... are at the origin of a great competition for the property of lands. The Rhône valley, above Lyons, presents a structure in which agriculture, whose property is too often very small, is on the defensive face to face others activities. The valley of Po, beetween Cremona and Parma, is a land of great properties and agriculture is always master of soils.

The flash-flood of the upper Loire on September 20th 1980 (3 500 m3/s at Bas-en-Basset) had just a weak impact on the bed-rock in the narrows. The flood overran alluvial deposits in the small basins, undermined the overhanging banks and modified the river design on few limited sectors. Vegetation has played a prominent part in its differential effects as a great part of the kinetic energy is consumed by riparian growths. During the next winters, the eroded banks collapsed in many sectors.

After the last alpine glaciation of the Würm the over deepened glacial troughs of the Dauphiné piedmont have been filled in with glacio-lacustrine clay and coarse materials from local streams. The Rhone river has occupied lately the basin of the Basses Terres at the Southern part of the Jura Mountains because the pleistocene alluvium had first to fill in the upper basins ; new artificial cuts have shown the destruction of a peat of Atlantic age (6330 ± 140 B.P.) by the fluviatile erosion. The coarse-grained sediment of the Rhone river has the shape of a wedge and has been aggrading from an upstream control. Around the 6th century AD. the Rhone river self-captured by dumping through a fossile glacial through. These recent events explain the braiding pattern of the new section which is quickly aggrading and lighten several enigmatic historical facts.