The saturated region immediately surrounding the stream channel has been characterized as a zone of active mixing, but the dynamics of this process are not accounted for in conventional hydrograph separations. We employ a simple mixing model to explore the possible consequences. Chemically distinct hillslope and groundwater runoff components are routed through a near-stream reservoir, representing the channel and surrounding alluvium, to produce streamflow. For physically plausible near-stream zones, groundwater contributions estimated from hydrograph separation on streamflow ranged from 45 to 80 percent larger than the true inflow values on the rising limb, and 19 to 57 percent larger at peak flow. The overestimates resulted from a near-stream reservoir-induced lag in the hillslope chemical signature relative to increases in hillslope water flux, and were consistent for reservoir dynamics ranging from constant-volume to linear. The results suggest that caution be exercised when interpreting hydrograph separations without knowledge of the hydrology of the near-stream zone.