The importance of P-flux on aquatic ecosystems bears out the study of P dynamics in river catchments. Aqueous P geochemistry depends on biogeochemical processes and Fe geochemistry. Our study focused on P and Fe behaviour at the groundwater-river water boundary. We monitored the distribution of dissolved and particulate P and Fe in the Leyre river from order 0 streams to order 4 main-river. We also analysed porewaters from river bed sediments and from wells located in the two main land-use types (pine forest and corn field). We determined the particulate speciation of P using chemical extractions. The particulate-dissolved P exchange capacity was measured using in-vitro experiments and 32 P radioactivity. Results show that the studied catchment groundwaters supplied dissolved Fe and P with a high Fe/P ratio. Dissolved Fe was subjected to oxidative precipitation at the groundwater/river water interface. This oxidation was enhanced by aquatic plants. P was trapped on Fe(III) particles so much so that dissolved P remained at concentrations >0.2 µM. This means that the coastal river outlet receives particulate P, which can be solubilized after anoxic diagenetic processes.