AbstractStreams and rivers act as landscape-scale bioreactors processing large quantities of terrestrial particulate organic matter (POM). This function is linked to their flow regime, which governs residence times, shapes organic matter reactivity and controls the amount of carbon (C) exported to the atmosphere and coastal oceans. Climate change impacts flow regimes by increasing both flash floods and droughts. Here, we used a modelling approach to explore the consequences of lateral hydrological contraction, that is, the reduction of the wet portion of the streambed, for POM decomposition and transport at the river network scale. Our model integrates seasonal leaf litter input as generator of POM, transient storage of POM on wet and dry streambed portions with associated decomposition and ensuing changes in reactivity, and transport dynamics through a dendritic river network. Simulations showed that the amount of POM exported from the river network and its average reactivity increased with lateral hydrological contraction, due to the combination of (1) low processing of POM while stored on dry streambeds, and (2) large shunting during flashy events. The sensitivity analysis further supported that high lateral hydrological contraction leads to higher export of higher reactivity POM, regardless of transport coefficient values, average reactivity of fresh leaf litter and differences between POM reactivity under wet and dry conditions. Our study incorporates storage in dry streambed areas into the pulse-shunt concept (Raymond and others in Ecology 97(1):5–16, 2016. https://doi.org/10.1890/14-1684.1), providing a mechanistic framework and testable predictions about leaf litter storage, transport and decomposition in fluvial networks.