To gain understanding on how alluvial zones modify water and nutrient export from semiarid catchments, we compared monthly discharge as well as stream chloride, carbon, and nitrogen dynamics between a hillslope catchment and a valley-bottom catchment with a well-developed alluvium. Stream water and solute fluxes from the hillslope and valley-bottom catchments showed contrasting patterns between hydrological transitions and wet periods, especially for bio-reactive solutes. During transition periods, stream water export decreased > 40% between the hillslope and the valley bottom coinciding with the prevalence of stream-to-aquifer fluxes at the alluvial zone. In contrast, stream water export increased by 20-70% between the hillslope and valley-bottom catchments during wet periods. During transition periods, stream solute export decreased by 34-97% between the hillslope and valley-bottom catchments for chloride, nitrate, and dissolved organic carbon. In annual terms, stream nitrate export from the valley-bottom catchment (0.32 +/- 0.12 kg N ha(-1) yr(-1) [average +/- standard deviation]) was 30-50% lower than from the hillslope catchment (0.56 +/- 0.32 kg N ha(-1) yr(-1)). The annual export of dissolved organic carbon was similar between the two catchments (1.8 +/- 1 kg C ha(-1) yr(-1)). Our results suggest that hydrological retention in the alluvial zone contributed to reduce stream water and solute export from the valley-bottom catchment during hydrological transition periods when hydrological connectivity between the hillslope and the valley bottom was low.