On the evening of the 7 August 1996 an intense storm occurred over the Ara s catchment near Biescas in the central Pyrenees. Eighty-seven people were killed as a result of the subsequent ¯ood, which hit a campsite located on the alluvial fan at the outlet of the 18 . 8 km 2 catchment. This paper presents the main results of a hydromorphological study of the event. The Bete s subcatchment received the most intense rainfall, estimated at somewhat in excess of 250 mm, which resulted in a peak ¯ow from this tributary of 300 m 3 s À1 . Just downstream from the Bete s river junction, ¯ow in the main channel reached 400 m 3 s À1 , increasing to around 500 m 3 s À1 further downstream. Rainfall in the larger Aso tributary was less intense, and in the head reaches ¯ow remained within-bank, representing a one in two-year return period event. Flow from this tributary did not exceed 100 m 3 s À1 , indicating that the Bete s subcatchment supplied some 75% of the ¯ow from just 28 . 7% of the catchment area. The extreme ¯ows caused the collapse of a series of sediment trap dams in the Ara s channel downstream of the Bete s junction. This resulted in the addition of 68 000 m 3 of sediment to an already disastrous event. Data from other rain gauges in the area showed both the extremely local nature of the event, and the problems of return period analysis for such storms, whose peaks are rarely observed at gauges. Together with the high geomorphological risks of the zone, this leads to the conclusion that a new method of spatial and temporal risk analysis is required for infrastructure planning.