 The catchment of this system is on the Lévézou plateau- which is at the western end of the Grandes Causses.The plateau lies between 700 m and 900 m and consist of granite rocks.It is a landscape of gentle valleys, drained to the north by the Ceor, Viaur, Bage and Violou which flow towards Rodez, and to the south by the Alrance and the Ruisseau de Asseynes which fall steeply into the Tarn.