Développement végétal dans le lit endigué de l’Isère en amont de Grenoble : du diagnostic à l’évaluation des pratiques de gestion

Abstract

In a channelized 50 km reach of the Isère river, studies of changes in bedforms and vegetation were undertaken from a series of vertical aerial photographs between 1948 and 1996 and from field measurements of geomorphological and ecological parameters. Results showed a radical change of channel landscape and geomorphological activity during the last decades. Two different periods have been identified. During the first period (pre-1 970), geomorphological activity of Isère channel was intense, bedforms were continually removed, and pioneer vegetation uprooted during frequent floods. During the second period (post-1970), geomorphological activity slowed down, leading to the stabilisation of bedforms (lateral gravel bars) and development of vegetation. Vegetation rate which was less than 5% in 1970 is presently by 20% and new successional mature stages with hardwood species (Fraxinus excelsior, Robinia pseudacacia, Ulmus minor) have progressively replaced pioneer communities and softwood woodlands. Several causes explain this change. The main cause was the drastic decrease in the gravel bedload supply (from 150,000 m3 before 1950 to 40,000 m3 in 1960 and 5,000 m3 in 2000) which resulted in lateral bars pavement processes and narrowing of the active bedload band. Secondary causes such as gravel extraction (responsible for 1-3 meters deep channel incision) and absence of large floods over the last decades have added their effects, favouring establishment of mature plant communities. The vegetation inside the dykes infers new risks for riverside residents : increase of roughness and elevation of flood water levels by promoting denser growth of woody floodplain species within the dykes, and logjams formation in the downstream reach in Grenoble in case of a very large flood. In an effort to reduce the risk of these vegetated bars, river managers propose two types of maintenance. The first one consists in a simple clear cutting of the vegetation. Fine sediments which recover the gravel bars (the thickness is 1 to 3 meters) are left in place as well as tree stumps and roots. In the second type, clear-cutting is followed by additional disturbances such as burning of branches, scraping of the ground, and scouring of fine sediment deposits. First records of species richness and distribution of community diversity and geomorphological changes permitted comparisons between the both techniques. The first one (clear cutting) encouraged bed-forms stabilisation, rapid and denser coppice regrowth, increase of channel roughness, increase of fine sediment deposits during floods, decrease in plant diversity and invasion by exotic species such as Faloppia japonica. As a consequence, this type of operation needs to be systematically repeated with a 3-year rotation. The second technique (clear cutting and scouring of fine sediment deposits) is particularly effective because it favoured the restoration of geomorphological processes (erosion-deposit). Gravel bars are regularly eroded during frequent floods and migrate downstream. The reactivation of geomorphological processes led to the restoration of favourable conditions for regeneration of vegetation in the active channel. Biodiversity is enriched with presence of pioneer species, typical of the alpine rivers with high geomorphological activity (Typha minima, Calamagrostis epigejos, Salix, Populus, Alnus).