Leaf litter can be retained in ﬂoodplains for several months before it enters rivers as lateral inputs. During this period, the environmental conditions on the ﬂoodplain can alter leaf litter chemistry and, consequently, affect its subsequent processing in the river. We analysed the effect of contrasting ﬂoodplain conditions on the chemical composition of leaf litter and its leachates, and how this affected their biodegradability and processing in rivers. To do so, we placed reed leaf litter (Phragmites australis) in open- and closed-canopy habitats of three ﬂoodplain sites with contrasting climates (semiarid Mediterranean, humid Mediterranean and continental) for 105 days. We then used litterbags in a river to examine the decomposition of preconditioned leaf litter in comparison with a control (nonpreconditioned litter), and laboratory assays to examine the biodegradation of their leachates. Contrasting conditions on the ﬂoodplain prompted differences in the nutrient content of leaf litter among ﬂoodplain sites. Preconditioning caused a generalized decline in the C content and an increase in the lignin content of leaf litter. Even so, preconditioning did not affect litter decomposition rates in the river, although it did reduce decomposition efﬁciency and biodegradability of leachates. Shredder colonization of litter was variable and generally higher on preconditioned litter, but not signiﬁcantly so. Different ﬂoodplain conditions had no inﬂuence on the aquatic processing of preconditioned litter. Our results demonstrate that the retention of leaf litter in terrestrial environments can affect C budgets of ﬂuvial ecosystems and the recipient food web by reducing the input and the biodegradability of C and nutrients.