The river has its source 1,100 meters above sea level in the Julian Alps to the west of Mount Triglav, 2,864 meters high, in the Trenta valley. It is 136 km long and drains an area of 3,400 km², of which 1,150 km² in Italy. Some people call it the emerald beauty because of the bright green colour of its waters. Its average flow (170 m³ / s per year at the mouth) is, after that of the Adige, the highest of the rivers in the Triveneto, remaining high even in summer (it never drops below 50 m³ / s) thanks to its karst and lowland springs. During the rainy season, it is instead subject to massive floods: its maximum flow was recorded in 1924 in Solkan (now a suburb of Nova Gorica on the Slovene-Italian border) and totalled 2,500 m³ / s. Currently, the flow of the Isonzo also depends on the opening or closing of the many dams built along its course in Slovenia.