Palaeogeography of a large groundwater-fed river: the Stella River (Po plain, Italy)

1. Background Stella River is a groundwater-fed or spring river. This type of river is common along the northern fringe of the Po-plain, where Pleistocene fluvial gravels originating from the Alps wedge out on the underlying Pleistocene flood-basin clays. Groundwater stagnates on the poorly permeable fine-grained fluvial deposits at the toe of the gravel-rich deposits (Figure 1). The roughly west-east oriented line with a length of about 500 km, along which these rivers emerge is called the spring line. Stella River has an average annual discharge of 40 m3/s, making it one of the largest spring rivers in Italy and, probably Europe. The river has its source about 30 km north of the Adriatic Sea coast. The discharge of the river is fairly constant over the year due to the absence of a catchment and the relatively steady supply of groundwater (Fontana, 2006). Because of this the influence of relative sea-level change dominates erosion and deposition, especially in the downstream realm. 2. Palaeogeography The palaeogeographic development of Stella River was mainly controlled by relative sea-level change. During the last glacial, relative sea-level lowstand resulted in incision into the Pleistocene substrate by a large predecessor of Stella River. This created an incised valley lined with fluvial terraces. As a result of postglacial relative sea-level rise, the valley was progressively filled with 5-10 m of fine-grained valley-fill deposits. The build-up of the coastal area follows the typical pattern for such coastal sites with barriers The Holocene valley-fill history would theoretically result merely from the interplay between groundwater-fed discharge, which is affected by climate, and by relative sea-level rise. However, data suggest that the incised valley has been occupied several times by neighbouring Tagliamento River during the Holocene as a result of avulsions. These phases led to the accumulation of allochthonous sediment from the Tagliamento catchment. The avulsions and related occupation of Stella valley, may be correlated with climatically induced changes in the Alpine hinterland. Besides climate, the presence of Man in the area since at least the Neolithic has probably affected the discharge (deforestation) and course of Stella River. The presented palaeogeographic maps will shed light on the different evolutionary steps of this interesting fluvial system and the influence on it by the main forcing factors of sea level and neighbouring river systems. 3. Future work The results show an interplay between the baseline activity of the groundwater-fed river and the major disturbances by other fluvial systems. Based on our data we envisage to proceed by: • Compiling available data to enhance the comprehension of groundwater-fed rivers, which are the dominant river type along the northern fringe of the Po Plain, by examining Stella River; • Constraining Tagliamento River occupation of Stella valley to understand timing and causes of major Tagliamento flooding and avulsion; • Correlating the downstream valley-fill sequence with regional relative sea-level history and palaeogeography. This study will serve as an example for groundwater-fed rivers which are also present along the Himalayan mountains, where they drain large sectors of the alluvial megafans characterizing that area. The situation there is comparable to the one in the Venetian-Friulian Plain, with the distinction that rivers flow directly into the Adriatic sea here. References Fontana, A. (2006). Evoluzione geomorfologica della bassa pianura friulana e sue relazioni con le dinamiche insediative antiche. Monografie Museo Friulano Storia Naturale 47, Udine, 288 pp. (With enclosed the Geomorphological map of the Friulian low plain, scale 1:50,000).