Decomposition of dry and fresh willow leaves and the dynamics of invertebrates during this decomposition were followed in upstream and downstream Oued Zegzel, Eastern Morocco. A seasonal comparison of the two studied sites stressed the importance of the structure of the benthic fauna in the process of decomposition. The rate of decomposition was faster in summer, following an increase in the biomass of invertebrates. It was slower in winter, when abundances of Chironomidae reached their maxima. The decomposition of willow leaves was faster upstream, where invertebrates biomass was greater. The artificial drying of leaves before their immersion had a positive effect on the activity of invertebrates. Whereas Melanopsis praemorsa preferred the dry leaves during the four seasons in upstream Zegzel, the preference of Hydropsyche maroccana for the fresh leaves in the downstream station changed according to the season. The seasonal variation of leaves quality seems to especially influence invertebrate colonisation, and thereafter the decomposition process in downstream Zegzel.