Relationships between discharge and suspended sediment are very complex in most Mediterranean catchments. In the case of the Arnás catchment (Central Spanish Pyrenees), with a long history of human activity, the main factors that explain the variability of suspended sediment concentration (SCC) during floods are the peak flow and the intensity of precipitation. A cluster analysis distinguishes four types of floods according to different characteristics of precipitation, discharge, suspended sediment transport and antecedent moisture conditions. G1 and G2 floods occur under dry conditions (which prevail most of the year), with moderate rainfall and low precipitation intensity; the discharge and suspended sediment response are very fast but limited in intensity. This suggests that the origin of water and sediment is restricted to areas located very close to the channel. G3 floods also occur under dry conditions, although during intense rainfall events; then the response in both discharge and suspended sediment is very high, showing an enlargement of the contributing areas. Under very wet conditions (G4 floods) relatively moderate precipitation produces a very high response in discharge, but suspended sediment concentration records moderate values due to the effect of dilution when the entire catchment is contributing.

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