The Eastern Friulian and Venetian Plain

The Eastern Friulian and Venetian Plain (PFV) stretches from the course of the Livenza River in the west to the rocky shores of the Karst in the east. Although it belongs to the Po Valley, of which it represents the easternmost portion, this plain stands out geographically because the hydrographic system of the Veneto-Friulian region flows directly into the Adriatic Sea instead of the Po River. Another unique aspect of the morphology of this plain is that the distance between the pre-Alpine edge and the coast gradually decreases from west to east, disappearing completely in the area of the Trieste Karst, where the mountains drop steeply into the sea.

The Isonzo, Natisone, Torre, Cormor, Tagliamento, Meduna, and Cellina rivers are the main alpine watercourses that traverse this plain. Their basins drain materials from mountains mainly composed of carbonate rocks found in the Carnic-Julian Prealps and the adjacent western Slovenian area.

The main factors that have guided the formation, evolution, and shaping of the late Pleistocene-Holocene part of the Eastern Friulian and Venetian Plain are linked to:

During the recent Quaternary, the rivers of the Eastern Friulian and Venetian Plain have formed depositional systems that extend continuously from the pre-alpine valley outlet to the Adriatic coast, creating a series of regional-scale or megafan alluvial cones (Fontana et al., 2008). These depositional systems exhibit marked longitudinal differentiation: the first tens of kilometers from the valley outlet to the spring area are gravelly with slopes ranging from 7 to 3‰, forming the High Plain. Moving away from the pre-alpine zone, the decrease in slope (below 3‰) has led to a decrease in sediment transport capacity by the rivers, allowing them to carry progressively finer sediments downstream, forming the Low Plain with silty-clayey flood deposits and sandy channel bodies. These differences in hydrodynamic character are also reflected in the distinct channel forms that characterize the rivers of the High and Low Plains. The High Plain features braided channel forms, while the Low Plain exhibits meandering single-channel forms (Fontana et al., 2004).

Among the most characteristic elements of the Eastern Friulian and Venetian Plain are the spring areas and the spring-fed rivers that feed them. The emergence of the water table occurs along a zone of variable width, ranging from 2 to 10 km, which defines an upper and a lower limit. The spring area thus separates the gravelly high plain, almost devoid of surface drainage, from the low plain, rich in silty-clayey soils and water. The upper limit of the springs corresponds to the intersection of the water table with the ground surface, while the lower limit is marked by the outcrop of impermeable clayey bodies. While the lower limit remains nearly constant, the upper limit is affected by variations in the water table surface, shifting upstream as the water table level rises and downstream as it falls.

Because spring-fed rivers are not directly fed by precipitation but indirectly by the water table, they are characterized by nearly constant flow throughout the year. Moreover, since they are not sourced from a mountainous basin, they have very limited sediment yields, which derive only from the erosive power they can exert on the plain. For these reasons, spring-fed rivers typically follow existing topographic depressions, such as abandoned paleo-channels of alpine rivers.

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