Contenu du sommaire : Enfoncement des lits fluviaux : processus naturels et impacts des activités humaines

Channel entrenchment in small mountain valleys of the western USA can potentially be triggered by removal of beaver dams (by humans or by storm surges of water and sediment), severe overgrazing by livestock, or by clearcut logging. Beaver dams are removed by humans because they are perceived as a nuisance or because beaver can be used as bait for hunting bear. Storm surges of water and sediment can breach beaver dams, leading to a domino-effect on downstream dams. Beaver have been successfully transplanted to restore streams which have experienced entrenchment. Overgrazing by in upland areas accelerates runoff ; overgrazing in riparian zones reduces resistance of the channel bed and banks. Entrenchment caused by overgrazing can be prevented by altering grazing strategies. Clearcut logging is known to increase peak flows, but as with removal of beaver dams and grazing, only leads to channel entrenchment if the added runoff overcomes the resistance of mountain valley-bottom alluvium and vegetation. If channel entrenchment is allowed to progress, the water table will drop, riparian vegetation will change to less desirable species, fish habitat will be lost, and downstream sedimentation will occur.

The floodplains of the Lower Salzach are gradually drying out due to the rarefaction of floods and and the lowering of the groundwater table. This latter feature has been caused by incision of the river's bed as a consequence of channel rectification, bed load removal, and upstream impoundment. In 1991 a selection of 45 water environements was characterized by the analysis of 11 different macro-invertebrate groups leading to the bio- indication of 5 types of former channels and backwaters ; this analysis was more accurate than an earlier study (1989) identifying 3 water environment- types using only aquatic molluscs. The result demonstrates that, other than the absence of floods, the groundwater influx becomes the most dominant factor for aquatic life in the remaining water environments. Their water supply depends on their depth, meaning that if their base level lies above the groundwater table's amplitude, they become desiccated permanently.

Sand and gravel harvesting, which peaked in 1979, are responsible for major problems in the Loire valley. The region of Centre alone provides 6.4 MT while the computed transit of alluvial sediments averages only 0.5 - 0.7 MT, i.e. 1 MT per year. The water level dropped and the degradation of the bed (up to 3 m) induced serious ecological damage and threatened engineering works such as that Tours bridge which collapsed in 1978. Moreover, the comparison of water profiles since 1864 reveals that floods reach higher levels due to the aggradation of islands and banks. However, gravel harvesting is not the only cause of bed degradation: river rectification works of the XlXth century which increased stream power and reservoirs which control load transit play a significant role. Gravel harvesting ceased in the river bed in 1992 except in some minor places. Extraction is now located in the floodplain and on the edge of the valley.

This study analyses recent river-bed incision in the northern Alps (France). This incision began after the second world war and, initially, affected mainly the river channels of large alpine valleys ; incision exists also on many mountain rivers but is less extensive because of the bed-rock outcrop. The river-bed incision is very severe : at present, many rivers are incised to 3-4 metres and the long profile lowering of several rivers exceeds locally 8 to 14 metres ; rates of incision exceeding 50 cm/year were frequent during the period 1950- 1970. The channel incision results from the interaction between : a) a natural decrease of the bed- load supply after the Little-Ice- Age ; b) a reduction of bank erosion due to the longitudinal embankment of fluvial channels ; c) a control of torrent dynamics by reforestation and contruction of weirs between 1870 and 1920 ; d) a drastic change of hydrology and sediment transport after dam construction ; e) major instability of river beds due to excessive gravel extraction ; f) a trapping of the bed-load by weirs built to prevent the lowering of long profiles.The future of rivers flowing in the northern Alps relates not only to river-bed stabilization by civil engineering works but also to the systemic management of water discharge and sediment supply to restore the longitudinal connection between the different reaches of rivers.

The temporal analysis of the fluvial forms of the Buëch River shows a substantial change in dynamics over the last 35 years. A severe entrenchment is observed in some sections of the channel, and is equally apparent in sections where dikes were built as in sections preserved from embankments. Specific methods are used to study the fluvial forms and the grain-size distribution of the sediments on the bars ; they reveal the dramatic impact of the gravel extraction in the process of bed degradation which has been occuring over the last 40 years. However, the extent and rapidity of the incision process are also affected by the effects of natural factors : the steep slope of the fluvial bed, and a significant decrease of the sediment delivery due to the reforestation of the catchments of the Buëch tributaries.

Over the period since the 1920s,the average values of entrenchment of the river beds of the Ardèche (middle reach) and the Drôme (lower and middle reaches) are respectively 0.43 and 1.68 metres. The main cause has been gravel extraction from the river bed during the 1970's. It seems that man-made incision has been increased by engineering works (dams, river embankments), the agricultural crisis of the 1950's and the natural peak flow reduction since the 1800's. Consequently, various reasons explain river bed degradation. Today, reversibility is the main problem of management. The damage to different uses (agriculture, water supply) and the deterioration of ecological diversity are well known. On the Drôme River, the planners have decided to halt incision because of the falling water table. Unfortunately, on the Ardèche River, continuing dredging to limit flooding and preserve tourist activities in the floodplain is likely to favour increased incision.

Although forecast by the National Company of the Rhône, the floods of February 1990, caused by the overflowing of the 'old Rhône', produced unusual afects on the Chautagne marsh. An hypothesis was formulated that movements of the bed of the old Rhône were responsible for this condition. This led the Syndicate of Defense of Embankments and Borders of the Upper Rhône to request a morphodynamic study, which is presented here in condensed manner. The hydroelectric diversion scheme has little effect on the transit of flood flows in the old Rhône but has stopped the transit of bed load, which had been left strongly reduced by the works completed upriver prior to the 20th century. Analysis of available data shows evidence of a lowering of the longitudinal profile, rendered complex by the fact that the Rhone's course is influenced by a morphology of fluvial braiding moderately affected by old dykes. Sediment transport is initiated by a flow of approximately 400 m3/s and occurs on average only three days per year. The bed degradation extends itself progressively through space whenever the flood discharge occurs, but with a reduced recurrence interval. The analysis demonstrates that the movement of coarse material will continue into the future. In order to maintain the flooding of the marsh, the water line for minimum discharge and the level of the water table in the old Rhône, civil engineering work is recommended. Its purpose is to minimize the transport that affects the final supply of bedload and to ensure a sustainable development of the floodplain.