The Litani River lies entirely within the internationally recognized boundaries of Lebanon. It rises in the Bekaa Valley, a short distance west of Baalbek (only a few kilometers southwest of the headwaters of the Orontes), and flows south down the axis of the Bekaa Valley between the Lebanon Mountains to the west and the Anti-Lebanon Mountains to the east. The two mountain systems are underlain by deformed Mesozoic carbonate rocks; the Bekaa Valley itself is developed in a fault-bounded depression filled with younger sediments. The discharge of the stream is derived from winter rainfall over the higher portions of the flanking mountain ranges, supplemented by groundwater contributions to base flow carried to the late-Cenozoic alluvial fill on the floor of the Bekaa Valley from the slopes of the mountains. At Qirawn the Litani enters a gorge between the Lebanon Mountains and the massif of Mount Hermon; near Nabatiya the river turns sharply to the west and flows to the Mediterranean through the Galilean Uplands, a hilly terrain underlain by Mesozoic and early Cenozoic carbonate rocks.