The lake Lugano (or Ceresio) is a pre-Alpine lake of glacial origin formed at the end of the last glaciation about ten thousand years ago, branched between Italy and Switzerland: in detail, it extends between the Canton of Ticino (the most extended and central part of the basin, with the district of Lugano and the center of Morcote), the province of Como (the north-eastern part with the centers of Porlezza and Valsolda, as well as Campione d'Italia, a historic Italian exclave surrounded by Switzerland) and the province of Varese (the south-western part with the center of Porto Ceresio). The main tributaries are the Cassarate, the Vedeggio and the Cuccio, the minor ones are the Bolletta, Laveggio, Magliasina, Mara, Rezzo, Telo di Osteno and Trallo streams. From its western branch begins the Tresa river (emissary), which flows into Lake Maggiore and therefore belongs to the catchment area of ​​the Ticino river. The dam bridge, over which the A2 national highway, the cantonal road and the railway passes: it connects the towns of Melide and Bissone, which overlook opposite shores of the lake. This construction rests its foundations on morainal deposits left after the retreat of the glaciers in prehistoric times.