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Changes in DOC concentrations within interstitial waters of a gravel bar were assayed in a 6th order stream, Stillaguamish River, Washington, USA. Subsurface water and surface water were sampled 3 times during high-water periods and 3 times during low-water periods. Throughout the study, subsurface flow was fed only by surface water infiltrations, which occurred at the head of the gravel bar. DOC concentrations of the subsurface flow were positively correlated with discharge and decreased with distance from the head of the gravel bar. DOC filtration was efficient within 60 m, with concentration in the first piezometer 50% higher than those in the fith. During relatively low-water periods, interstitial DOC contents were low at the first piezometer (85% of surface water concentrations).