Hyporheic fluxes intensity is expected to control for a great part the importance of self purification processes in stream which have porous bottoms, like sandy and gravel beds. A number of experiments on the field have led to qualitatively demonstrate the existence of inwelling, outwelling, upwelling and downwelling of water movements in a stream channel. A more quantitative approach is now required to build a hydraulic model of the hyporheic zone functioning. It would help to better understand the impact of combined sewer overflows on receiving streams. From a field experiment, first results are presented and discussed in the aim to build such a model.