In High Resolution Stereo Camera (HRSC) images of the Mars Express Mission a 130 km long interior channel is identified within a 400 km long valley network system located in the Lybia Montes. Ages of the valley floor and the surroundings as derived from crater counts define a period of ~350 Myrs during which the valley might have been formed. Based on HRSC stereo measurements the discharge of the interior channel is estimated at ~4800 m3/s, corresponding to a runoff production rate of ~1 cm/day. Mass balances indicate erosion rates of a few cm/year implying the erosion activity in the valley to a few thousand years for continuous flow, or one or more orders of magnitude longer time spans for more intermittent flows. Therefore, during the Hesperian, relatively brief but recurring episodes of erosion intervals are more likely than sustained flow.