The Nahr el Kabir el Janoubi river is a shared coastal river tracing the north-south border between Lebanon and the Syrian Arab Republic. It rises from karstic springs and wadis in the northern part of the Lebanon Mountain range, reaches the intra-mountainous Bqaiaa plain, then flows westward through a basaltic central plateau where it forms steep gorges. It then meanders through the extensive alluvial flatlands of the coastal Akkar/Hamidiye plain and discharges into the Mediterranean Sea near the Lebanese town of Arida (figure 1). Several tributaries discharge into the Nahr el Kabir el Janoubi on both sides of its course, including the Wadi al Atchane, the Nassiriya and the Arous on the Syrian side. On the Lebanese side, the main tributaries are the Wadi Khaled, Es-Safa and Chadra. Qarnat Araba, constitutes the highest point of the catchment with an altitude of 2,215 m asl.

The basin is divided into four geomorphological zones: the mountain region in the upper catchment shared between Lebanon and the Syrian Arab Republic, the intra-mountainous cross-border Bqaiaa plain, the central plateau/gorge area running along the border, and the coastal cross-border Akkar/Hamidiye plain. The river basin covers 954 km2, of which 26 per cent lie in Lebanon and 74 per cent lie in the Syrian Arab Republic and is characterized by Mediterranean winter precipitation with increasing volumes and intensity from the coastal plain towards the mountainous areas and dry, hot summers.