Experiments were performed to determine the mechanisms and rates of solute exchange between a flowing stream and porous streambed. Mass balance, flow visualization, and depth profiles of concentration were used to investigate the exchange of an inert tracer in a laboratory flume. The measured mass transfer was compared to the exchange predicted by models of exchange related to bed forms, which are presented in a companion paper. For the initial stages with stationary bed forms or slowly moving bed forms the net exchange was predicted satisfactorily and was dominated by bed form--induced interstitial flows (pumping). In the initial stages with rapidly moving bed forms the exchange was dominated by scour/deposition as bed forms propagated (turnover). At later times the models of bed form--related exchange significantly underpredicted the measured exchange. The additional exchange, and the exchange to a flat bed, may be related to bed inhomogeneity or irregular variations in pressure at the bed surface which are not related to bed forms. Such effects are likely to be more pronounced in the field situation.¿ 1997 American Geophysical Union