This paper deals with channel evolution over the past 200 yr in 12 selected streams in northern and central Italy and aims at reconstructing the evolutionary trends (e.g., trends of channel width and bed elevation) and understanding the causes of channel adjustments. The selected streams have been studied using various sources and methods (historical maps, aerial photographs, topographic surveys, and geomorphological surveys). The selected rivers have undergone almost the same processes in terms of temporal trends; however, the magnitude of adjustments varies according to several factors, such as original channel morphology. Initially, river channels underwent a long phase of narrowing (up to 80%) and incision (up to 8–10 m), which started at the end of the nineteenth century and was intense from the 1950s to the 1980s. Then, over the last 15–20 yr, channel widening and sedimentation, or bed-level stabilization, have become the dominant processes in most of the rivers. Different human interventions have been identifi ed as the causes of channel adjustments in Italian rivers (sediment mining, channelization, dams, reforestation, and torrent control works). Such interventions have caused a dramatic alteration of the sediment regime, whereas effects on channel-forming discharges have seldom been observed. Some notable implications for river management and restoration are (1) the state of rivers before major human disturbances and channel adjustments can rarely be taken as a reference, as at present rivers are far from their pristine condition; and (2) sediment management is and will be a key issue in such fl uvial systems

A field experiment is described for continuous recording of bed load transport rates in a coarse-grained alluvial channel, using instruments typically available to seismologists. The experiment was carried out in 1990 in the Italian Alps, in a small basin equipped since 1982 with instruments conventionally used in hydrological surveys and bed load transport measurements. In July 1990, six seismometers were placed along the alluvial channel for up to about 150 m upstream of the flow gauging station. Continuous microseismic recordings of seven flood flows were analysed, five being associated with coarse sediment delivery. The mechanism of bed load transport was inferred from two or three microseismic impulse peaks occurring before and after the discharge peak shown in the hydrograph. Those microseismic peaks are thought to reflect the pulsed nature of bed load transport while the periods elapsing between them are interpreted as indicating the duration of the transport process and appeared independent of peak discharges. Surveillance continue de la charge de fond dans un cours d'eau au lit graveleux par sismographesRésumé Un réseau de sismographes a été expérimenté en 1990 pour la surveillance continue du transport par charriage sur le cours d'eau principal d'un petit bassin dans les Alpes italiennes. Dans le même cours d'eau, des mesures conventionnelles sur les débits liquides et les volumes de transport solide sont assurées depuis 1982 par une station limnigraphique équipée d'un piège a sédiments. Jusqu'à 150 m a l'amont de la station limnigraphique, six sismographes ont été installés en juillet 1990 dans le lit graveleux et sur les berges. L'enregistrement continu des sollicitations microsismiques a été analysé en relation avec sept écoule-ments de crue, cinq étant des écoulements avec charge solide importante. Les détections des sismographes ont mis en évidence que la charge de fond est caractérisé par deux ou trois pointes d'activité microsismique, se produisantes avant et après le débit de pointe, telles qu'une nature pulsative du transport par charriage peut en être déduite. Les pulsions ont montré des successions à laps de temps variables qui correspondent a la durée du charriage, sans rapport avec le débit de pointe.

Sediment gravity flows are very common sedimentary processes in the Alpine region and are often characterized by rapid deposition of large amounts of material. Hazard evaluation in such mountainous areas depends on proper identification of the dominant sedimentary processes, interpreted both from modern and ancient sedimentary facies and their distribution. Three main groups of alluvial fans, characterized by different dominant sedimentary processes, have been distinguished on the basis of lithological characteristics of the catchment area. The dominant catchment lithologies are: 1 massive and/or crudely stratified carbonate rocks (dolomite and massive limestones); 2 fine-grained sedimentary and metamorphic rocks (schists, calc-schists, mica schists, slate, phyllites and quartzites); 3 massive crystalline rocks (granites, granodiorites). Their main characteristics are illustrated by three case studies concerning large debris-flow events that occurred in the recent past. The comparison of sediment texture and grain-size distribution indicates that important differences in the sedimentological features of debris flows are generated by the three different rock types in the catchments. Colluvium lithology strongly controls the grain-size distribution of the debris available on the catchment that is mobilized, transported and accumulated on the fan during catastrophic flood events. The proportion of fine-grained particles (clay and fine silt) within the colluvium plays a key role in controlling the dominant primary sedimentary processes. The study of 23 flood events over the past 30 yr indicates that the catchments of group 1 and 2 fans produce large amounts of clay and fine silt, which typically can lead to the generation of cohesive sediment gravity flows. Group 3 fan catchments produce colluvium free of clay and fine silt that can be mobilized and transported by water flow processes, and which in extreme flood events usually are associated with non-cohesive sediment gravity flows.

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