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Landslide dams are common features in Alpine areas; they may act as large sediment traps in upper catchments. To document their influence on sediment fluxes, the authors applied a sediment budget approach. The extent of landslide deposits was reconstructed from geomorphic survey and mapping; the cross-section of the landslide mass in its valley was established and the material characterized. The geometry of the reservoir created upstream from the dam and the volume of trapped sediments were assessed with the use of a DEM within a GIS. Subsequently, the geometry of post-depositional erosional features was used to calculate the volume of the eroded landslide mass. Two study cases were investigated, according to their position in the French Southern Alps and to their initiating slope-failure mechanism: the Fontfroide rock-avalanche site, and the early Holocene Chenaillet earth-flow site. From the calculated volumes of sediment filling and dam erosion, it appears that the degree of river incision across the dam mainly varies as a function of local parameters. Landslide dams are persistent features, controlling sediment fluxes long after their occurrence. [ABSTRACT FROM AUTHOR]

Deglaciation in high mountain areas triggers a series of adjustments affecting the various geomorphic components of formerly glaciated catchments. Active processes, part of the 'paraglacial' open system, release and transport sediments from formerly glacierized sources to catchment sinks. Such cascade sedimentary systems may be disrupted by temporary sediments traps. To evaluate the consequences of such traps on sediment export, several test areas were selected, where former glacial geometries and glacial retreat chronology were reconstructed from field data. Special attention is given to the link between the glacial margins and the glacio-fluvial systems. During glacial retreat, the presence of morainic ridges may temporarily interrupt the sedimentary cascade system, thus forcing local aggradation and change in the glacio-fluvial pattern. The volume of trapped sediments is controlled by the volume and the position of morainic ridges, while the storage residence time is strongly dependent upon the rate of moraine erosion. The ice surface elevation and its lowering control ice-contact storages, as well as sedimentary and water fluxes. The time lag between the rate of sediment accumulation on the valley floor and the rate of glacial shrinking influences the pattern and behaviour of drainage many decades after the peak in glacial melting. [ABSTRACT FROM AUTHOR]