Alluvial fans and megafans along the southern side of the Alps

The foreland basin of the southern European Alps is characterized by large fan-shaped alluvial systems fed by the main montane valleys and these depositional systems present an extent of 300–3000 km2, with a length of 30–70 km. Most of them are megafans, characterized by evident longitudinal differentiation in which steep piedmont sector consists of amalgamated gravels, while the distal portion has a gradient < 2‰ and is dominated by fine sediments. The major depositional phase occurred between 26 and 19 kyr cal BP during the Last Glacial Maximum (LGM) marine lowstand, when the Alpine glaciers reached the plain and fed the related fluvioglacial systems. The easternmost megafans also partly extended on the Adriatic shelf whereas, west from Garda Lake, their downstream development was limited by the Po River plain. The thickness of LGM alluvial sedimentation ranges between 30 and 15 m and pinches out about 25 km off the present coast. Soon after ice decay, after 19–17 kyr cal BP, sediment delivery from Alpine catchments to the plain dramatically decreased and in the central Alps large intramontane lakes formed, trapping almost all the bedload. Thus, the ratio between sediment and water discharge dramatically decreased and an erosive phase affected the LGM megafans and fans, leading the rivers to entrench for tens of meters. The funneling effect created by the fluvial incisions allowed the gravels to arrive tens of kilometers further downstream than in the LGM. In the Venetian–Friulian megafans a single valley formed in the piedmont sector, while 2–5 incised valleys developed in the distal sector. These latter valleys have been almost completely filled by a depositional lobe formed in the last 8 kyr, partly triggered by Holocene sea-level rise. The Alpine tributaries of the river Po still flow along a single incised valley from their megafan apex to the junction with the Po and they have not yet been affected by sea-level influence.