Riparian vegetation is of paramount importance for stream ecology but the underlying mechanisms are still poorly understood. For instance, riparian vegetation plays a key role in determining the nature and amount of autochthonous and allochthonous organic matter entering the streams. In temperate regions, both aquatic primary production and terrestrial organic matter are strongly influenced by the seasonal patterns displayed by riparian vegetation: i.e. leaf emergence in spring and leaf fall in late autumn. Still it remains unclear, how these seasonal patterns affect the contributions of autochthonous and allochthonous organic matter to stream food webs. Here, we shed light on the seasonal variations of stream food webs in relation with the processes of leaf emergence and leaf fall. Using stable isotope analysis of nitrogen, carbon and hydrogen, we studied stream food webs in four sites differing in their riparian vegetation (from meadow to forest) at four different dates (before leaf emergence, just after leaf emergence, before leaf fall and just after leaf fall). All sites were located maximum 50 km from each others, so that they display homogenous geological and climatic conditions. Following a quantitative sampling, we estimated the biomasses of macroinvertebrates and fish. Accordingly, we analysed the isotope ratios of the representative taxa (1531 values) from basal resources (algae, macrophytes, biofilm, suspended organic matter and leaf litter) to consumers (macroinvertebrates and fish). In a first step, we described the food web trajectories across seasons, using circular statistics on different food web metrics. In a second step, we estimated the contributions of autochthonous and allochthonous organic matter to the food webs, running isotopic mixing models. Given the riparian vegetation differences between sites, we expected a sharp contrast in terms of food web functioning, with sites mainly influenced by aquatic primary production (meadow sites), and sites mainly influenced by allochthonous organic matters (forested sites). In addition to these differences between sites, we expected to observe a seasonal variation of food web at each site, in relation with light availability and terrestrial inputs of organic matter.