Experiments were conducted in a recirculating flume to elucidate the fundamental physical and chemical processes which control the stream-subsurface exchange of colloids. Results are presented on the rate of exchange of colloids (kaolinite clay) and a conservative solute (lithium) from a stream to a sand streambed covered by stationary bed forms (dunes, ripples). Kaolinite and lithium were added to the recirculating stream, and their exchange with the bed was observed over time. Kaolinite was observed to be much more extensively trapped in the streambed than lithium owing to nonconservative processes. By the end of most experiments, essentially all added kaolinite was taken up by the streambed. The observed exchange rates can be explained by analyzing the solute and particle fluxes through the stream-subsurface interface and the physicochemical interactions between transported kaolinite and the bed sediment. The colloid pumping model predicts particle exchange based on pumping hydraulics, particle settling in the bed, and filtration by the bed sediments. Observed colloid and solute exchanges were successfully predicted by the process-based models without the use of fitting coefficients. Hydraulic parameters measured in the flume and particle parameters measured in separate experiments were used as model inputs. The successful prediction of experimental results validates the modeling approach of combining a fundamental hydraulic exchange model with a physicochemical model for colloid transport and filtration in the streambed. Further, because colloid transport behavior was interpreted in terms of basic exchange and trapping processes, the results of this study are expected to be directly applicable to the analysis of fine sediment dynamics in natural streams. ¿ 2000 American Geophysical Union