Lago Trasimeno is situated between Perugia, Cortona, and Chiusi. Since the Etruscan period, those three cities have exerted their influence over the territory of the lake. The Trasimeno Regional Archaeology Project is investigating the archaeological heritage of the territory west of Lago Trasimeno, principally within the Comune of Castiglione del Lago. This region is characterized by a narrow plain that gives way to a series of low hills that form the eastern edge of the Valdichiana, which corresponds to the modern border between Umbria and Tuscany. The southern limit of the territory is marked by Lago di Chiusi and the now dry river valley of the Chiana (Clanis) as it moves eastward across the plain of Tresa. To the west of the Valdichiana, Mount Amiata rises in the background and Mount Cetona can be seen to the southwest.
The topographical features of this region were formed during the late Pliocene to middle Plesitocene, ca. 3.60-0.78 million years ago, and consist of marine and continental fluvial deposits that alternate between clays and sand. Tectonic uplift has caused the terrain to angle downward from northwest to southeast with riverine tracks that drain southward (Bizarri, et al. 2011). As uplift, drainage, and drying progressed, remnant pockets of deeper water formed the district’s lakes. The line of low hills marking the eastern limit of the Valdichiana forms the west-east watershed between the Chiana River valley to the west and Lago Trasimeno to the east. Since the Etruscan era this ridge served as a communication route between Chiusi and Cortona.
The current shape of the terrain largely resembles that in place ca. 500 BC (Battino 2014). However, during the Etruscan and Roman periods, the river Clanis ran sluggishly southward, widening into areas of open water today called Lago/Chiaro di Montepulciano and Lago/Chiaro di Chiusi. The Clanis eventually continued into the Tiber river just to the southeast of Velzna/Volsinii Veteres (Orvieto), on its way to Rome (Talbert 2000, Map 42).
In 1780 the Papal States and the Grand Duchy of Tuscany established the Argine di Separazione, “Separation Levee,” at Chiusi Scalo, and the waters of Lago di Chiusi and the northern section of the Chiana now drain northward into the Arno River as engineered by the massive 19th century land reclamation project directed by Fossombroni and Manetti (Fossombroni 1835).
Batino, S., 2014, “Letture geoarcheologiche nel paesaggio urbano e periurbano della Val di Chiana chiusina,” in Atti del Convegno ‘Dialogo intorno al Paesaggio’, Culture, Territori, Linguaggi 4: 184-95.
Bizzarri, R., Baldanza, A., and Argenti, P., 2011, “Plio-Quartenary paleoenvironmental evolution across western Umbria and Tuscany,” Il Quaternario 24: 81-83.
Fossombroni, V., 1835, Memorie idraulico storiche sopra la Val-di-Chiana. 2 vols., 3rd Ed. Montepulciano, Angiolo Fiumi.Talbert, R.J.A., ed., 2000. Barrington Atlas of the Greek and Roman world. Princeton, Princeton University Press.