Riparian zones are known to control the hydrology and biogeochemistry of forest headwater catchments. Some evidence suggests that these riparian-stream connections are shaped by a relatively small volume of soil, or dominant source layer (DSL), through which most water and solutes are routed laterally. However, the hydrological and biogeochemical significance of the DSL has not been broadly evaluated. We compiled data from four forest headwaters, each from different European sites (boreal, temperate, subhumid Mediterranean, semiarid Mediterranean) to test whether DSL dimensions and biogeochemical characteristics vary predictably across ecoregions based on differences in hydroclimate, topography, and soil features. Boreal DSLs were shallow and thin, whereas small-scale topographic heterogeneity shaped DSL dimensions at the temperate site. In the Mediterranean sites, DSLs were deeper and thicker, but upper riparian layers that seldomly connected to the streams had a large influence on the overall lateral flux. Contrasting hydroclimates and soils led to high dissolved organic carbon concentrations in riparian solutions in both boreal and Mediterranean sites. By contrast, nitrate concentrations were driven by differences in soil saturation, being orders of magnitude higher in dry Mediterranean than in wet temperate and boreal riparian soils. Notably, stream chemistry did not consistently reflect riparian DSL chemistry across flow conditions and ecoregions. We hypothesize that ecoregion-specific water sources bypassing the riparian zone, as well as ecoregion-specific in-stream biogeochemical processes could explain these discrepancies. Overall, conceptualizing the varied roles of the DSL across diverse systems can aid in both scientific assessments and management of land-water connectivity in river networks.