The Tarnon has its spring in the Aigoual massif (1,565 m) in the Bassurels district. 38.9 km later, it flows into the Tarn downstream of Florac, having already flown together with the Mimente. It separates the Causse Méjean from the Cévennes. Upstream of Rousses, the Trépalous flows into the Tarnon. That river has carved out the Gorges du Tapoul, which are famous for canyoning. The Tarnon is a Cévenol river: very strong and sudden floods can occur during intense weather events.